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Measles vaccine immunogenicity and antibody persistence in 12 vs 15-month old infants
Vaccine 18 (2000) 2411±2415
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Measles vaccine immunogenicity and antibody persistence in 12 vs 15-month old infants Candice E. Johnson a,*, Anil Darbari b, Deepika S. Darbari b, David Nalin c, Judy Whitwell b, Linda W. Chui d, Mario A. Cleves e, Mary L. Kumar b a Department of Pediatrics, University of Colorado, Denver, CO 80218, USA Department of Pediatrics, Case Western Reserve University School of Medicine at Metro Health Medical Center, Cleveland, OH, USA c Merck Vaccine Division, West Point, PA, USA d Department of Microbiology and Public Health at the University of Alberta, Edmonton, Alberta, Canada e Department of Epidemiology and Biostatistics, Case Western Reserve University School of Medicine at Metro Health Medical Center, Cleveland, OH, USA b
Received 30 August 1999; accepted 2 December 1999
Abstract Hypothesis: Maternal measles immunity in the United States today is primarily vaccine induced, with corresponding lower antibody titers in infants, as compared to infants born in an earlier era to mothers with naturally acquired measles immunity. We hypothesized that, due to lower titer of passively transferred maternal measles antibody, administration of measles vaccine at 12 months of age would result in seroconversion and antibody persistence comparable to vaccination at 15 months of age. Population: Children at both an urban hospital and a suburban clinic. Methods: Informed consent was obtained from mothers for the infants to receive M-M-R1II vaccine at either 12 or 15 months and to have serum samples obtained before vaccination and 4 weeks post-vaccination (PV). Between 9 and 39 months PV, a third serum sample was obtained from 28% of seroconverters. A diary of adverse experiences was kept for 3 weeks PV. Sera were assayed by a microneutralization assay (NT) and an enzyme immunoassay (EIA) for measles antibody. Results: Both age groups tolerated vaccination well with minor and transient side eects. Forty-four of 47 (94%) 12-monthold infants seroconverted by NT, compared to 45 of 46 (98%) 15-month-olds ( p = NS). There was no statistically signi®cant decline in median NT titers or EIA titers in nineteen 12-month-olds and thirteen 15-month olds followed for 9±39 months PV. Conclusion: This study showed comparable serologic responses in 12- vs 15-month-old infants born to measles vaccineimmune mothers; however, the sample size was too small to have adequate power and further study is indicated. Titers of antibody were constant in both the 12-month-old and the 15-month-old infants, over a 9±39 month period, suggesting that waning immunity over this period of time is not a problem in either age group. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Measles vaccine; Vaccine immunogenicity
1. Introduction Between 1989 and 1991, the United States experienced a measles epidemic with over 55,000 cases and * Corresponding author. Tel.: +1-303-861-6007; fax: +1-303-8747062. E-mail address: [email protected] (C.E. Johnson).
132 deaths [1]. Since then, the implementation of a two-dose policy has essentially eliminated indigenous cases of measles [2]. Only 100 cases of measles occurred in 1998, which were all probably linked to imported cases [2]. Worldwide, however, measles remains an important public health problem [3]. Following the 1989 epidemic, there was increased awareness of measles susceptibility in infants less than 15 months of age. Fifteen percent of all measles cases
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C.E. Johnson et al. / Vaccine 18 (2000) 2411±2415
reported to CDC in 1994 occurred in children <15 months of age, and the age-speci®c incidence was highest for children aged <1 yr [4]. Increased susceptibility to measles in infants <15 months of age has occurred as a result of the shift in maternal measles immunity from naturally acquired to vaccine-induced. Infants of mothers with vaccine acquired immunity have lower antibody titers at birth, which decline rapidly to nonprotective levels, leaving young infants susceptible to measles within the ®rst year of life [5±7]. Historically, when measles vaccine was introduced in 1963, it was administered at 9 months of age. In 1965, the recommended age was increased to 12 months, and in 1976, further extended to 15 months, based on studies which demonstrated lower rates of seroconversion due to interfering maternally derived antibody persisting through 14 months of age. In recognition of altered maternal immunity during three decades of vaccine use, the Advisory Committee on Immunization Practices (ACIP) in 1994 revised the recommended age for the ®rst dose of measles vaccine, dropping it back to 12±15 months [8]. This recommendation is appropriately based on public health concerns, but there are limited data which directly compare measles antibody response in infants receiving vaccine at 12 months to those vaccinated at 15 months of age [9]. The objectives of this study were: (1) to compare seroconversion rates in infants after M-M-R1II vaccination at 12 months vs 15 months of age; (2) to compare incidence of adverse PV events in the above groups; and (3) to compare antibody persistence for up to 39 months PV in the two groups. 2. Methods 2.1. Recruitment and study design Mothers and their healthy term infants 12±15 months old, attending well child care visits at an urban hospital based clinic and a suburban pediatric practice, were included in the study. The study nurse approached each family prior to the infant's ®rst birthday. If parents declined to participate at 12 months, infants were not eligible at 15 months. Inclusion Criteria included: (1) mother born and raised in the United States; (2) mother E30 years of age at delivery of the child; (3) mother reports having received measles vaccine in childhood; (4) healthy term infant; (5) no clinical history of naturally acquired measles, mumps or rubella, or known exposure to measles, mumps or rubella within the previous two weeks; and (6) no history for the infant of previous vaccination with measles, mumps or rubella vaccine in monovalent or combined form. Exclusion Criteria were: (1) sensitivity to neomycin; (2) immune globulin or any blood pro-
duct during the previous 6 months or plans to receive any during the study; (3) chronic disease, such as cyanotic heart diseases or renal failure; (4) any other vaccine in the previous 2 weeks. 2.2. Procedures This study was approved by the MetroHealth Medical Center Institutional Review Board. Informed consent was obtained from all mothers. After enrollment pre-vaccination serum samples were obtained and measles antibody measured using a previously described microneutralization assay (NT) [10] and an enzyme immunoassay (EIA) using Measelisa-II1, (MA Bioproducts). Sera providing positive NT results at dilutions e1:10 were considered positive. The EIA values were interpreted as follows: e0.16 seropositive; 0.14± 0.15 equivocal; and E0.13 seronegative. The NT assay and EIA were performed in the same laboratories over the entire time period of the study, and World Health Organization control sera were used to standardize the NT assay with each run. Twenty-three 15-month old children, ful®lling the inclusion criteria for the study, had participated in a previously published study of measles vaccine where Attenuvax1 [10]; the monovalent measles vaccine, was administered [10]; twentyfour additional 15-month-old and forty-seven 12month old infants were enrolled speci®cally for this study. After drawing pre-vaccination sera, 0.5 cc MM-R1II (Merck & Co. Inc.) was given subcutaneously. NT and EIA titers were measured in all children at 4 weeks PV, both in the previous study and in the new enrollees. A diary was provided for recording adverse experiences within 3 weeks after vaccination. At least 9 months after completion of the original study, as many parents as possible were contacted, and informed consent requested for a third serum sample. Parents of 36 study infants were located, and all consented. Since enrollment had spanned 2.5 years, the persistence samples ranged from 9 to 39 months PV. 2.3. Statistical methods Data was analyzed using the STATA1 statistical package (STATA Corporation, College Station, TX) and the Statistical Analysis System (SAS Institute Inc., Cary, NC). Unless otherwise noted, the Fisher's Exact Test was used to analyze and compare categorical data. Within age group, changes from 4 weeks PV to 9±39 months PV in the natural logs of serologic titers were compared using the paired t-test and a two factor test repeated in one ANOVA. Con®dence intervals were calculated using the exact binomial method.
C.E. Johnson et al. / Vaccine 18 (2000) 2411±2415
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Forty-seven infants received measles immunization at 12 months of age, as did forty-six 15-month old infants. All infants in both groups returned for the 4 week PV serum. For antibody persistence, 19 infants from the 12-month old (40%) and 17 from the 15month old (37%) cohorts returned 9±39 months after vaccination. Three of these 17 infants could only have an EIA done due to the small volume of serum, and one 15-month old infant was dropped from the followup analysis because he did not respond to the ®rst dose of M-M-R1II, and received a second dose of the vaccine, 2 months later. After the second dose, he had a normal seroconversion. He was not seropositive by either EIA or NT before vaccination.
3.4.2. Antibody persistence The mean time interval for the persistence sampling was 20.3 months (Table 1). All children remained both measles EIA and NT positive. There was no signi®cant dierence between the 9 and 39 month mean NT titers for the two groups ( p = 0.676). The EIA titers in the 12-month old group were 17 high-positive, one medium-positive and one low-positive. In the 15-month group, eight were high-positive, three medium-positive, and one low-positive. To see if antibody titers decline over time, the change in the neutralization titer (expressed logarithmically) compared to the initial titers at 4 weeks PV was calculated, and the mean dierence for both groups was close to zero. All values were within two standard deviations of the mean dierence for their group, indicating that titers did not change signi®cantly over periods between 9 and 39 months PV.
3.2. Adverse experiences
4. Discussion
There were few reported adverse experiences with vaccination, and all were transient and mild. There were no statistically signi®cant dierences in adverse experience rates between the 12- and the 15-month olds.
We demonstrate in this study that 12-month old infants born to mothers with vaccine induced measles immunity show measles antibody response to M-MR1II vaccine similar to 15-month old infants, given the small sample in this study. Our study, however, had only 47 and 46 patients in each age group, thus limiting the statistical power to detect a dierence. The data indicate that a sample size of 408 per group would be needed if evaluated using the NT method and 270 per group if the EIA method is used (with 80% power and p-value less than 0.5). Markowitz et al. showed a seropositive response of 98% in 12month old infants (sample size Ð 192) using the M-M-R1II vaccine, but their study did not directly compare the results with those in 15-month old infants [7]. Enzyme immunoassay is generally less sensitive than microneutralization [11], and antibody as measured by EIA tends to become undetectable earlier as compared to neutralizing antibody. Our ®nding of the pre-vaccination positive EIA titers in six NT-negative infants
3. Results 3.1. Enrollment of patients
3.3. Comparison of prevaccination serologic results All forty-seven 12-month olds and forty-six 15month olds were seronegative by neutralization assay before vaccination. Among the 12-month old infants 45 of 47 (96%) were negative by EIA before vaccination, compared to 42 of 46 (91%) 15-month olds, p = NS. The seropositivity by EIA was believed to be due to cross-reacting antibody or false positivity, because the NT assay is more sensitive than the EIA. 3.4. Post-vaccination serologic results 3.4.1. 4 weeks PV Forty-one of forty-seven 12-month old infants (87%, 95% C.I. 74±95%) became seropositive by EIA 4 weeks PV, while two had equivocal results; 45 of 46 15-month-olds (98%, 95% C.I. 88±100%) did so. Forty-four of the 47 (94%, 95% C.I. 82±99%) 12month old infants developed neutralization titers e10, while 45 of the 46 (98%, 95% C.I. 88±100%) of the 15-month-olds had NT e10. Comparing the 4-week post-vaccination results between 12- and 15-month old infants, the proportion of infants becoming seropositive was not statistically dierent by NT ( p = 0.317) or by EIA ( p = 0.174) using a two-tailed Fisher exact test and excluding the two infants with equivocal results.
Table 1 Geometric means and ranges of NT titers by age and time Age (months)
N
4 weeks PV
9±39 months PV
p-valuea
12
19 13
46.3 (20±60) 62.5 (10±160) 0.676
0.262
15
38.6 (20±80) 53.8 (10±320) 0.317
p-valueb
0.671
a Paired t-test comparing if mean dierence within the same age group equals zero. b t-test comparing means between the two age groups. All comparison tests were performed using arithmetic means of log (NT).
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C.E. Johnson et al. / Vaccine 18 (2000) 2411±2415
was unexpected. All six of these infants (two 12-month olds and four 15-month olds) became seropositive by NT and their EIA titers boosted in their 4 week PV samples. In our study, we followed a subset from each cohort, 9±39 months PV. No local outbreak of measles occurred during this period. There was no evidence of antibody decline in either group. All infants remained positive by NT and EIA. The only child with a decline in NT over 39 months was in the 15-month cohort. These data are reassuring as to the equivalence of antibody response to M-M-R1II vaccine in 12- vs 15month old infants. Further long-term follow-up of infants immunized at 12 months will be important to document that serologic protection persists throughout early childhood and that immune memory, evidenced by responses of the two cohorts to a school-entry booster, is equivalent. The second dose of measles vaccine, now recommended at 4±6 years or 11±13 years of age, should further guarantee measles immunity for older children and adults [12,13]. The level of antibody required for protection from measles infection and disease is an important issue. The NT assay detects antibodies speci®c for measles virus envelope glycoproteins H and F, which are involved in the neutralization of viral infectivity [14]. Several studies have addressed the relationship between antibody level measured by plaque neutralization (PN) and protection [15,16]. We have previously reported that an NT titer of 10 in the assay used in this study corresponds approximately to a PN titer of 120 mIU [17]. Chen et al. have previously reported that a PN titer of >120 mIU provided protection from measles in college outbreak investigation [15]. However, in a recent study from Senegal, Samb et al. reported high protection from natural measles in vaccinated children, even in children with titers <120 mIU by PN assay [16]. The authors suggested that anamnestic antibody responses or sucient cellular immunity provided protection against natural measles infection in the absence of detectable antibodies in individuals with vaccineinduced immunologic memory. Up to 50% of seronegative immunized children appeared protected, thus demonstrating the complexity of de®ning protection from measles serologically. In a study reported from isolated Paci®c islands which have been measles-free for many years [18], one dose of measles vaccine was approximately 85±90% protective. Two doses were 100% protective for intervals as long as 27 years, suggesting no decline in measles protection, independent of any antibody titer declines which might have occurred. Previously, reducing the age of vaccination for measles raised the concern of susceptibility to measles due to primary or secondary vaccine failure in infants vaccinated when maternal antibody was present. How-
ever, studies indicating higher risk of measles in later childhood in infants who had received measles vaccination at 12±14 months included infants born to mothers with naturally-acquired, not vaccine-induced immunity [19±22]. Previously published studies have demonstrated declining maternal measles antibody titers over time, as measured in cord sera [5,6]. Vaccine induced maternal measles antibodies have been shown to disappear earlier in a child's life than natural measles antibodies, making such infants susceptible to measles at a younger age [23,24]. Identi®cation of mothers with low measles antibody titers and early vaccination of their infants has been suggested as a strategy [6], but would be logistically dicult. Since the majority of infants are now born to mothers with only vaccine-induced immunity, our data suggest that lowering the age of routine measles vaccination to 12 months should not increase the number of primary vaccine failures and appears to provide serologic immunity comparable to that achieved when vaccination occurs at 15 months. A larger study should be performed, perhaps including 9-month old infants, to answer this question.
References [1] Centers for Disease Control. Measles Ð United States, 1996, and the interruption of indigenous transmission. MMWR 1997;46(11):242±6. [2] Centers for Disease Control. Epidemiology of measles. MMWR 1999;48:749±53. [3] Guris D, Auerbach SB, Vitek C, et al. Measles outbreak in Micronesia, 1991 to 1994. Pediatr Infect Dis J 1998;17:33±9. [4] Centers for Disease Control. Measles Ð United States, 1994. MMWR 1995;44(26):486±94. [5] Yeager AS, Harvey B, Crosson Jr FJ, Davis JH, Ross LA, Halonen PE. Need for measles revaccination in adolescents: correlation with birth date prior to 1972. J Pediatr 1983;102:191±5. [6] Lennon JL, Black FL. Maternally derived measles immunity in era of vaccine-protected mothers. J Pediatr 1986;108(1):671±6. [7] Markowitz LE, Albrecht P, Rhodes P, et al. Changing levels of measles antibody titers in women and children in the United States: impact on response to vaccination. Pediatrics 1996;97:53±8. [8] Center for Disease Control. General recommendation on immunization: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1994;43(RR-1):1±38. [9] King G, Markowitz L, Burns S, Health J, Nordin J, Bellini W. A comparison of seroconversion rates at 9, 12 or 15 months of age. In: Presented at the 34th Interscience Conference on Antimicrobial Agents and Chemotherapy; 4±7 October, 1994; Orlando, FL, 1994. [10] Johnson CE, Nalin DR, Chui LW, Whitwell J, Marusyk RG, Kumar ML. Measles vaccine immunogenicity in 6- versus 15month-old infants born to mothers in the measles vaccine era. Pediatrics 1994;93:939±44. [11] Boteler WL, Luipersbeck PM, Fuccilo DA, O'Beirne AJ. Enzyme-linked immunosorbent assay for detection of measles antibody. J Clin Microbiol 1983;17:814±8. [12] Center for Disease Control. Measles prevention: recommen-
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[13] [14] [15] [16] [17]
[18]
[19]
dation of the immunization practices advisory committee (ACIP). MMWR 1989;38(S-9):1±13. Katz SL. Harmony on the second dose of measles vaccine. Pediatrics 1997;100:891±2. Chui LW-L, Marusyk RG, Pabst HF. Measles virus speci®c antibody in infants in a highly vaccinated society. J Med Virol 1991;33:199±204. Chen RT, Markowitz LE, Albrech P, et al. Measles antibody: Reevaluation of protective titers. J Infect Dis 1990;162:1036±42. Samb B, Aaby P, Whittle HC, et al. Serologic status and measles attack rate among vaccinated and unvaccinated children in rural Senegal. Pediatr Infect Dis J 1995;14:203±9. Johnson CE, Kumar ML, Whitwell JK, et al. Antibody persistence after primary measles-mumps-rubella vaccine and response to a second dose given at four to six vs eleven to thirteen years. Pediatr Infect Dis J 1996;15:687±92. Guris D, McCready J, Watson JC, et al. Measles vaccine eectiveness and duration of vaccine induced immunity in the absence of boosting from exposure to measles virus. Pediatr Infect Dis J 1996;15:1082±6. Hersh BS, Markowitz LE, Homan RE, et al. A measles out-
[20]
[21]
[22]
[23]
[24]
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break at a college with a prematriculation immunization requirement. Am J Public Health 1991;81:360±4. Mast EE, Berg JL, Hanrahan LP, Wassel JT, Davis JP. Risk factors for measles in a previously vaccinated population and cost-eectiveness of revaccination strategies. JAMA 1990;264:2529±33. Shelton JD, Jacobson JE, Orenstein WA, Schultz KF, Donnell Jr HD. Measles vaccine ecacy: in¯uence of age at vaccination versus duration of time since vaccination. Pediatrics 1978;62:961±4. Hutchins SS, Markowitz LE, Mead P, et al. A school based measles outbreak: the eect of a selective revaccination policy risk factors for vaccine failure. Am J Epidemiol 1990;132:157± 68. Christenson B, Bottiger M. Measles antibody: comparison of long-term titers, early vaccination titers and naturally acquired immunity to and booster eects on the measles virus. Vaccine 1994;12:129±33. Pabst HF, Spady DW, Marusyk RG, et al. Reduced measles immunity in infants in a well vaccinated population. Pediatr Infect Dis J 1992;11:525±9.
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Measles vaccine immunogenicity and antibody persistence in 12 vs 15-month old infants Candice E. Johnson a,*, Anil Darbari b, Deepika S. Darbari b, David Nalin c, Judy Whitwell b, Linda W. Chui d, Mario A. Cleves e, Mary L. Kumar b a Department of Pediatrics, University of Colorado, Denver, CO 80218, USA Department of Pediatrics, Case Western Reserve University School of Medicine at Metro Health Medical Center, Cleveland, OH, USA c Merck Vaccine Division, West Point, PA, USA d Department of Microbiology and Public Health at the University of Alberta, Edmonton, Alberta, Canada e Department of Epidemiology and Biostatistics, Case Western Reserve University School of Medicine at Metro Health Medical Center, Cleveland, OH, USA b
Received 30 August 1999; accepted 2 December 1999
Abstract Hypothesis: Maternal measles immunity in the United States today is primarily vaccine induced, with corresponding lower antibody titers in infants, as compared to infants born in an earlier era to mothers with naturally acquired measles immunity. We hypothesized that, due to lower titer of passively transferred maternal measles antibody, administration of measles vaccine at 12 months of age would result in seroconversion and antibody persistence comparable to vaccination at 15 months of age. Population: Children at both an urban hospital and a suburban clinic. Methods: Informed consent was obtained from mothers for the infants to receive M-M-R1II vaccine at either 12 or 15 months and to have serum samples obtained before vaccination and 4 weeks post-vaccination (PV). Between 9 and 39 months PV, a third serum sample was obtained from 28% of seroconverters. A diary of adverse experiences was kept for 3 weeks PV. Sera were assayed by a microneutralization assay (NT) and an enzyme immunoassay (EIA) for measles antibody. Results: Both age groups tolerated vaccination well with minor and transient side eects. Forty-four of 47 (94%) 12-monthold infants seroconverted by NT, compared to 45 of 46 (98%) 15-month-olds ( p = NS). There was no statistically signi®cant decline in median NT titers or EIA titers in nineteen 12-month-olds and thirteen 15-month olds followed for 9±39 months PV. Conclusion: This study showed comparable serologic responses in 12- vs 15-month-old infants born to measles vaccineimmune mothers; however, the sample size was too small to have adequate power and further study is indicated. Titers of antibody were constant in both the 12-month-old and the 15-month-old infants, over a 9±39 month period, suggesting that waning immunity over this period of time is not a problem in either age group. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Measles vaccine; Vaccine immunogenicity
1. Introduction Between 1989 and 1991, the United States experienced a measles epidemic with over 55,000 cases and * Corresponding author. Tel.: +1-303-861-6007; fax: +1-303-8747062. E-mail address: [email protected] (C.E. Johnson).
132 deaths [1]. Since then, the implementation of a two-dose policy has essentially eliminated indigenous cases of measles [2]. Only 100 cases of measles occurred in 1998, which were all probably linked to imported cases [2]. Worldwide, however, measles remains an important public health problem [3]. Following the 1989 epidemic, there was increased awareness of measles susceptibility in infants less than 15 months of age. Fifteen percent of all measles cases
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C.E. Johnson et al. / Vaccine 18 (2000) 2411±2415
reported to CDC in 1994 occurred in children <15 months of age, and the age-speci®c incidence was highest for children aged <1 yr [4]. Increased susceptibility to measles in infants <15 months of age has occurred as a result of the shift in maternal measles immunity from naturally acquired to vaccine-induced. Infants of mothers with vaccine acquired immunity have lower antibody titers at birth, which decline rapidly to nonprotective levels, leaving young infants susceptible to measles within the ®rst year of life [5±7]. Historically, when measles vaccine was introduced in 1963, it was administered at 9 months of age. In 1965, the recommended age was increased to 12 months, and in 1976, further extended to 15 months, based on studies which demonstrated lower rates of seroconversion due to interfering maternally derived antibody persisting through 14 months of age. In recognition of altered maternal immunity during three decades of vaccine use, the Advisory Committee on Immunization Practices (ACIP) in 1994 revised the recommended age for the ®rst dose of measles vaccine, dropping it back to 12±15 months [8]. This recommendation is appropriately based on public health concerns, but there are limited data which directly compare measles antibody response in infants receiving vaccine at 12 months to those vaccinated at 15 months of age [9]. The objectives of this study were: (1) to compare seroconversion rates in infants after M-M-R1II vaccination at 12 months vs 15 months of age; (2) to compare incidence of adverse PV events in the above groups; and (3) to compare antibody persistence for up to 39 months PV in the two groups. 2. Methods 2.1. Recruitment and study design Mothers and their healthy term infants 12±15 months old, attending well child care visits at an urban hospital based clinic and a suburban pediatric practice, were included in the study. The study nurse approached each family prior to the infant's ®rst birthday. If parents declined to participate at 12 months, infants were not eligible at 15 months. Inclusion Criteria included: (1) mother born and raised in the United States; (2) mother E30 years of age at delivery of the child; (3) mother reports having received measles vaccine in childhood; (4) healthy term infant; (5) no clinical history of naturally acquired measles, mumps or rubella, or known exposure to measles, mumps or rubella within the previous two weeks; and (6) no history for the infant of previous vaccination with measles, mumps or rubella vaccine in monovalent or combined form. Exclusion Criteria were: (1) sensitivity to neomycin; (2) immune globulin or any blood pro-
duct during the previous 6 months or plans to receive any during the study; (3) chronic disease, such as cyanotic heart diseases or renal failure; (4) any other vaccine in the previous 2 weeks. 2.2. Procedures This study was approved by the MetroHealth Medical Center Institutional Review Board. Informed consent was obtained from all mothers. After enrollment pre-vaccination serum samples were obtained and measles antibody measured using a previously described microneutralization assay (NT) [10] and an enzyme immunoassay (EIA) using Measelisa-II1, (MA Bioproducts). Sera providing positive NT results at dilutions e1:10 were considered positive. The EIA values were interpreted as follows: e0.16 seropositive; 0.14± 0.15 equivocal; and E0.13 seronegative. The NT assay and EIA were performed in the same laboratories over the entire time period of the study, and World Health Organization control sera were used to standardize the NT assay with each run. Twenty-three 15-month old children, ful®lling the inclusion criteria for the study, had participated in a previously published study of measles vaccine where Attenuvax1 [10]; the monovalent measles vaccine, was administered [10]; twentyfour additional 15-month-old and forty-seven 12month old infants were enrolled speci®cally for this study. After drawing pre-vaccination sera, 0.5 cc MM-R1II (Merck & Co. Inc.) was given subcutaneously. NT and EIA titers were measured in all children at 4 weeks PV, both in the previous study and in the new enrollees. A diary was provided for recording adverse experiences within 3 weeks after vaccination. At least 9 months after completion of the original study, as many parents as possible were contacted, and informed consent requested for a third serum sample. Parents of 36 study infants were located, and all consented. Since enrollment had spanned 2.5 years, the persistence samples ranged from 9 to 39 months PV. 2.3. Statistical methods Data was analyzed using the STATA1 statistical package (STATA Corporation, College Station, TX) and the Statistical Analysis System (SAS Institute Inc., Cary, NC). Unless otherwise noted, the Fisher's Exact Test was used to analyze and compare categorical data. Within age group, changes from 4 weeks PV to 9±39 months PV in the natural logs of serologic titers were compared using the paired t-test and a two factor test repeated in one ANOVA. Con®dence intervals were calculated using the exact binomial method.
C.E. Johnson et al. / Vaccine 18 (2000) 2411±2415
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Forty-seven infants received measles immunization at 12 months of age, as did forty-six 15-month old infants. All infants in both groups returned for the 4 week PV serum. For antibody persistence, 19 infants from the 12-month old (40%) and 17 from the 15month old (37%) cohorts returned 9±39 months after vaccination. Three of these 17 infants could only have an EIA done due to the small volume of serum, and one 15-month old infant was dropped from the followup analysis because he did not respond to the ®rst dose of M-M-R1II, and received a second dose of the vaccine, 2 months later. After the second dose, he had a normal seroconversion. He was not seropositive by either EIA or NT before vaccination.
3.4.2. Antibody persistence The mean time interval for the persistence sampling was 20.3 months (Table 1). All children remained both measles EIA and NT positive. There was no signi®cant dierence between the 9 and 39 month mean NT titers for the two groups ( p = 0.676). The EIA titers in the 12-month old group were 17 high-positive, one medium-positive and one low-positive. In the 15-month group, eight were high-positive, three medium-positive, and one low-positive. To see if antibody titers decline over time, the change in the neutralization titer (expressed logarithmically) compared to the initial titers at 4 weeks PV was calculated, and the mean dierence for both groups was close to zero. All values were within two standard deviations of the mean dierence for their group, indicating that titers did not change signi®cantly over periods between 9 and 39 months PV.
3.2. Adverse experiences
4. Discussion
There were few reported adverse experiences with vaccination, and all were transient and mild. There were no statistically signi®cant dierences in adverse experience rates between the 12- and the 15-month olds.
We demonstrate in this study that 12-month old infants born to mothers with vaccine induced measles immunity show measles antibody response to M-MR1II vaccine similar to 15-month old infants, given the small sample in this study. Our study, however, had only 47 and 46 patients in each age group, thus limiting the statistical power to detect a dierence. The data indicate that a sample size of 408 per group would be needed if evaluated using the NT method and 270 per group if the EIA method is used (with 80% power and p-value less than 0.5). Markowitz et al. showed a seropositive response of 98% in 12month old infants (sample size Ð 192) using the M-M-R1II vaccine, but their study did not directly compare the results with those in 15-month old infants [7]. Enzyme immunoassay is generally less sensitive than microneutralization [11], and antibody as measured by EIA tends to become undetectable earlier as compared to neutralizing antibody. Our ®nding of the pre-vaccination positive EIA titers in six NT-negative infants
3. Results 3.1. Enrollment of patients
3.3. Comparison of prevaccination serologic results All forty-seven 12-month olds and forty-six 15month olds were seronegative by neutralization assay before vaccination. Among the 12-month old infants 45 of 47 (96%) were negative by EIA before vaccination, compared to 42 of 46 (91%) 15-month olds, p = NS. The seropositivity by EIA was believed to be due to cross-reacting antibody or false positivity, because the NT assay is more sensitive than the EIA. 3.4. Post-vaccination serologic results 3.4.1. 4 weeks PV Forty-one of forty-seven 12-month old infants (87%, 95% C.I. 74±95%) became seropositive by EIA 4 weeks PV, while two had equivocal results; 45 of 46 15-month-olds (98%, 95% C.I. 88±100%) did so. Forty-four of the 47 (94%, 95% C.I. 82±99%) 12month old infants developed neutralization titers e10, while 45 of the 46 (98%, 95% C.I. 88±100%) of the 15-month-olds had NT e10. Comparing the 4-week post-vaccination results between 12- and 15-month old infants, the proportion of infants becoming seropositive was not statistically dierent by NT ( p = 0.317) or by EIA ( p = 0.174) using a two-tailed Fisher exact test and excluding the two infants with equivocal results.
Table 1 Geometric means and ranges of NT titers by age and time Age (months)
N
4 weeks PV
9±39 months PV
p-valuea
12
19 13
46.3 (20±60) 62.5 (10±160) 0.676
0.262
15
38.6 (20±80) 53.8 (10±320) 0.317
p-valueb
0.671
a Paired t-test comparing if mean dierence within the same age group equals zero. b t-test comparing means between the two age groups. All comparison tests were performed using arithmetic means of log (NT).
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was unexpected. All six of these infants (two 12-month olds and four 15-month olds) became seropositive by NT and their EIA titers boosted in their 4 week PV samples. In our study, we followed a subset from each cohort, 9±39 months PV. No local outbreak of measles occurred during this period. There was no evidence of antibody decline in either group. All infants remained positive by NT and EIA. The only child with a decline in NT over 39 months was in the 15-month cohort. These data are reassuring as to the equivalence of antibody response to M-M-R1II vaccine in 12- vs 15month old infants. Further long-term follow-up of infants immunized at 12 months will be important to document that serologic protection persists throughout early childhood and that immune memory, evidenced by responses of the two cohorts to a school-entry booster, is equivalent. The second dose of measles vaccine, now recommended at 4±6 years or 11±13 years of age, should further guarantee measles immunity for older children and adults [12,13]. The level of antibody required for protection from measles infection and disease is an important issue. The NT assay detects antibodies speci®c for measles virus envelope glycoproteins H and F, which are involved in the neutralization of viral infectivity [14]. Several studies have addressed the relationship between antibody level measured by plaque neutralization (PN) and protection [15,16]. We have previously reported that an NT titer of 10 in the assay used in this study corresponds approximately to a PN titer of 120 mIU [17]. Chen et al. have previously reported that a PN titer of >120 mIU provided protection from measles in college outbreak investigation [15]. However, in a recent study from Senegal, Samb et al. reported high protection from natural measles in vaccinated children, even in children with titers <120 mIU by PN assay [16]. The authors suggested that anamnestic antibody responses or sucient cellular immunity provided protection against natural measles infection in the absence of detectable antibodies in individuals with vaccineinduced immunologic memory. Up to 50% of seronegative immunized children appeared protected, thus demonstrating the complexity of de®ning protection from measles serologically. In a study reported from isolated Paci®c islands which have been measles-free for many years [18], one dose of measles vaccine was approximately 85±90% protective. Two doses were 100% protective for intervals as long as 27 years, suggesting no decline in measles protection, independent of any antibody titer declines which might have occurred. Previously, reducing the age of vaccination for measles raised the concern of susceptibility to measles due to primary or secondary vaccine failure in infants vaccinated when maternal antibody was present. How-
ever, studies indicating higher risk of measles in later childhood in infants who had received measles vaccination at 12±14 months included infants born to mothers with naturally-acquired, not vaccine-induced immunity [19±22]. Previously published studies have demonstrated declining maternal measles antibody titers over time, as measured in cord sera [5,6]. Vaccine induced maternal measles antibodies have been shown to disappear earlier in a child's life than natural measles antibodies, making such infants susceptible to measles at a younger age [23,24]. Identi®cation of mothers with low measles antibody titers and early vaccination of their infants has been suggested as a strategy [6], but would be logistically dicult. Since the majority of infants are now born to mothers with only vaccine-induced immunity, our data suggest that lowering the age of routine measles vaccination to 12 months should not increase the number of primary vaccine failures and appears to provide serologic immunity comparable to that achieved when vaccination occurs at 15 months. A larger study should be performed, perhaps including 9-month old infants, to answer this question.
References [1] Centers for Disease Control. Measles Ð United States, 1996, and the interruption of indigenous transmission. MMWR 1997;46(11):242±6. [2] Centers for Disease Control. Epidemiology of measles. MMWR 1999;48:749±53. [3] Guris D, Auerbach SB, Vitek C, et al. Measles outbreak in Micronesia, 1991 to 1994. Pediatr Infect Dis J 1998;17:33±9. [4] Centers for Disease Control. Measles Ð United States, 1994. MMWR 1995;44(26):486±94. [5] Yeager AS, Harvey B, Crosson Jr FJ, Davis JH, Ross LA, Halonen PE. Need for measles revaccination in adolescents: correlation with birth date prior to 1972. J Pediatr 1983;102:191±5. [6] Lennon JL, Black FL. Maternally derived measles immunity in era of vaccine-protected mothers. J Pediatr 1986;108(1):671±6. [7] Markowitz LE, Albrecht P, Rhodes P, et al. Changing levels of measles antibody titers in women and children in the United States: impact on response to vaccination. Pediatrics 1996;97:53±8. [8] Center for Disease Control. General recommendation on immunization: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1994;43(RR-1):1±38. [9] King G, Markowitz L, Burns S, Health J, Nordin J, Bellini W. A comparison of seroconversion rates at 9, 12 or 15 months of age. In: Presented at the 34th Interscience Conference on Antimicrobial Agents and Chemotherapy; 4±7 October, 1994; Orlando, FL, 1994. [10] Johnson CE, Nalin DR, Chui LW, Whitwell J, Marusyk RG, Kumar ML. Measles vaccine immunogenicity in 6- versus 15month-old infants born to mothers in the measles vaccine era. Pediatrics 1994;93:939±44. [11] Boteler WL, Luipersbeck PM, Fuccilo DA, O'Beirne AJ. Enzyme-linked immunosorbent assay for detection of measles antibody. J Clin Microbiol 1983;17:814±8. [12] Center for Disease Control. Measles prevention: recommen-
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