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Serological evaluation of a simplified immunization schedule using quadruple DPT-polio vaccine in Burkina Faso
Serological evaluation of a simplified immunization schedule using quadruple DPT-polio vaccine in Burkina Faso Hans C. Riimke *°°, Martin Schlumberger t, Bruno Floury ~, Jaap Nagel* and Bert van Steenis* The immunogenicity of quadruple DPT-polio vaccine used in a two-dose regimen was investigated in a cross-sectional serological survey involving 355 children under 5 years o f age. This schedule is currently applied in the E P I programme in three provinces in Burkina Faso, West Africa. It was found that two doses of quadruple DPT-polio vaccine induced antibodies at protective levels to diphtheria and tetanus toxin, and to the three types of polioviruses in over 90% of 179 children. The persistence o f antibodies to tetanus and polioviruses is good, since over 90% of the children studied still had antibodies more than 2 years after their last vaccination. The antibodies to diphtheria toxin tend to decline in the first 6 months after vaccination, which is not uncommon. However, our data indicate clearly that a very high percentage (98%) of children have been primed to diphtheria toxin. O f 176 non-vaccinated children, up to 25% of the older ones had antibodies to polioviruses, most of them only to one type. This appears to be a sensitive parameter for the circulation of wildpolioviruses in the environment. As the vaccination coverage in the study area was low ( < 60% ), it was to be expected that the circulation o f polioviruses in the community could not be interrupted. The present study demonstrates the applicability o f a two-dose strategy for primary immunization with a quadruple DPT-polio vaccine especially for poliovirus components and the toxoids. To induce pertussis immunity, however, a third vaccination is recommended. Keywords: Immunization; EPI; simplified immunization schedule; inactivated polio vaccine; enhanced-potency IPV; DPT-polio vaccine
In many developing countries a major obstacle for the Expanded Programme on Immunization (EPI) is to reach an acceptable vaccination coverage, especially in giving children a complete course of primary immunizations. For this, five immunization contacts per child in the first year of life are required to give three doses of the triple DPT vaccine against diphtheria, pertussis and tetanus, three (or four) doses of live oral polio vaccine (OPV), one BCG vaccination and one measles vaccination (the general immunization schedule as advised by WHO and UNICEF). Simplified immun*RIVM: National institute of Public Health and Environmental Protection, Bilthoven, The Netherlands. tAPMP: Association pour la Promotion de la M~dicine Pr6ventive, Paris, France/ Bobo-Dioulasso, Burkina Faso. tCentre Muraz, Unit6 de Vaccinologie, OCCGE (Organisation de Coordination et de Cooperation pour la lutte contre les Grandes End~mies), Bobo-Dioulasso, Burkina Faso. Present address: Mission Franqaise de Cooperation, BP 91 Lomb, Togo. ~°To whom correspondence should be addressed at: National Institute of Public Health and Environmental Protection (Medical Center Immunizations), PO Box 1, 3720 BA, Bilthoven, The Netherlands. ( Received 17 J u ly 1992; revised 28 October 1992; accepted 3 December 1992) 0264-410X/93/11/1113-06 © 1993 Butterworth-Heinemann Lid
ization schedules have been designed to reach acceptable levels of population immunity with less operational constraints. From 1980 and 1982, such programmes have been operational in parts of two West African countries, Senegal and Burkina Faso respectively. Here, immunizations are given in only two visits, targeted at the ages of 3-8 and 9-14 months, respectively. Vaccines used are quadruple DPT-polio vaccine, given twice, containing an inactivated polio virus vaccine component with enhanced potency (IPV) 1-4. Other vaccines are simultaneously given against tuberculosis, measles and yellow fever. Although DPT-polio is expensive, the total costs of such a simplified programme per fully immunized child are less than that of a conventional programme because the logistic costs are lowered by the reduced number of immunization contacts 5. The effectiveness of the simplified immunization programme in three provinces of Burkina Faso was investigated in 1989. In the first phase we studied the impact of the programme on the epidemiology of poliomyelitis, neonatal tetanus, measles and pertussis, by interviewing parents of 17 154 children under the age of 10 among a total population of 763 000, in 90 randomly chosen clusters. The results have been described elsewhere 6'7. With regard to poliomyelitis and pertussis,
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Immunogenicity of two doses of DPT IPV. H.C. ROmke et al.
the findings are summarized here. Only 21 new probable cases of poliomyelitis had occurred since the start of the vaccination programme in 1982, the most recent case in 1986. Five of them had been immunized twice: two after the onset of paralysis, one child had one injection before and one after, and two had the two injections in the same year as the onset of paralysiS, but at unknown dates. Thus, in the worst view, only the latter two cases might be regarded as failures of the two-dose IPV strategy. With regard to pertussis, 2483 (14.5% of the 17 154 children) probable cases were found to have had whooping cough at some time in their life. Of these, 9% had been vaccinated twice according to the current EPI activities. Apparently there was no contribution to herd immunity against pertussis, which is not unexpected given the low vaccination coverage of about 50%. In this article we report results on the serological effectiveness of the programme, obtained in the second phase of this evaluation (April 1989). In a cross-sectional serological survey, antibody levels to diphtheria and tetanus toxins, and the three types of poliovirus in immunized children are compared with those in non-immunized children, as a function of their age. Also the persistence of antibody levels is analysed by stratification of intervals between the second vaccination and the blood sampling.
MATERIALS AND METHODS
Study area and immunization programme The Expanded Programme of Immunization (EPI) in Centre Nord, Burkina Faso, is sponsored and organized by the Netherlands foundation 'Stichting Redt de Kinderen' (Save the Children Fund). Centre Nord is a region of three contiguous provinces with a population of 763000 inhabitants. Since 1982, immunizations are given either by mobile teams using jet injectors (except for BCG), or at fixed health centres where needles and syringes are used. The simplified vaccination schedule in Centre Nord is: Visit 1 at age 3 - 8 months: first D P T polio, BCG Visit 2 at age 9 14 months: second D P T - p o l i o , measles, yellow fever. The two doses of D P T - p o l i o vaccine are given 2 to 6 months apart, which could be confirmed during the present survey. Individual vaccinations are recorded on vaccination certificates, provided at low cost for each child during the first session. Live oral polio vaccines have not been used at all in Centre Nord since 1982. The study has been performed at two study sites (Tourcoing-Bam and Zeguedeguin), involving a total of ten villages surrounding these places. All previous immunizations in these villages were done by mobile teams using jet injectors. The study has been approved by the Ministry of Health in Ouagadougou, Burkina Faso. In Centre Nord, the D P T polio vaccine used was produced by RIVM, The Netherlands (lot numbers 301 310), The polio component is of the enhanced-type inactivated polio vaccine (IPV) 8. One dose (0.5 ml) of the vaccine contained: diphtheria toxoid 15 Lf, pertussis 16 IOU, tetanus toxoid 10 Lf, polioviruses type 1 : 40 D antigen units (DU), type 2 : 8 DU, type 3 : 3 2 DU, and aluminium phosphate 3 mg. This quadruple vaccine was
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Vaccine, Vol. 11, Issue 11, 1993
produced following studies showing the effectiveness ol two doses of D P T and IPV as separate vaccines '~ ~3
Study design and population In the present cross-sectional study, antibodies to diphtheria and tetanus toxin, polioviruses and measles have been measured in sera from twice-immunized children at the age of 0 to 4 years. For comparison, non-immunized children have also been studied, as antibodies to some micro-organisms (polioviruses, measles virus) can be acquired rapidly during the first years of life by natural immunization by wild, circulating micro-organisms. Parents were invited to bring children under 5 years of age to take part in the study, and to be given vaccinations if needed. Village chiefs and health workers were visited I week and 1 day before the study sessions to obtain their cooperation. For each child, personal data were obtained from the parents and from vaccination cards: birth date, sex, location, village, province, dates of previous vaccinations, presence of BCG scar as a marker of an immunization contact. To improve the validity of these data villagers assisted in filling up the forms, especially in verifying the children's names recorded on the vaccination certificates. Vaccinations were only accepted by objective proof such as personalized vaccination cards and BCG scars. Blood samples were taken from a total of 367 children. Of these, 355 could be grouped according to their age and immunization status: 176 non-vaccinated children (age 3 8 months: 56; 9 15 months; 57; and 16 48 months: 63 children), and 179 children vaccinated twice (age 3 8 months; 9; 9 15 months, 28; and 16-48 months; 142 children ). In the three age groups of children without documented D P T polio vaccinations 12, 13 and 13 children had a BCG scar, respectively. Only five of the 13 children in the oldest age group appeared to have antibodies to the toxoids and polio antigens. Although these five children had apparently been vaccinated, they are included in the tables and calculations with the data on the non-vaccinated children, because their vaccination status was not properly documented. For the other eight children, the antibody levels to these antigens did not indicate that they had been vaccinated previously. The vaccinated children could also be grouped according to the interval between the second D P T polio vaccination and blood sampling. This group includes also seven more children older than 4 years, but included above. Of these 186 children, 48 were studied within 6 months of the second D P T polio dose, 23 between 7 and 12 months, 54 between 13 and 18 months, 23 between 19 and 24 months, and 38 more than 25 months after. The average ages at which the D P T - polio vaccinations were given are:first D P T - p o l i o : 185 days (s.d. 128 days); second D P T polio: 321 days (s.d. 181 days); measles: 356 days (s.d. 136 days). In all, 72% of the children in the study sample received the second dose before the age of 12 months, 89% before 18 months. As expected, these ages are clearly different from the recommended ages for vaccinations according to EPI.
Samples, coding and determination of antibody levels Venous blood samples were obtained, and kept at ambient midday temperature (42°C) for a period of 4 h or less. After centrifugation on the same day the sera were kept at 2-8"C, and transported under code to
Immunogenicity of two doses of DPT-IPV: H.C. ROmke et al.
RIVM for antibody titrations. The code list was held by Organisation de Coordination et de Coop6ration pour la lutte contre les Grandes End6mies (OCCGE). The codes were broken after finishing the titrations. Neutralizing tetanus and diphtheria antitoxins were quantified by a toxin-binding inhibition test 14'~5, and expressed as International Units (IU) per ml. Polio antibodies were quantified by a standard microneutralization test, using Vero cells as target cells for unneutralized wild poliovirus. Polio antibody titres are given as 2log reciprocal values, indicating the lowest of a twofold dilution series showing neutralizing activity. Antibodies to pertussis antigens have not been measured, as their relevance for protection against pertussis is not known. In addition, the antibody status would be influenced very much by the fact that pertussis was a prevalent disease, maintaining antibody levels by boosting through circulating bacteria. In earlier surveys it was found that DPT-polio vaccine induced significant levels of antibodies against pertussis antigens (not shown). Epidemiological data are regarded as more important to evaluate pertussis vaccine effectiveness. Results of antibody determinations beyond the detection limits have been adapted such that they could be processed numerically. Diphtheria and tetanus antibodies <0.03 IU ml- ~ (or <0.06 U ml- t in case of sera diluted because of small quantity) were calculated as 0.01 IUm1-1, a value >16.00 was processed as 20.00IUm1-1. Polio antibodies: a 2log titre <1 (or another value in the case of diluted or toxic serum) was calculated as 0, a value > 12 (maximum titre in assay) as 13. Calculation of geometric mean titres and ranges, as well as the statistical analyses are performed after these alterations. Work-up of data and statistical analysis were done using Lotus 1-2-3 and Epi Info software.
Table 1 Tetanus and diphtheria antibody titres according to vaccination status and age Tetanus Age group
(Y'
2
Diphtheria 0
2
3 4 months Titre 0.05+ 0.1 + 1.0 + GMT Range n
10 (18) 3 (5) 1 (2) 0.02 0.01-0.06 56
9 (100) 9 (100) 6 (67) 2.04 0.79-5.27 9
2 (4) 1 (2) 0 (0) 0.01 0.01-0.03 56
9 (100) 7 (78) 2 (22) 0.36 0.13-1.04 9
9-15 months Titre 0.05+ 0.1 + 1.0 + GMT Range n
1 (2) 0 (0) 0 (0) 0.01 0.01-0.02 57
27 (96) 25 (89) 15 (24) 1.10 0.21-5.79 28
1 (2) 0 (0) 0 (0) 0.01 0.01-0.02 57
24 (86) 21 (75) 7 (25) 0.33 0.05-2.14 28
1~48 months Titre 0.05+ 0.1 + 1.0 + GMT Range n
8 (13) 138 (97) 8 (13) 130 (92) 2 (3) 55 (39) 0.02 0.70 0.01-0.07 0.18-2.66 63 142
8 (13) 99 (70) 4 (6) 72 (51) 1 (2) 5 (4) 0.02 0.11 0.01-0.04 0.03-0.46 63 142
19 (11) 172 (96) 11 (6) 162 (91) 3 (2) 77 (43) 0.02 0.79 0.01-0.05 0.19-3.22 176 179
11 (6) 142 (79) 5 (3) 100 (56) 1 (1) 14 (8) 0.01 0.14 0.01-0.03 0.03-0.66 176 179
All ages Titre 0.05+ 0.1 + 1.0 + GMT Range n
The number (percentage) of children with a minimum antibody level of 0.05, 0.1 and 1.0 IU m1-1, respectively, as well as the geometric mean titres (GMT) and range (__+1 s.d.) are given per group. Within each age group, ;(= tests give significantly different (p < 0.0001 ) antibody levels between non-vaccinated and twice-vaccinated children. Differences between age groups were not significant (p > 0.01), except for diphtheria antitoxins in the twice-vaccinated groups of 9-15 months versus 16-48 months of age, respectively (p < 0.0001 ) aNo. of doses of DPT-polio vaccine
RESULTS The results are presented in relation to vaccination status. Tetanus antibodies are found only after vaccination or passive immunization (e.g. by placental transfer). In older children, their presence may therefore discriminate between non-vaccinated and (twice-) vaccinated. In the present survey they have been used as a check of the validity of the assessment of vaccination status. Within the group of non-vaccinated children there was no difference in antibody titres between children with or without a BCG scar (data not shown). Inclusion or exclusion of the five children without written documentation of immunization who had a BCG scar and tetanus antibodies made no difference between the data of the non- and twice-vaccinated children (not shown). Because of the lack of written documentation of vaccination they have been included in the 'non-vaccinated group'.
Tetanus antibodies
Of the twice-immunized children in the various age groups, 96% had antibodies at a protective level ( i>0.05 IU ml- 1), while of the non-immunized children 11% had tetanus antibodies at this level, as shown in Table 1. In the youngest non-vaccinated children, 10 out of 56 (18 % ) probably had maternally derived antibodies
(EPI aims at immunizing women of child-bearing age with tetanus toxoid). The 9-15 month age groups has lost maternal antibodies, only one child of 57 was antibody-positive. In the oldest age group of reportedly non-vaccinated children, eight out of 63 (13%) had antibodies, most probably due to non-recorded vaccinations. A BCG scar was found in five of these children (it is possible that some children received only BCG, without DPT-polio at the same vaccination session). On the basis of these figures (not corrected for BCG-positive children), the predictive value of the presence of tetanus antibodies ~>0.1IUm1-1 for a reportedly vaccinated child was 92% (sensitivity 93.7%, and specificity 91.7%). At a lower discriminative level (0.05 IU ml-1 ) the predictive value was 97% (sensitivity 90.2%, and specificity 96.9%). Thus, in the present survey the assessment of vaccination status can be regarded as sufficient. When the children are grouped according to the interval between the last DPT-polio injection and bleeding, the persistence of tetanus antibodies appeared satisfactory. More than 2 years after the second DPT-polio immunization all of the 38 children studied still had anti-tetanus antibody levels />0.1 IUm1-1 ( Table 2 ).
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Immunogenicity of two doses of DPT IPV. H.C. Rumke et al. Table 2 Tetanus and diphtheria antibody titres according to interval between second vaccination and blood sampling
Interval
Tetanus
Diphtheria
0 6 months 0.05+ 0.1 + 1.0 + GMT Range (n = 48)
48 (100) 47 (98) 41 (85) 2.18 0.68 7.02
47 (98) 43 (90) 15 (31) 0.45 0.11 1.9
7 - 1 2 months 0.05+ 0.1 + 1.0 + GMT Range (n = 23)
22 (96) 21 (91) 6 (26) 0.35 0.08-1.52
19 (83) 13 (57) 0 (0) 0.07 0.02-0.22
13 18 months 0.05 + 0.1 + 1.0 + GMT Range (n 54)
53 (98) 50 (93) 20 (37) 0.58 0.16-2.16
43 (80) 35 (65) 3 (6) 0.09 0.02 0.39
19 24 months 0.05+ 0.1 + 1.0 + GMT Range (n = 23)
23 (100) 22 (96) 9 (39) O.58 0.18 1.94
18 (78) 13 (57) 0 (0) 0.09 0.02-0.35
~>25 months 0.05+ 0.1 + 1.0 + GMT Range (n = 38)
38 (100) 38 (100) 16 (42) 0.62 0.21-1.89
Table 3
31 (82) 22 (58) 1 (3) 0.09 0.02-0.41
The number (percentage) of children with a minimum antibody level of 0.05, 0.1 and 1.0 IU m1-1, respectively, as well as the geometric mean titres (GMT) and range (_+1 s.d.) are given per group. ~ tests gave significantly different (p < 0.0001) diphtheria antibody levels between 0 6 months versus 7 12 months after the second vaccination. Differences between other groups were not significant
Diphtheria antibodies As shown in Table 1, 11 of the 176 non-vaccinated children ( 6 % ) h a d antibodies />0.05 IU ml-1. Of these 11, eight belonged to the 63 children of the 16 48 months age group. Of the 179 twice-vaccinated children, 142 (79%) had antibodies above this level. It appeared that the antibody levels declined rapidly with time after vaccination (Table 2). In the first 6 months after the second vaccination 47 out of 48 children (98%) were antibody-positive, indicating a high seroconversion rate. Children studied after a longer interval following the second injection had lower antibody levels: about 80% still have circulating diphtheria antitoxins, but at a stable level. Thus, the finding of a substantial percentage (21% ) of children lacking diphtheria antibodies is probably caused by the relatively short persistence of these antibodies after the second vaccination. However, it can be assumed that nearly all twice-vaccinated children in this study have been primed for diphtheria toxoid, which is the goal of primary immunization. Polio antibodies More than 95% of the vaccinees had antibodies to
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poliovirus types l and 2 ( Table 3 ). The immunogenicity of the polio type 3 component in the quadruple vaccine appeared somewhat lower (about 93% positive after two vaccinations) than that of types 1 and 2. Although we regard the presence of any neutralizing activity (values in tables: 1 + ) in a given serum against the virus strains used in the test as protective, we used a safety margin (values 3 + ) to correct for non-specificity, aberrant virus strains, and in our hands small-laboratory inaccuracy. Both 1 + and 3 + values are shown. As compared with diphtheria and tetanus antibodies, polio antibodies tended to persist longer after two vaccinations ( Table 4 ), but this may be due to boosting by naturally circulating wild virus. This is suggested by the findings in non-vaccinated children. In 20-.25% of these we also found polio antibodies. In the youngest age group these could be maternal antibodies. In the oldest group only a few of these children had antibodies to all three types of polioviruses and to the toxoids, suggesting unrecorded prior vaccination. The majority of the other children had antibodies to only one of the three virus types. These must have been induced by natural contact with polioviruses circulating in the study area. It was not possible to allocate children with a hagh antibody titre to only one type of poliovirus to specific villages (data not shown).
V a c c i n e , Vol. 11, I s s u e 11, 1993
Polio antibody titres according to vaccination status and age Type 1 0a
2
Type 2 0
2
Type 3 0
2
3-8 months 1+ 19 (35) 3+ 4 (7) 6+ 2 (4) Mean 0.8 S.d. 1.6 n 55
9 (100) 9 (100) 7 (78) 7.3 2.1 9
23 (42) 10 (18) 0 (0) 1.0 1.3 55
9 (100) 9 (100) 8 (89) 9.0 2.3 9
17 (31) 3 (5) 0 (0) 0.6 1.1 55
8 (89) 8 (89) 3 (33) 5.1 3.0 9
9-15 months 1+ 3 (5) 3+ 2 (4) 6+ 2 (4) Mean 0.3 S.d. 1.2 n 57
28 (100) 28 (100) 23 (82) 7.3 2.3 28
1 (2) 1 (2) 1 (2) 0.1 0.9 57
28 (100) 28 (100) 27 (96) 9.1 2.1 28
5 (9) 3 (5) 2 (4) 0.5 2.0 57
26 (93) 21 (75) 12 (43) 5.4 3.6 28
16 48 months 1+ 17 (27) 136 (96) 3+ 15 (24) 123 (87) 6+ 8 (13) 99 (70) 7.2 Mean 1.5 3.4 S.d. 3.1 142 n 63
19 (30) 141 (99) 16 (25) 141 (99) 11 (17) 119 (84) 2.1 8.4 3.7 2.8 63 142
22 (35) 132 (93) 19 (30) 111 (78) 11 (17) 84 (59) 1.9 5.9 3.0 3.5 63 142
All 1+ 39 (22) 3+ 19 (11) 6+ 12 (6) Mean 0.9 S.d. 2.2 n 175
173 (97) 169 (94) 129 (72) 7.2 3.2 179
43 (25) 178 (99) 27 (15) 177 (99) 12 (7) 154 (86) 1.1 8.5 2.6 2.7 175 179
44 (25) 25 (14) 13 (7) 1.1 2.3 175
166 (93) 140 (78) 99 (55) 5.8 3.5 179
The number (percentage) of children with a minimum neutralizing antibody level of 1, 3 and 6 (1/2, 1/8 and 1/64), respectively, as well as the mean titres and standard deviation (s.d.) are given per group. Within each age group, ;(2 tests give significantly different (p < 0.0001) antibody levels between non-vaccinated and twice-vaccinated children. Per vaccine group (0 or 2 doses) there are no significant differences (p > 0.01 ) between the age groups, with exception of those between the levels of type 1 antibodies (p = 0.0003), and for type 2 antibodies (p < 0.001) both of the youngest and middle age groups aNo. of doses of DPT-polio vaccine
Immunogenicity of two doses of DPT-IPV: H.C. ROmke et al. Table 4 Polio antibody titres according to interval between second vaccination and blood sampling Interval
Type 1
Type 2
Type 3
0-6 months 1+ 3+ 6+ Mean S.d. (n = 48)
45 (94) 43 (90) 32 (67) 7.1 3.5
48 (100) 47 (98) 44 (92) 9.3 2.5
43 (90) 33 (69) 19 (40) 4.7 3.3
7-12 months 1+ 3+ 6+ Mean S.d. (n = 54)
23 (100) 23 (100) 15 (65) 6.3 2.4
23 (100) 23 (100) 20 (87) 8.0 2.5
21 (91) 20 (87) 10 (43) 5.4 3.2
13-18 months 1+ 3+ 6+ Mean S.d. (n = 54)
51 (94) 50 (93) 39 (72) 6.9 3.2
54 (100) 54 (100) 47 (87) 8.6 2.5
51 (94) 40 (74) 30 (56) 5.8 3.5
19-24 months 1+ 3+ 6+ Mean S.d. (n = 23)
23 (100) 22 (96) 18 (78) 7.5 2.9
22 (96) 22 (96) 18 (78) 8.0 3.4
21 (91) 20 (87) 15 (65) 6.3 3.3
/>25 months 1+ 3+ 6+ Mean S.d. (n = 38)
38 (100) 38 (100) 32 (84) 8.8 3.0
38 (100) 38 (100) 32 (84) 8.5 2.6
37 (97) 34 (89) 31 (82) 7.6 3.3
The number (percentage) of children with a minimum neutralizing antibody level of 1, 3 and 6 (1/2, 1/8 and 1/64), respectively, as well as the mean titres and standard deviation (s.d.) are given per group. Differences between groups according to interval were not significant (p > 0.01) for either type of polio virus antibody
DISCUSSION In the present study it was found that two doses of quadruple DPT-polio vaccine induce antibodies at protective levels to diphtheria and tetanus toxin, and to the three types of polioviruses in over 90% of children. Antibodies to tetanus and polioviruses persist for more than 2 years after their last vaccination in the majority of children. The diphtheria antitoxin levels tend to decline after 6 months, and appear to remain stable thereafter. Waning of diphtheria antitoxins is not uncommon 16--18. Our data indicate clearly that nearly all children had been primed for diphtheria toxoid. Of course, it is desirable that vaccinated children have protective levels of antitoxins for a long period, but it is likely that the absence of demonstrable antibodies does not automatically imply that the vaccinee is not protected. Immunological memory without detectable antibodies may be sufficient for protection against clinical diphtheria, and is likely to be boosted upon a revaccination or natural (skin) infection. Regarding the immunity induced against poliovirus by two doses of DPT-polio the results are in line with observations from the Kolda region in Senegal. In this region, two doses of a comparable DPT-polio vaccine
gave a high level of protection during the polio type 1 virus epidemic in 1986 (vaccine efficacy 89%) 19 21. Owing to the low vaccination coverage in that area the epidemic could not be prevented. Because of the high seroconversion rates in the present study, we feel confident that a similar high percentage of the vaccinated children here is well protected. The epidemiological survey done 3 months before the present serological survey in the same area 6'7 revealed no recent clinical cases (last case in 1986). After the survey, no polio cases become known through routine reporting systems in the study region up to July 1992. Since among non-immune individuals infected with poliovirus only a few will develop paralysis (1:100 to 1:1000) cases may not have become apparent, although the magnitude of this survey with data on 17 154 children justifies confidence in the absence of recent polio patients. Nevertheless, at the low vaccination coverage found during the same survey (40-60%) 6, silent transmission of wild poliovirus probably still continues, as indicated by the presence of antibodies in non-vaccinated children (and perhaps the stability of antibody levels in vaccinated children). This appears a more sensitive parameter for the circulation of polioviruses than the finding of paralytic patients, although it is impossible to estimate the extent of virus circulation. Hence, if the vaccination coverage remains at its present level (irrespective of which type of vaccine is used), non-vaccinated individuals will hardly be protected by herd immunity. It is therefore reassuring that in the years after the present surveys the vaccination coverage has increased to > 70%. No serological data for pertussis were presented. During the epidemiological survey done in the same year as the present study, it was found that pertussis could not be fully prevented in the study region. It would probably need a much higher vaccination coverage to reduce the number of pertussis patients. An investigation of a small pertussis outbreak in a village in one of the study provinces suggested that the proportion of cases among younger vaccinees was lower than that among the non-vaccinated 22. These findings are in agreement with a study from Kenya, which also showed that after some years pertussis antibodies waned faster in a two-dose schedule group than in a three-dose group. Also the incidence of pertussis cases appeared higher (not significant) in the two-dose group 23. It has to be concluded that the protection given by two doses of pertussis vaccine is suboptimal. In conclusion, this study shows that the simplified immunization schedule, with only two visits for injections of DPT polio vaccine, can be used for primary immunization against diphtheria, tetanus and the three poliovirus antigens. Neutralizing antibodies to tetanus toxin and the three polioviruses are detectable more than 2 years after the second vaccination, while diphtheria neutralizing antibodies tend to persist for less time. Although not the subject of the present study, for protection against pertussis two doses is regarded as suboptimal. Therefore, to induce stronger and longer lasting immunity a third vaccination is recommended. The immunization programme in the area studied here is now being changed accordingly. ACKNOWLEDGEMENTS The authors are grateful to the staff of the provincial EPI
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I r n m u n o g e n i c i t y of t w o d o s e s o f D P T IPV: H.C. REimke et al.
teams, technicians of the Centre Muraz, the technical assistants of APMP, Burkina Faso, and the staff of the Stichting Redt de Kinderen (The Hague/Ouagadougou) for their technical support in the execution of this survey.
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REFERENCES
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1 Stoeckel, P. Simplifying immunizations schedules in West Africa. In: Universal Child Immunization (Ed. Mandl, P.-E.) Unicef Assignment Children 1985, 89/72, 329-344 2 Stuyt, L.B.J. Vaccinaties in Afrika; ervaringen in Burkina Faso. Ned. Tijdschr. Geneeskd. 1988, 132, 2375-2380 3 Stoeckel, P., Schlumberger, M, Parent, G., Maire, B., Van Wezel, A.L., Van Steenis, G. and Salk, D. Use of killed polio virus vaccine in a routine immunization program in West Africa. Rev. Infect. Dis. 1984, 6, $463-466 4 Cohen, H. and Nagel, J. Two injections of Diphtheria-TetanusPertussis-Polio Vaccine as the backbone of a simplified immunization schedule in developing countries. Rev. Infect. Dis. 1984, 6, $350-351 5 De Champeaux, A. and Martin, B.-J. Evaluation comparative de I'efficience de deux strategies de vaccination au Burkina Faso. In: Economic and Financial Aspects of the Expanded Program of Immunization, Centre International de I'Enfance, Paris, France, 1991 6 Schwoebel, V. and Da Silva, A. (Eds) Evaluation ~pid6miologique du Programme Elargi des Vaccinations des Provinces du Barn, Sanmatenga et Namentenga, Burkina Faso. Rapport final. July 1989. APMP, Paris 7 Schwoebel, V., Dauvisis, A.-V., Helynck, B., Gomes, E., Drejer, G.F., Schlumberger, M. et al. Community-based evaluation survey of immunizations in Burkina Faso. Buff. World Health Organ. 1992, 70, 583-590 8 Van Wezel, A.L, Van Steenis, G., Hannik, C.A. and Cohen, H. New approach to the production of concentrated and purified inactivated polio and rabies tissue culture vaccines. Dev. Biol. Stand. 1978, 41, 159 168 9 Mangay-Angara, A., Fulgencio, L., Casabal, G., Gudani, L., Sumpaico, J., Ocampo, A. et al. A two-dose schedule for immunization of infants using a more concentrated DPT-vaccine. Dev. Biol. Stand. 1978, 41, 15-22 10 Mahieu, J.M., Muller, A.S., Voorhoeve, A.M. and Dikken, H. Pertussis in a rural area of Kenya: epidemiology and a preliminary report of a vaccine trial. Buff. World Health Organ. 1978, 56, 773-780
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Salk, J., Cohen, H., Fillastre, C., Stoeckel, P., Rey, J.-L., Schiumberger, M. et al. Killed polio virus antigen titrations in humans. Dev. Biol. Stand. 1978, 41, 119 132 Salk, J., Stoeckel, P., Van Wezel, A.L., Lapinleimu, K. and Van Steenis, G. Antigen content of inactivated polio virus vaccine for use in a one- or two-dose regimen. Ann. Clin. Res. 1982, 14, 204-212 Salk, D., Van Wezel, A.L. and Salk, J. Induction of long term immunity to paralytic poliomyelitis by use of a non-infectious vaccine. Lancet 1984, ii, 1317 1321 Hendriksen, C.F.M., Van der Gun, J.W., Nagel, J. and Kreeftenberg, J.G. The toxin binding inhibition test as a reliable in vitro alternative to the toxin neutralization test in mice for the estimation of tetanus antitoxin in human sera. J. Biol. Stand. 1988, 16, 287-297 Hendriksen, C.FM., Van der Gun, J.W. and Kreeftenberg, J.G. Combined estimation of tetanus and diphtheria antitoxin in human sera by in vitro toxin-binding inhibition (ToBI). J. Biol. Stand. 1989, 17, 191-200 Simonsen, O. Vaccination against tetanus and diphtheria. Evaluation of immunity in the Danish population, guidelines for revaccination, and methods for control of vaccination programs. Dan. Med. Bull. 1989, 36, 24-47 BjTrkholm, B., Granstr6m, M., Wahl, M., Hedstrom, C.-E. and Hagberg, L. Increased dose of diphtheria toxoid for basic immunization of adults. Eur. J. Clin. Microbiol. Infect. Dis. 1989, 8, 701 705 Larsen, K., UIIberg-Olsson, K., Wickbom, B. and Hederstedt, B. The immunization of children with combined diphtheria and tetanus vaccine in Sweden. J. Biol. Stand. 1987, 15, 103-107 Centers for Disease Control. Preliminary report: paralytic poliomyelitis Senegal, 1986. MMWR Morb. Mortal. Wkly Rep. 1987, 36, 387-390 Centers for Disease Control. Paralytic poliomyelitis Senegal 1986 1987: update on the N-IPV efficacy study. MMWR Morb. Mortal. Wkly Rep. 1988, 37, 257-258 Robertson, S.E., Traverso, H.P., Drucker, J.A., Rovira, E.Z., Fabre-Teste, B., Sow, A. et al. Clinical efficacy of a new enhanced potency inactivated polio virus vaccine. Lancet 1988, I, 897 899 R0mke, H.C., Oostvogel, P.M. and Schlumberger, M. Investigation of a Pertussis Outbreak in the Province of Bam, Burkina Faso, January 1991. Report 928702.002, RIVM, Bilthoven, The Netherlands Muller, A.S., Leeuwenburg, J. and Voorhoeve, A.M. Pertussis in a rural area of Kenya; epidemiology and results of a vaccine trial. Bull. World Health Organ. 1984, 62, 899-908
In many developing countries a major obstacle for the Expanded Programme on Immunization (EPI) is to reach an acceptable vaccination coverage, especially in giving children a complete course of primary immunizations. For this, five immunization contacts per child in the first year of life are required to give three doses of the triple DPT vaccine against diphtheria, pertussis and tetanus, three (or four) doses of live oral polio vaccine (OPV), one BCG vaccination and one measles vaccination (the general immunization schedule as advised by WHO and UNICEF). Simplified immun*RIVM: National institute of Public Health and Environmental Protection, Bilthoven, The Netherlands. tAPMP: Association pour la Promotion de la M~dicine Pr6ventive, Paris, France/ Bobo-Dioulasso, Burkina Faso. tCentre Muraz, Unit6 de Vaccinologie, OCCGE (Organisation de Coordination et de Cooperation pour la lutte contre les Grandes End~mies), Bobo-Dioulasso, Burkina Faso. Present address: Mission Franqaise de Cooperation, BP 91 Lomb, Togo. ~°To whom correspondence should be addressed at: National Institute of Public Health and Environmental Protection (Medical Center Immunizations), PO Box 1, 3720 BA, Bilthoven, The Netherlands. ( Received 17 J u ly 1992; revised 28 October 1992; accepted 3 December 1992) 0264-410X/93/11/1113-06 © 1993 Butterworth-Heinemann Lid
ization schedules have been designed to reach acceptable levels of population immunity with less operational constraints. From 1980 and 1982, such programmes have been operational in parts of two West African countries, Senegal and Burkina Faso respectively. Here, immunizations are given in only two visits, targeted at the ages of 3-8 and 9-14 months, respectively. Vaccines used are quadruple DPT-polio vaccine, given twice, containing an inactivated polio virus vaccine component with enhanced potency (IPV) 1-4. Other vaccines are simultaneously given against tuberculosis, measles and yellow fever. Although DPT-polio is expensive, the total costs of such a simplified programme per fully immunized child are less than that of a conventional programme because the logistic costs are lowered by the reduced number of immunization contacts 5. The effectiveness of the simplified immunization programme in three provinces of Burkina Faso was investigated in 1989. In the first phase we studied the impact of the programme on the epidemiology of poliomyelitis, neonatal tetanus, measles and pertussis, by interviewing parents of 17 154 children under the age of 10 among a total population of 763 000, in 90 randomly chosen clusters. The results have been described elsewhere 6'7. With regard to poliomyelitis and pertussis,
Vaccine, Vol. 11, Issue 11, 1993
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Immunogenicity of two doses of DPT IPV. H.C. ROmke et al.
the findings are summarized here. Only 21 new probable cases of poliomyelitis had occurred since the start of the vaccination programme in 1982, the most recent case in 1986. Five of them had been immunized twice: two after the onset of paralysis, one child had one injection before and one after, and two had the two injections in the same year as the onset of paralysiS, but at unknown dates. Thus, in the worst view, only the latter two cases might be regarded as failures of the two-dose IPV strategy. With regard to pertussis, 2483 (14.5% of the 17 154 children) probable cases were found to have had whooping cough at some time in their life. Of these, 9% had been vaccinated twice according to the current EPI activities. Apparently there was no contribution to herd immunity against pertussis, which is not unexpected given the low vaccination coverage of about 50%. In this article we report results on the serological effectiveness of the programme, obtained in the second phase of this evaluation (April 1989). In a cross-sectional serological survey, antibody levels to diphtheria and tetanus toxins, and the three types of poliovirus in immunized children are compared with those in non-immunized children, as a function of their age. Also the persistence of antibody levels is analysed by stratification of intervals between the second vaccination and the blood sampling.
MATERIALS AND METHODS
Study area and immunization programme The Expanded Programme of Immunization (EPI) in Centre Nord, Burkina Faso, is sponsored and organized by the Netherlands foundation 'Stichting Redt de Kinderen' (Save the Children Fund). Centre Nord is a region of three contiguous provinces with a population of 763000 inhabitants. Since 1982, immunizations are given either by mobile teams using jet injectors (except for BCG), or at fixed health centres where needles and syringes are used. The simplified vaccination schedule in Centre Nord is: Visit 1 at age 3 - 8 months: first D P T polio, BCG Visit 2 at age 9 14 months: second D P T - p o l i o , measles, yellow fever. The two doses of D P T - p o l i o vaccine are given 2 to 6 months apart, which could be confirmed during the present survey. Individual vaccinations are recorded on vaccination certificates, provided at low cost for each child during the first session. Live oral polio vaccines have not been used at all in Centre Nord since 1982. The study has been performed at two study sites (Tourcoing-Bam and Zeguedeguin), involving a total of ten villages surrounding these places. All previous immunizations in these villages were done by mobile teams using jet injectors. The study has been approved by the Ministry of Health in Ouagadougou, Burkina Faso. In Centre Nord, the D P T polio vaccine used was produced by RIVM, The Netherlands (lot numbers 301 310), The polio component is of the enhanced-type inactivated polio vaccine (IPV) 8. One dose (0.5 ml) of the vaccine contained: diphtheria toxoid 15 Lf, pertussis 16 IOU, tetanus toxoid 10 Lf, polioviruses type 1 : 40 D antigen units (DU), type 2 : 8 DU, type 3 : 3 2 DU, and aluminium phosphate 3 mg. This quadruple vaccine was
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Vaccine, Vol. 11, Issue 11, 1993
produced following studies showing the effectiveness ol two doses of D P T and IPV as separate vaccines '~ ~3
Study design and population In the present cross-sectional study, antibodies to diphtheria and tetanus toxin, polioviruses and measles have been measured in sera from twice-immunized children at the age of 0 to 4 years. For comparison, non-immunized children have also been studied, as antibodies to some micro-organisms (polioviruses, measles virus) can be acquired rapidly during the first years of life by natural immunization by wild, circulating micro-organisms. Parents were invited to bring children under 5 years of age to take part in the study, and to be given vaccinations if needed. Village chiefs and health workers were visited I week and 1 day before the study sessions to obtain their cooperation. For each child, personal data were obtained from the parents and from vaccination cards: birth date, sex, location, village, province, dates of previous vaccinations, presence of BCG scar as a marker of an immunization contact. To improve the validity of these data villagers assisted in filling up the forms, especially in verifying the children's names recorded on the vaccination certificates. Vaccinations were only accepted by objective proof such as personalized vaccination cards and BCG scars. Blood samples were taken from a total of 367 children. Of these, 355 could be grouped according to their age and immunization status: 176 non-vaccinated children (age 3 8 months: 56; 9 15 months; 57; and 16 48 months: 63 children), and 179 children vaccinated twice (age 3 8 months; 9; 9 15 months, 28; and 16-48 months; 142 children ). In the three age groups of children without documented D P T polio vaccinations 12, 13 and 13 children had a BCG scar, respectively. Only five of the 13 children in the oldest age group appeared to have antibodies to the toxoids and polio antigens. Although these five children had apparently been vaccinated, they are included in the tables and calculations with the data on the non-vaccinated children, because their vaccination status was not properly documented. For the other eight children, the antibody levels to these antigens did not indicate that they had been vaccinated previously. The vaccinated children could also be grouped according to the interval between the second D P T polio vaccination and blood sampling. This group includes also seven more children older than 4 years, but included above. Of these 186 children, 48 were studied within 6 months of the second D P T polio dose, 23 between 7 and 12 months, 54 between 13 and 18 months, 23 between 19 and 24 months, and 38 more than 25 months after. The average ages at which the D P T - polio vaccinations were given are:first D P T - p o l i o : 185 days (s.d. 128 days); second D P T polio: 321 days (s.d. 181 days); measles: 356 days (s.d. 136 days). In all, 72% of the children in the study sample received the second dose before the age of 12 months, 89% before 18 months. As expected, these ages are clearly different from the recommended ages for vaccinations according to EPI.
Samples, coding and determination of antibody levels Venous blood samples were obtained, and kept at ambient midday temperature (42°C) for a period of 4 h or less. After centrifugation on the same day the sera were kept at 2-8"C, and transported under code to
Immunogenicity of two doses of DPT-IPV: H.C. ROmke et al.
RIVM for antibody titrations. The code list was held by Organisation de Coordination et de Coop6ration pour la lutte contre les Grandes End6mies (OCCGE). The codes were broken after finishing the titrations. Neutralizing tetanus and diphtheria antitoxins were quantified by a toxin-binding inhibition test 14'~5, and expressed as International Units (IU) per ml. Polio antibodies were quantified by a standard microneutralization test, using Vero cells as target cells for unneutralized wild poliovirus. Polio antibody titres are given as 2log reciprocal values, indicating the lowest of a twofold dilution series showing neutralizing activity. Antibodies to pertussis antigens have not been measured, as their relevance for protection against pertussis is not known. In addition, the antibody status would be influenced very much by the fact that pertussis was a prevalent disease, maintaining antibody levels by boosting through circulating bacteria. In earlier surveys it was found that DPT-polio vaccine induced significant levels of antibodies against pertussis antigens (not shown). Epidemiological data are regarded as more important to evaluate pertussis vaccine effectiveness. Results of antibody determinations beyond the detection limits have been adapted such that they could be processed numerically. Diphtheria and tetanus antibodies <0.03 IU ml- ~ (or <0.06 U ml- t in case of sera diluted because of small quantity) were calculated as 0.01 IUm1-1, a value >16.00 was processed as 20.00IUm1-1. Polio antibodies: a 2log titre <1 (or another value in the case of diluted or toxic serum) was calculated as 0, a value > 12 (maximum titre in assay) as 13. Calculation of geometric mean titres and ranges, as well as the statistical analyses are performed after these alterations. Work-up of data and statistical analysis were done using Lotus 1-2-3 and Epi Info software.
Table 1 Tetanus and diphtheria antibody titres according to vaccination status and age Tetanus Age group
(Y'
2
Diphtheria 0
2
3 4 months Titre 0.05+ 0.1 + 1.0 + GMT Range n
10 (18) 3 (5) 1 (2) 0.02 0.01-0.06 56
9 (100) 9 (100) 6 (67) 2.04 0.79-5.27 9
2 (4) 1 (2) 0 (0) 0.01 0.01-0.03 56
9 (100) 7 (78) 2 (22) 0.36 0.13-1.04 9
9-15 months Titre 0.05+ 0.1 + 1.0 + GMT Range n
1 (2) 0 (0) 0 (0) 0.01 0.01-0.02 57
27 (96) 25 (89) 15 (24) 1.10 0.21-5.79 28
1 (2) 0 (0) 0 (0) 0.01 0.01-0.02 57
24 (86) 21 (75) 7 (25) 0.33 0.05-2.14 28
1~48 months Titre 0.05+ 0.1 + 1.0 + GMT Range n
8 (13) 138 (97) 8 (13) 130 (92) 2 (3) 55 (39) 0.02 0.70 0.01-0.07 0.18-2.66 63 142
8 (13) 99 (70) 4 (6) 72 (51) 1 (2) 5 (4) 0.02 0.11 0.01-0.04 0.03-0.46 63 142
19 (11) 172 (96) 11 (6) 162 (91) 3 (2) 77 (43) 0.02 0.79 0.01-0.05 0.19-3.22 176 179
11 (6) 142 (79) 5 (3) 100 (56) 1 (1) 14 (8) 0.01 0.14 0.01-0.03 0.03-0.66 176 179
All ages Titre 0.05+ 0.1 + 1.0 + GMT Range n
The number (percentage) of children with a minimum antibody level of 0.05, 0.1 and 1.0 IU m1-1, respectively, as well as the geometric mean titres (GMT) and range (__+1 s.d.) are given per group. Within each age group, ;(= tests give significantly different (p < 0.0001 ) antibody levels between non-vaccinated and twice-vaccinated children. Differences between age groups were not significant (p > 0.01), except for diphtheria antitoxins in the twice-vaccinated groups of 9-15 months versus 16-48 months of age, respectively (p < 0.0001 ) aNo. of doses of DPT-polio vaccine
RESULTS The results are presented in relation to vaccination status. Tetanus antibodies are found only after vaccination or passive immunization (e.g. by placental transfer). In older children, their presence may therefore discriminate between non-vaccinated and (twice-) vaccinated. In the present survey they have been used as a check of the validity of the assessment of vaccination status. Within the group of non-vaccinated children there was no difference in antibody titres between children with or without a BCG scar (data not shown). Inclusion or exclusion of the five children without written documentation of immunization who had a BCG scar and tetanus antibodies made no difference between the data of the non- and twice-vaccinated children (not shown). Because of the lack of written documentation of vaccination they have been included in the 'non-vaccinated group'.
Tetanus antibodies
Of the twice-immunized children in the various age groups, 96% had antibodies at a protective level ( i>0.05 IU ml- 1), while of the non-immunized children 11% had tetanus antibodies at this level, as shown in Table 1. In the youngest non-vaccinated children, 10 out of 56 (18 % ) probably had maternally derived antibodies
(EPI aims at immunizing women of child-bearing age with tetanus toxoid). The 9-15 month age groups has lost maternal antibodies, only one child of 57 was antibody-positive. In the oldest age group of reportedly non-vaccinated children, eight out of 63 (13%) had antibodies, most probably due to non-recorded vaccinations. A BCG scar was found in five of these children (it is possible that some children received only BCG, without DPT-polio at the same vaccination session). On the basis of these figures (not corrected for BCG-positive children), the predictive value of the presence of tetanus antibodies ~>0.1IUm1-1 for a reportedly vaccinated child was 92% (sensitivity 93.7%, and specificity 91.7%). At a lower discriminative level (0.05 IU ml-1 ) the predictive value was 97% (sensitivity 90.2%, and specificity 96.9%). Thus, in the present survey the assessment of vaccination status can be regarded as sufficient. When the children are grouped according to the interval between the last DPT-polio injection and bleeding, the persistence of tetanus antibodies appeared satisfactory. More than 2 years after the second DPT-polio immunization all of the 38 children studied still had anti-tetanus antibody levels />0.1 IUm1-1 ( Table 2 ).
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Immunogenicity of two doses of DPT IPV. H.C. Rumke et al. Table 2 Tetanus and diphtheria antibody titres according to interval between second vaccination and blood sampling
Interval
Tetanus
Diphtheria
0 6 months 0.05+ 0.1 + 1.0 + GMT Range (n = 48)
48 (100) 47 (98) 41 (85) 2.18 0.68 7.02
47 (98) 43 (90) 15 (31) 0.45 0.11 1.9
7 - 1 2 months 0.05+ 0.1 + 1.0 + GMT Range (n = 23)
22 (96) 21 (91) 6 (26) 0.35 0.08-1.52
19 (83) 13 (57) 0 (0) 0.07 0.02-0.22
13 18 months 0.05 + 0.1 + 1.0 + GMT Range (n 54)
53 (98) 50 (93) 20 (37) 0.58 0.16-2.16
43 (80) 35 (65) 3 (6) 0.09 0.02 0.39
19 24 months 0.05+ 0.1 + 1.0 + GMT Range (n = 23)
23 (100) 22 (96) 9 (39) O.58 0.18 1.94
18 (78) 13 (57) 0 (0) 0.09 0.02-0.35
~>25 months 0.05+ 0.1 + 1.0 + GMT Range (n = 38)
38 (100) 38 (100) 16 (42) 0.62 0.21-1.89
Table 3
31 (82) 22 (58) 1 (3) 0.09 0.02-0.41
The number (percentage) of children with a minimum antibody level of 0.05, 0.1 and 1.0 IU m1-1, respectively, as well as the geometric mean titres (GMT) and range (_+1 s.d.) are given per group. ~ tests gave significantly different (p < 0.0001) diphtheria antibody levels between 0 6 months versus 7 12 months after the second vaccination. Differences between other groups were not significant
Diphtheria antibodies As shown in Table 1, 11 of the 176 non-vaccinated children ( 6 % ) h a d antibodies />0.05 IU ml-1. Of these 11, eight belonged to the 63 children of the 16 48 months age group. Of the 179 twice-vaccinated children, 142 (79%) had antibodies above this level. It appeared that the antibody levels declined rapidly with time after vaccination (Table 2). In the first 6 months after the second vaccination 47 out of 48 children (98%) were antibody-positive, indicating a high seroconversion rate. Children studied after a longer interval following the second injection had lower antibody levels: about 80% still have circulating diphtheria antitoxins, but at a stable level. Thus, the finding of a substantial percentage (21% ) of children lacking diphtheria antibodies is probably caused by the relatively short persistence of these antibodies after the second vaccination. However, it can be assumed that nearly all twice-vaccinated children in this study have been primed for diphtheria toxoid, which is the goal of primary immunization. Polio antibodies More than 95% of the vaccinees had antibodies to
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poliovirus types l and 2 ( Table 3 ). The immunogenicity of the polio type 3 component in the quadruple vaccine appeared somewhat lower (about 93% positive after two vaccinations) than that of types 1 and 2. Although we regard the presence of any neutralizing activity (values in tables: 1 + ) in a given serum against the virus strains used in the test as protective, we used a safety margin (values 3 + ) to correct for non-specificity, aberrant virus strains, and in our hands small-laboratory inaccuracy. Both 1 + and 3 + values are shown. As compared with diphtheria and tetanus antibodies, polio antibodies tended to persist longer after two vaccinations ( Table 4 ), but this may be due to boosting by naturally circulating wild virus. This is suggested by the findings in non-vaccinated children. In 20-.25% of these we also found polio antibodies. In the youngest age group these could be maternal antibodies. In the oldest group only a few of these children had antibodies to all three types of polioviruses and to the toxoids, suggesting unrecorded prior vaccination. The majority of the other children had antibodies to only one of the three virus types. These must have been induced by natural contact with polioviruses circulating in the study area. It was not possible to allocate children with a hagh antibody titre to only one type of poliovirus to specific villages (data not shown).
V a c c i n e , Vol. 11, I s s u e 11, 1993
Polio antibody titres according to vaccination status and age Type 1 0a
2
Type 2 0
2
Type 3 0
2
3-8 months 1+ 19 (35) 3+ 4 (7) 6+ 2 (4) Mean 0.8 S.d. 1.6 n 55
9 (100) 9 (100) 7 (78) 7.3 2.1 9
23 (42) 10 (18) 0 (0) 1.0 1.3 55
9 (100) 9 (100) 8 (89) 9.0 2.3 9
17 (31) 3 (5) 0 (0) 0.6 1.1 55
8 (89) 8 (89) 3 (33) 5.1 3.0 9
9-15 months 1+ 3 (5) 3+ 2 (4) 6+ 2 (4) Mean 0.3 S.d. 1.2 n 57
28 (100) 28 (100) 23 (82) 7.3 2.3 28
1 (2) 1 (2) 1 (2) 0.1 0.9 57
28 (100) 28 (100) 27 (96) 9.1 2.1 28
5 (9) 3 (5) 2 (4) 0.5 2.0 57
26 (93) 21 (75) 12 (43) 5.4 3.6 28
16 48 months 1+ 17 (27) 136 (96) 3+ 15 (24) 123 (87) 6+ 8 (13) 99 (70) 7.2 Mean 1.5 3.4 S.d. 3.1 142 n 63
19 (30) 141 (99) 16 (25) 141 (99) 11 (17) 119 (84) 2.1 8.4 3.7 2.8 63 142
22 (35) 132 (93) 19 (30) 111 (78) 11 (17) 84 (59) 1.9 5.9 3.0 3.5 63 142
All 1+ 39 (22) 3+ 19 (11) 6+ 12 (6) Mean 0.9 S.d. 2.2 n 175
173 (97) 169 (94) 129 (72) 7.2 3.2 179
43 (25) 178 (99) 27 (15) 177 (99) 12 (7) 154 (86) 1.1 8.5 2.6 2.7 175 179
44 (25) 25 (14) 13 (7) 1.1 2.3 175
166 (93) 140 (78) 99 (55) 5.8 3.5 179
The number (percentage) of children with a minimum neutralizing antibody level of 1, 3 and 6 (1/2, 1/8 and 1/64), respectively, as well as the mean titres and standard deviation (s.d.) are given per group. Within each age group, ;(2 tests give significantly different (p < 0.0001) antibody levels between non-vaccinated and twice-vaccinated children. Per vaccine group (0 or 2 doses) there are no significant differences (p > 0.01 ) between the age groups, with exception of those between the levels of type 1 antibodies (p = 0.0003), and for type 2 antibodies (p < 0.001) both of the youngest and middle age groups aNo. of doses of DPT-polio vaccine
Immunogenicity of two doses of DPT-IPV: H.C. ROmke et al. Table 4 Polio antibody titres according to interval between second vaccination and blood sampling Interval
Type 1
Type 2
Type 3
0-6 months 1+ 3+ 6+ Mean S.d. (n = 48)
45 (94) 43 (90) 32 (67) 7.1 3.5
48 (100) 47 (98) 44 (92) 9.3 2.5
43 (90) 33 (69) 19 (40) 4.7 3.3
7-12 months 1+ 3+ 6+ Mean S.d. (n = 54)
23 (100) 23 (100) 15 (65) 6.3 2.4
23 (100) 23 (100) 20 (87) 8.0 2.5
21 (91) 20 (87) 10 (43) 5.4 3.2
13-18 months 1+ 3+ 6+ Mean S.d. (n = 54)
51 (94) 50 (93) 39 (72) 6.9 3.2
54 (100) 54 (100) 47 (87) 8.6 2.5
51 (94) 40 (74) 30 (56) 5.8 3.5
19-24 months 1+ 3+ 6+ Mean S.d. (n = 23)
23 (100) 22 (96) 18 (78) 7.5 2.9
22 (96) 22 (96) 18 (78) 8.0 3.4
21 (91) 20 (87) 15 (65) 6.3 3.3
/>25 months 1+ 3+ 6+ Mean S.d. (n = 38)
38 (100) 38 (100) 32 (84) 8.8 3.0
38 (100) 38 (100) 32 (84) 8.5 2.6
37 (97) 34 (89) 31 (82) 7.6 3.3
The number (percentage) of children with a minimum neutralizing antibody level of 1, 3 and 6 (1/2, 1/8 and 1/64), respectively, as well as the mean titres and standard deviation (s.d.) are given per group. Differences between groups according to interval were not significant (p > 0.01) for either type of polio virus antibody
DISCUSSION In the present study it was found that two doses of quadruple DPT-polio vaccine induce antibodies at protective levels to diphtheria and tetanus toxin, and to the three types of polioviruses in over 90% of children. Antibodies to tetanus and polioviruses persist for more than 2 years after their last vaccination in the majority of children. The diphtheria antitoxin levels tend to decline after 6 months, and appear to remain stable thereafter. Waning of diphtheria antitoxins is not uncommon 16--18. Our data indicate clearly that nearly all children had been primed for diphtheria toxoid. Of course, it is desirable that vaccinated children have protective levels of antitoxins for a long period, but it is likely that the absence of demonstrable antibodies does not automatically imply that the vaccinee is not protected. Immunological memory without detectable antibodies may be sufficient for protection against clinical diphtheria, and is likely to be boosted upon a revaccination or natural (skin) infection. Regarding the immunity induced against poliovirus by two doses of DPT-polio the results are in line with observations from the Kolda region in Senegal. In this region, two doses of a comparable DPT-polio vaccine
gave a high level of protection during the polio type 1 virus epidemic in 1986 (vaccine efficacy 89%) 19 21. Owing to the low vaccination coverage in that area the epidemic could not be prevented. Because of the high seroconversion rates in the present study, we feel confident that a similar high percentage of the vaccinated children here is well protected. The epidemiological survey done 3 months before the present serological survey in the same area 6'7 revealed no recent clinical cases (last case in 1986). After the survey, no polio cases become known through routine reporting systems in the study region up to July 1992. Since among non-immune individuals infected with poliovirus only a few will develop paralysis (1:100 to 1:1000) cases may not have become apparent, although the magnitude of this survey with data on 17 154 children justifies confidence in the absence of recent polio patients. Nevertheless, at the low vaccination coverage found during the same survey (40-60%) 6, silent transmission of wild poliovirus probably still continues, as indicated by the presence of antibodies in non-vaccinated children (and perhaps the stability of antibody levels in vaccinated children). This appears a more sensitive parameter for the circulation of polioviruses than the finding of paralytic patients, although it is impossible to estimate the extent of virus circulation. Hence, if the vaccination coverage remains at its present level (irrespective of which type of vaccine is used), non-vaccinated individuals will hardly be protected by herd immunity. It is therefore reassuring that in the years after the present surveys the vaccination coverage has increased to > 70%. No serological data for pertussis were presented. During the epidemiological survey done in the same year as the present study, it was found that pertussis could not be fully prevented in the study region. It would probably need a much higher vaccination coverage to reduce the number of pertussis patients. An investigation of a small pertussis outbreak in a village in one of the study provinces suggested that the proportion of cases among younger vaccinees was lower than that among the non-vaccinated 22. These findings are in agreement with a study from Kenya, which also showed that after some years pertussis antibodies waned faster in a two-dose schedule group than in a three-dose group. Also the incidence of pertussis cases appeared higher (not significant) in the two-dose group 23. It has to be concluded that the protection given by two doses of pertussis vaccine is suboptimal. In conclusion, this study shows that the simplified immunization schedule, with only two visits for injections of DPT polio vaccine, can be used for primary immunization against diphtheria, tetanus and the three poliovirus antigens. Neutralizing antibodies to tetanus toxin and the three polioviruses are detectable more than 2 years after the second vaccination, while diphtheria neutralizing antibodies tend to persist for less time. Although not the subject of the present study, for protection against pertussis two doses is regarded as suboptimal. Therefore, to induce stronger and longer lasting immunity a third vaccination is recommended. The immunization programme in the area studied here is now being changed accordingly. ACKNOWLEDGEMENTS The authors are grateful to the staff of the provincial EPI
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teams, technicians of the Centre Muraz, the technical assistants of APMP, Burkina Faso, and the staff of the Stichting Redt de Kinderen (The Hague/Ouagadougou) for their technical support in the execution of this survey.
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