The status of live viral vaccination in early life

Vaccine 31 (2013) 2531–2537

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Review

The status of live viral vaccination in early life Hayley A. Gans ∗ Department of Pediatrics, Stanford University School of Medicine, Stanford, CA, United States

a r t i c l e

i n f o

Article history: Received 22 January 2012 Received in revised form 17 August 2012 Accepted 17 September 2012 Available online 28 September 2012 Keywords: Live viral vaccine Infant vaccination

a b s t r a c t The need for neonatal vaccines is supported by the high disease burden during the first year of life particularly in the first month. Two-thirds of childhood deaths are attributable to infectious diseases of which viruses represent key pathogens. Many infectious diseases have the highest incidence, severity and mortality in the first months of life, and therefore early life vaccination would provide significant protection and life savings. For some childhood viral diseases successful vaccines exist, such as against measles, mumps, rubella, varicella, influenza poliovirus, and rotavirus, but their use in the first year particularly at birth is not yet practiced. Vaccines against other key pathogens continue to elude scientists such as against respiratory syncytial virus. The obstacles for early and neonatal vaccination are complex and include host factors, such as a developing immune system and the interference of passively acquired antibodies, as well vaccine-specific issues, such as optimal route of administration, titer and dosing requirements. Importantly, additional host and infrastructure barriers also present obstacles to neonatal vaccination in the developing world where morbidity and mortality rates are highest. This review will highlight the current live viral vaccines and their use in the first year of life, focusing on efficacy and entertaining the barriers that exist. It is important to understand the successes of current vaccines and use this knowledge to determine strategies that are successful in young infants and for the development of new vaccines for use in early life. © 2012 Elsevier Ltd. All rights reserved.

Contents 1.

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3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2531 1.1. Why neonatal vaccines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2532 1.2. General considerations and immunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2532 1.3. Interference by passive antibodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2532 Individual vaccines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2533 2.1. Measles, mumps, rubella, and varicella . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2533 2.2. Rotavirus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2533 2.3. Influenza . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2535 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2535 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2535

1. Introduction The high disease burden during the first year of life underscores the need for neonatal vaccination. Recent estimates show that over 3 million deaths occur during the first month of life and approximately 5.5 million within the first year. Two-thirds of childhood deaths are attributable to infectious diseases and thus are targets for prevention through immunization [1].

∗ Tel.: +1 650 723 5682; fax: +1 650 725 8040. E-mail address: [email protected] 0264-410X/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vaccine.2012.09.043

Respiratory and diarrheal diseases are the leading cause of childhood deaths followed by measles. Respiratory syncytial virus (RSV), parainfluenza virus and influenza virus are key respiratory viral pathogens [1]. Vaccines against RSV and parainfluenza elude scientists, and despite the existence of vaccines against influenza, use in young infants is not currently practiced. However new strategies are on the horizon and will be mentioned below. In contrast, for viral diseases such as measles, mumps, rubella, varicella, polio, and rotavirus, vaccines are available for use in childhood [2]. These immunizations provide valuable protection and serve as examples for future vaccines.

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Nonetheless obstacles remain that preclude effective vaccination in the very young when incidence and mortality are highest. Vaccine needs for developed countries are different than those in the developing world. The infrastructure demands in the developing world suggest that strategies which will significantly impact childhood health must include vaccines administered at birth. Often birth represents the only interaction with health providers until children are ill, and thus an important time for providing preventive health measures [3,4]. This article will review the current live viral vaccines and will highlight the known barriers to effective infant vaccination.

developing nations. Data from early measles immunization will be reviewed. Mumps and rubella are not diseases affecting infants and thus consideration of an early life vaccine is not supported. Further, data suggest that mumps immunity provided by MMR may not persist unlike measles and rubella and therefore an earlier dose may not be warranted [32]. Varicella is infrequently seen in neonates, and while severity of disease is higher than in older age groups mortality is not [33]. For these reasons these vaccines have not been tested in young infants and will not be discussed further. 1.2. General considerations and immunity

1.1. Why neonatal vaccines Vaccines provide preventive protection, and thus need to be given prior to exposure. Many infectious diseases have the highest incidence, severity and mortality in the first months of life. Respiratory viruses account for a significant disease burden in the very young. Of these RSV is the most common cause of acute lower respiratory tract infection, accounting for approximately 3.4 million hospitalizations and 200,000 deaths annually [5,6]. Rates in infants are estimated to be 2–3 times higher, peaking at 2–7 months of age. This is followed by influenza which results in approximately 1 million severe cases and 100,000 deaths annually [7]. Thus, the target age group for vaccination against these viruses is infants in the first weeks of life in order to establish protective immunity by the time exposure occurs. While vaccines for RSV are currently under investigation, their status will be reviewed, as well as potential efforts to immunize young infants against influenza using the current live viral vaccine. Despite advances to childhood health from the current viral vaccines, their use has led to epidemiological shifts that warrant exploration. Examples include rotavirus vaccine which has substantially reduced efficacy in the developing world compared with high and middle income nations and measles vaccines where the use of the vaccine for decades has created a new group of susceptible individuals, infants between 6 and 12 months of age. Rotavirus is the most common cause of severe gastroenteritis in children, causing approximately 500,000 deaths annually among children aged <5 [8]. Severe dehydrating gastroenteritis caused by rotavirus occurs primarily among children aged 4–23 months [9,10]. The current live viral vaccines have excellent efficacy against disease in developed countries resulting in 85–98% reductions in severe rotavirus disease [11–16]. In contrast these same vaccines have significantly reduced efficacy in the developing nations of 39–77% [17–19], the reasons for which are not fully understood but will be entertained below. During the nearly 5 decades of measles immunization, vaccine recommendations have been adjusted to meet emerging epidemiological shifts. Most recently outbreaks in the developed world have highlighted the susceptibility of infants <12 months, a shift which more closely approximates the disease burden in the developing world. Measles remains the leading cause of vaccine preventable childhood mortality globally with 164,000 deaths annually [20] and highest fatality rates occurring during the first year of life [21–23]. Recent epidemics in the United States have reported 21% of cases were in children <12 months who also represented 26% of measles hospitalizations [24]. This is a result of most mothers in the U.S. having vaccine-induced measles immunity which provides less measles antibody to their infants transplacentally. By six months these infants are susceptible to measles [25–29], a scenario which is paralleled in other developed countries who have long standing measles immunization programs [30,31]. Therefore a renewed interest in an early primary measles dose has arisen in the developed world, which aligns with the desire in

Live viral vaccines are among the most effective strategies for inducing lifelong immunity with as little as a single dose [34]. Currently 5 of the 16 vaccines used routinely are live viral vaccines providing protection against poliovirus, rotavirus, measles, mumps, rubella, varicella, and influenza [2]. Live viral vaccines have high efficacy rates resulting in 90–100% disease reduction since their introduction with the exception of rotavirus and influenza vaccines. How viral vaccines induce lasting immunity is not fully understood. It is clear that a strong innate immune response must be initiated which promotes the expansion of both CD4+ and CD8+ T lymphocytes [35–37]. T cells are important for viral clearance as well as inducing and maintaining lasting memory. Antibodies also play a role in protection against viral infection and the presence of antigen-specific antibodies has become the marker of vaccine efficacy [34], mainly due to the ease of their detection. Many studies have documented the persistence of antibody titers to viral vaccination for decades [34], but what factors contribute to lasting memory B cell immunity are unknown. Additionally, a discordance between B and T cell immunity induced by viral vaccines I seen, especially in the persistence of these responses [38]. T cell immunity lasts for long periods of time even in the absence of detectable antibody titers. In this context, humoral immunity is boosted upon revaccination [39,40], suggesting that T cells serve to boost the amnestic humoral response. The issue of host susceptibility in this situation is important but has not been established in humans since direct challenge studies are lacking. However, data from animals [41], and from areas where diseases are endemic therefore providing natural infectious exposures [42], suggest that these individuals are protected from viral infection, at least severe or symptomatic disease. It unclear what the best markers for viral vaccine efficacy are. Historically vaccine efficacy has been based on the identification of humoral immunity but recent data suggest that T cell immunity may be equally or more important. This is relevant when considering the immunogenicity of viral vaccines in infants who possess immunologic restrictions. It has become clear that the susceptibility to infection is difficult to determine using routine markers for live viral vaccine efficacy. What is known is that many factors influence the acquisition of lasting vaccine immunity including host factors, such as age, and vaccine-specific issues, such as vaccine titer and route of administration. 1.3. Interference by passive antibodies Nature provides infants with passive immunity to infections through the transplacental transfer of IgG during pregnancy and IgA in breast milk. Despite the obvious benefits to the young child, the levels of antibodies wane over time and are often not protective at the time of disease exposure. Unfortunately these titers even if non-protective may pose obstacles to some viral

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vaccines. This interference has been clearly demonstrated with measles immunization administered subcutaneously [43,44] and rotavirus vaccine administered orally [45]. In the presence of passive antibodies (PA) measles and rotavirus humoral immunity is diminished. However, at least for measles, T cell responses are preserved and serve to boost the humoral responses to antigen re-exposure. These boosted antibody responses are of high avidity suggesting an amnestic response likely to be quickly protective [46,47]. Research evaluating PA interference of infant responses to measles favors the hypothesis that PA directly masks specific B cell determinants, thus preventing antigen binding and recognition by infant B cells [48,49]. T cell immunity is spared because the binding of the PA to vaccine antigen results in antibody-antigen complexes which are processed by antigen presenting cells for T cell presentation [50]. This allows for adequate T cell but limited B cell immune responses after the first antigen exposure, and would allow for T cell priming of B cell responses to subsequent antigen exposures, as seen in vaccine studies [46,51]. An alternative mechanism, recently proposed, suggests that PA inhibits B cell activation through a negative feedback path when measles-specific IgG-measles virus complex binds to the B cell through that Fc receptor Fc␥RIIB, a known negative feedback mechanism for B cell activation [52]. Which mechanism is responsible for the blunting effects of PA in humans is not known, and would be important to understand if vaccines that can overcome this obstacle are to be developed.

2. Individual vaccines 2.1. Measles, mumps, rubella, and varicella Of the viral vaccines, immunization against measles, mumps, rubella and varicella is administered through the subcutaneous route. The first targeted antigen in the group was measles, which carries a high rate of mortality particularly in infants. Immunization in the developed countries is recommended at 12–15 months of age based on studies performed decades ago showing that infants in this age group lack PA and thus this interference avoided. Immunization of children 12 months or older will not be reviewed here, rather efforts for early immunization which is relevant to neonatal vaccination will be entertained. Current immunization strategies have failed to prevent many infant measles cases and deaths and in the developing countries immunization is initiated at 9 months of age [22]. Yet, even this adjustment falls short of preventing the high burden of measles disease and fatality in young infants. Therefore there has been interest in earlier vaccination in the developing world for decades. Recently interest has emerged in developed countries as a result of the shift in cases as younger infants are losing PA earlier and become susceptible to measles before routine immunization at 12 months of age. Previous measles immunization efforts in infants as young as 3 months of age have met with only partial success [27,53–56]. Historically measles vaccine failures were attributed to interference from PA, and more recently the additional impact of the developing immune system on immune responses has been reported [25,27]. Interestingly, mumps vaccination administered at 6 months of age did not show PA interference with humoral immunity [57], indicating that not all PA is equivalent in its inhibition. Interference is independent of titer, but avidity studies are lacking and thus interference may result from the quality rather than quantity of the antibodies present. In the presence of PA, measles-specific humoral immunity is blunted and the percentage of infants seroconverting is lower,

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findings that are independent on age at time of immunization [44,57,58]. Studies also indicate that mumps and measles specific humoral immunity, both neutralizing antibody titers and avidity, are diminished in young infants who are immunized in the absence of PA [44,47,57]. These limitations are age dependent and appear to mature around 9 months. Humoral immunity in both instances is boosted after subsequent doses of measles and mumps vaccine. In contrast, T cell immunity is present even in young infants in response to measles and mumps vaccination and is neither agedependent nor affected by PA (see Table 1 for summary). Efficacy of an early two-dose schedule has been documented, describing a prime-boost phenomenon. Robust humoral immunity did develop after revaccination suggesting an effective priming of immunity even when antibody titers after the first measles dose are lower in younger infants [39,53,59,60]. An early two-dose measles strategy was efficacious in a measles outbreak in the US, and in highly endemic countries [21,59]. Interest in an early two-dose immunization in the developing world is strengthened by the finding that children who receive two measles immunization before 12 months showed a mortality reduction greatly in excess of that expected by measles deaths alone [21,61]. How well measles-specific immunity persists in children given an early two dose vaccine regimen is an important question. Persistence of humoral immunity up to 6 years was demonstrated in children after an early two-dose regimen [39,62–64]. In these studies, infants who were initially vaccinated in the presence of PA had lower antibodies titers even after a booster, however, they had equivalent rates of seroconversion, and percentage of infants with titers considered seroprotective even one year after revaccination [26,39,63]. Importantly, studies of the early two-dose regimen show that the youngest infants developed measles T cell responses that were equivalent to those of older infants, regardless of the presence of PA [26]. In addition, one study showed the longevity of cellular immunity up to one year after revaccination in children receiving primary measles vaccination at 6 months of age [39]. Of interest, when T cell responses in infants are compared with those in adults to either measles or mumps antigen, relative limitations were documented in the infant memory antigen-specific T cell responses including the induction of key cytokines, such as interferon-␥ (IFN-␥) [43,44,57,65]. Infant T cell function was limited even in the memory effector cells, such as CD4+ CD45RO+ T cells which produced less IFN-␥ in response to viral stimulation in vitro. The clinical significance of this finding is not clear, and measles vaccination is efficacious in young infants and the induction of T cell immunity has primed humoral immunity even after immunization in young infants. Efforts are turning toward novel routes for measles vaccine administration in an attempt to overcome some of the obstacles associated with immunizing young infants. Measles vaccine has been administered in an aerosol form to infants as young as 9 months of age [66–68], in hopes that this will mimic the route of natural infection more precisely and avoid the issue of interference with PA. Aerosol immunization induces both T and B cell immunity comparable to the subcutaneous route, but unfortunately infants in these studies did not have PA thus this obstacle could not be tested. These studies have paved the way for larger on-going studies to evaluate if aerosol measles vaccination may be as effective for primary immunization and have already been used successfully for secondary immunization in older children [69]. This will be particularly important for developing countries where needleless immunizations have the potential to increase distribution efforts. 2.2. Rotavirus The only live viral vaccines that are routinely used in infancy are against rotavirus and poliovirus. Poliovirus vaccines are covered

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Table 1 Summary of live viral vaccines. Vaccine

Age of immunization of primary dose (months)

Immunization

Immune response

Obstacles

Measles Mumpsa

12–15 9b

Live attenuated measles, mumps, rubella (subcutaneous) Measles (aerosol)c

Humoral Lower antibody titer and avidity when given <12 months Titers and avidity boost after subsequent vaccine doses Cellular No age related differences when given <12 months Relatively diminished responses compared to adults

Passive antibodies-predominantly IgG transferred transplacentally Immature Immune system

Humoral IgG and IgA induced but lower if given in neonatal period and in developing countries Poorly correlated with vaccine efficacy Mucosal immunity established after one dose but requires priming Cellular Not studied

Passive antibodies-predominantly IgA from breast milk, but also serum IgG Competition from other gastrointestinal pathogens

Humoral More heterotypic in children compared with trivalent influenza vaccine Cellular Not studied

Not studied

Rotavirus

Influenza

1.75–2

6

Live attenuated or bovine reassortant rotavirus (oral)

Live attenuated intranasal vaccine (Intranasal)

Immature immune system

Not studied

Not studied

a Immunization provided as trivalent vaccine MMR (measles, mumps rubella) in developed countries and as monovalent measles in most developing countries, no mumps given in developing countries. b 12–15 months in primary dose in developed countries and 9 months primary measles dose in developing countries. c Routine immunization is via subcutaneous route although aerosol delivery has been studied as primary and secondary measles doses.

elsewhere and therefore will only be used to highlight particular contributions to viral immunity here. Two live-attenuated, orally administered rotavirus vaccines are licensed [14,16] and are in use in most high and medium income countries. In 2009, the WHO recommended the inclusion of rotavirus vaccine in national immunization programs, and several countries have adapted rotavirus policies [12]. In the developed world vaccine has reduced the incidence of severe disease by 85–98% in the first and second years following immunization [70], and in follow up studies, efficacy remained high 3 years after vaccination [71]. Attempts made to understand the vaccine efficacy have been largely unrevealing which is important since these vaccines are not as effective in the developing world, and yet understanding potential immune obstacles has been difficult. Unfortunately, immunogenicity studies from developed countries show that humoral immunity is not well correlated with vaccine efficacy since both neutralizing IgG and serum IgA levels fall below the documented rates of disease prevention [72] (see Table 1 for summary). Importantly mucosal immunity, evaluated by stool rotavirus specific IgA levels, increases after even the first dose of oral rotavirus vaccination, but requires multiple doses to induce responses in most infants [73]. T cell studies are lacking for rotavirus vaccine although have been shown to develop in young infants after rotavirus infection [74] and may represent a promising tool for assessing vaccine immunogenicity in the future. While early immunity is the goal, the current rotavirus vaccine schedule initiates vaccination at 6–8 weeks with the need for multiple doses to induce optimal immunity. This therefore delays full protection by several months. Efforts to address this suggest that an early neonatal dose may be protective and safe. Studies in fact indicate that protection against severe disease has been documented as early as 2 weeks after the first rotavirus vaccine dose administered at 6 weeks [75], indicating that these young infants are mounting protective responses. Further, infants given rotavirus vaccine at birth followed by doses at 2 and 4 months had few adverse events and mounted an immune response. Unfortunately this strategy was

not pursued further since the infants in the group receiving a neonatal dose were less likely to seroconvert and develop neutralizing antibody in response to the vaccine compared with infants given their first dose at 2 months [76]. The greatest promise for a neonatal rotavirus vaccine arose from 2 outbreaks in India [73,77]. In these outbreaks, 2 rotavirus strains, both naturally occurring bovine-human reassortants, were isolated from newborns. Neonates infected with these strains were asymptomatic, developed anti-viral immunity, and were protected against disease up to 2 years after infection. Clearly these strains were able to replicate in the presence of PA (since infection was incurred when infants had passive immunity from their mothers) likely because the PA was not matched to these strains due to their reassortant nature and because they are not widely circulating in the population. Since these strains exhibit many characteristics that make them ideal for neonatal vaccination, they are currently under development for use in the newborn period [73]. The advantages of orally administered live viral vaccines include viral replication at mucosal and luminal interfaces, providing immune responses that mimic natural infection [70]. These responses include secretory (s) IgA and serum neutralizing antigenspecific IgG and IgA responses. Serum and mucosal antibody responses are present in young infants, but data suggest that infants have reduced immunity compared with older children and adults. Further, younger children need priming with multiple vaccine doses to achieve comparable IgG titers. In contrast to IgG which is boosted by repeated exposures, IgA levels do not boost with subsequent vaccine doses remaining lower in the very young [78]. Additionally, there is discordance between the development of serum and mucosal humoral immunity, with the assumption that B cell homing receptors are not mature in infants. Further lymphoid maturation appears to be specific to location. For instance, in contrast to vaccines administered orally, nasal routes have induced strong sIgA responses in young children to influenza and poliovirus, indicating that nasal lymphoid tissue is functional in young infants whereas tonsilar tissue has a delayed maturation [79].

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It was hypothesized that the delivery of live viral vaccination via the mucosal route may have the additional advantage of bypassing the obstacle of PA suppression. This was based on the high vaccine efficacy observed in the developed world to poliovirus and rotavirus vaccines as well as the fact that breastfeeding was not found to have an effect on vaccine immunity to either vaccine in developed countries [80,81]. Unfortunately studies in developing countries have revealed potential interference with both sIgA and neutralizing IgG responses to live oral vaccines including both poliovirus and rotavirus. Studies have documented neutralization of vaccine virus with the higher titers of serum IgG and breast milk IgA as is seen in mothers in the developing world [82]. The inhibitory effects of breastfeeding on the acquisition of anti-viral immunity are highest in colostrum and decline as breast feeding progresses. Nonetheless, even when first doses are delayed until 6 weeks, antibody titers in the breast milk of woman in developing world were found to be inhibitory in vitro and clinical evidence suggests an inference with rotavirus vaccine efficacy [83]. Seroconversion rates and the level of Ab titers in response to vaccine are reduced when data are stratified according to countries’ per capita gross national incomes [84]. Further research is needed, in particular to determine whether the interval between breastfeeding and vaccination has any effect on immunity. It will also be important to determine if the oral route induces T cell immunity even after neonatal doses that can then prime humoral immunity in later life. Other obstacles exist when considering an oral route of viral vaccination. This includes competition from other GI pathogens [81,83,85–88]. Several studies have documented a lower immune response to the first dose of a rotavirus vaccine in infants who received OPV simultaneously compared with infants who did not supporting the theory of pathogen competition [83]. It is also hypothesized that the immune response to a live oral rotavirus vaccine might be diminished in the presence of pathogens causing asymptomatic infection [88,89]. Despite the complexity of immunizing infants in the developing world, the clear public health advantage to rotavirus vaccination, even at reduced efficacy, is significant. There is also a non-specific advantage with a reduction of all-cause diarrhea in infants receiving rotavirus vaccine [83]. This latter phenomenon is an important consideration for future vaccine use in neonates as it may signal the induction of the immune system that can then respond to other pathogens, and importantly has been observed with other vaccines, such as measles and BCG [61,90]. 2.3. Influenza The only currently licensed vaccine administered intranasally is the cold-adapted, live attenuated influenza virus vaccine (LAIV) which was first recommended for use in children >2 years in 2007, but clinical trials included infants as young as 6 months [91]. Overall in naïve children receiving 2 doses of vaccine as is currently recommended for children <9 years, the efficacy of LAIV against influenza disease was approximately 77% [91]. LAIV has revealed several advantages compared to historical vaccination with trivalent inactivated influenza vaccine (TIV) administered intramuscularly in young children. Comparisons in children 6–71 months who received two doses of either LAIV or TIV revealed 45–53% fewer cases of influenza illness in the LAIV recipients [92,93]. This advantage was independent of age. Further, LAIV showed disease protection for antigentically dissimilar strains of influenza A H3N2, suggesting that either the live attenuated nature of the vaccine or the nasal route of administration provided broadened immunity to vaccine recipients. Several mechanisms have been proposed for the higher efficacy of LAIV compared with TIV in children, including more vigorous mucosal immunity, induction of cellular immune responses, and serum antibodies that

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are heterotypic and more broadly reactive [94,95]. The antibody response to LAIV compared with TIV in young children has been shown to be more broadly reactive and as a result the efficacy of LAIV against drifted influenza has been demonstrated [96,97] (see Table 1 for summary). Despite the increased efficacy of LAIV in infants, there were safety concerns in infants 6–11 months of age who demonstrated increased rates of wheezing and hospitalization compared with infants receiving TIV and therefore LAIV is not currently licensed for infants [98]. Using the successes of the intranasal LAIV, live attenuated immunizations against RSV infection are being developed, including an intranasally administered cold passage live attenuated vaccine. Phase I studies showed immunogenicity and protection against subsequent disease when administered at 1–2 months, but future studies are needed before these will be licensed for use [99]. 3. Conclusions Live viral vaccines are one of the most effective public health measures. Nonetheless, decreased immunogenicity when administered in infancy has thrown doubt on their viability for protecting the very young. Despite the relatively limited immune responses, clinical benefit has been documented. Thus, we may need to adjust our expectations for early vaccination to one of preventing disease severity and mortality rather than producing sterilizing immunity. There are approaches to vaccination that are effective in the very young, such as prime boost strategies which utilize the functional components of the immune system and can activate the immune system for future exposures. As our understanding of the infant immune system expands, the development of new vaccines for use in young children will arise. Conflict of interest: The author has no conflicts of interest. References [1] WHO. In: Press W, editor. World Health Statistics. 2011. [2] MMWR. Vaccine-preventable diseases, immunizations, and MMWR—1961–2011. MMWR Morb Mortal Wkly Rep 2011;60:49–57. [3] Daniels D, Jiles RB, Klevens RM, Herrera GA. Undervaccinated African-American preschoolers: a case of missed opportunities. Am J Prev Med 2001;20:61–8. [4] Menzies R, Turnour C, Chiu C, McIntyre P. Vaccine preventable diseases and vaccination coverage in Aboriginal and Torres Strait Islander people, Australia 2003 to 2006. Commun Dis Intell 2008;32(Suppl.):S2–67. [5] Nair H, Nokes DJ, Gessner BD, Dherani M, Madhi SA, Singleton RJ, et al. Global burden of acute lower respiratory infections due to respiratory syncytial virus in young children: a systematic review and meta-analysis. Lancet 2010;375:1545–55. [6] Groothuis JR, Hoopes JM, Jessie VG. Prevention of serious respiratory syncytial virus-related illness. I. Disease pathogenesis and early attempts at prevention. Adv Ther 2011;28:91–109. [7] Nair H, Brooks WA, Katz M, Roca A, Berkley JA, Madhi SA, et al. Global burden of respiratory infections due to seasonal influenza in young children: a systematic review and meta-analysis. Lancet 2011;378:1917–30. [8] Parashar UD, Burton A, Lanata C, Boschi-Pinto C, Shibuya K, Steele D, et al. Global mortality associated with rotavirus disease among children in 2004. J Infect Dis 2009;200(Suppl. 1):S9–15. [9] Glass RI, Kilgore PE, Holman RC, Jin S, Smith JC, Woods PA, et al. The epidemiology of rotavirus diarrhea in the United States: surveillance and estimates of disease burden. J Infect Dis 1996;174(Suppl. 1):S5–11. [10] Kilgore PE, Holman RC, Clarke MJ, Glass RI. Trends of diarrheal disease—associated mortality in US children, 1968 through 1991. JAMA 1995;274:1143–8. [11] MMWR. Prevention of rotavirus gastroenteritis among infants and children recommendations of the advisory committee on immunization practices (ACIP). MMWR Recomm Rep 2009;58:1–25. [12] MMWR. Rotavirus vaccines: an update. MMWR Wkly Epidemiol Rec 2009;84:533–40. [13] MMWR. Rotavirus surveillance-worldwide, 2009. MMWR Morb Mortal Wkly Rep 2010;60:514–6. [14] Ruiz-Palacios GM, Perez-Schael I, Velazquez FR, Abate H, Breuer T, Clemens SC, et al. Safety and efficacy of an attenuated vaccine against severe rotavirus gastroenteritis. N Engl J Med 2006;354:11–22. [15] Tate JE, Parashar UD. Monitoring impact and effectiveness of rotavirus vaccination. Expert Rev Vaccines 2011;10:1123–5.

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[16] Vesikari T, Matson DO, Dennehy P, Van Damme P, Santosham M, Rodriguez Z, et al. Safety and efficacy of a pentavalent human-bovine (WC3) reassortant rotavirus vaccine. N Engl J Med 2006;354:23–33. [17] Armah GE, Sow SO, Breiman RF, Dallas MJ, Tapia MD, Feikin DR, et al. Efficacy of pentavalent rotavirus vaccine against severe rotavirus gastroenteritis in infants in developing countries in sub-Saharan Africa: a randomised, double-blind, placebo-controlled trial. Lancet 2010;376:606–14. [18] Madhi SA, Cunliffe NA, Steele D, Witte D, Kirsten M, Louw C, et al. Effect of human rotavirus vaccine on severe diarrhea in African infants. N Engl J Med 2010;362:289–98. [19] Zaman K, Dang DA, Victor JC, Shin S, Yunus M, Dallas MJ, et al. Efficacy of pentavalent rotavirus vaccine against severe rotavirus gastroenteritis in infants in developing countries in Asia: a randomised, double-blind, placebo-controlled trial. Lancet 2010;376:615–23. [20] CDC. Global measles mortality, 2000–2008. MMWR 2009;58:1321–6. [21] Aaby P, Andersen M, Sodemann M, Jakobsen M, Gomes J, Fernandes M. Reduced childhood mortality after standard measles vaccination at 4–8 months compared with 9–11 months of age. BMJ 1993;307:1308–11. [22] CDC. Progress in reducing global measles deaths, 1999–2004. MMWR 2006;55:247–9. [23] Zarocostas J. Mortality from measles fell by 91% in Africa from 2000 to 2006. BMJ 2007;335:1173. [24] CDC. Update: Measles—United States, January–July 2008. MMWR 2008;57:893–6. [25] Gans H, Arvin A, Galinus J, Logan L, DeHovitz R, Maldonado Y. Deficiency of the humoral immune response to measles vaccine in infants immunized at age 6 months. JAMA 1998;280:527–32. [26] Gans H, Yasukawa L, Alderson A, Rinki M, DeHovitz R, Maldonado Y, Beeler J, Audet S, Arvin AM. Humoral and cell-mediated immune responses to an early two-dose measles vaccination regimen in the United States. J Infect Dis 2004; 189. [27] Kumar ML, Johnson CE, Chui LW, Whitwell JK, Staehle B, Nalin D. Immune response to measles vaccine in 6-month-old infants of measles seronegative mothers [published erratum appears in Vaccine 1999 Apr 23;17(17):2206]. Vaccine 1998;16:2047–51. [28] Maldonado YA, Lawrence EC, DeHovitz R, Hartzell H, Albrecht P. Early loss of passive measles antibody in infants of mothers with vaccine-induced immunity. Pediatrics 1995;96:447–50. [29] Markowitz LE, Albrecht P, Rhodes P, Demonteverde R, Swint E, Maes EF, et al. Changing levels of measles antibody titers in women and children in the United States: impact on response to vaccination. Kaiser Permanente Measles Vaccine Trial Team. Pediatrics 1996;97:53–8. [30] Carson MM, Spady DW, Albrecht P, Beeler JA, Thipphawong J, Barreto L, et al. Measles vaccination of infants in a well-vaccinated population. Pediatr Infect Dis J 1995;14:17–22. [31] Leuridan E, Van Damme P. Passive transmission and persistence of naturally acquired or vaccine-induced maternal antibodies against measles in newborns. Vaccine 2007;25:6296–304. [32] Dayan GH, Rubin S. Mumps outbreaks in vaccinated populations: are available mumps vaccines effective enough to prevent outbreaks? Clin Infect Dis 2008;47:1458–67. [33] Khandaker G, Marshall H, Peadon E, Zurynski Y, Burgner D, Buttery J, et al. Congenital and neonatal varicella: impact of the national varicella vaccination programme in Australia. Arch Dis Child 2011;96:453–6. [34] Amanna IJ, Carlson NE, Slifka MK. Duration of humoral immunity to common viral and vaccine antigens. N Engl J Med 2007;357:1903–15. [35] Precopio ML, Betts MR, Parrino J, Price DA, Gostick E, Ambrozak DR, et al. Immunization with vaccinia virus induces polyfunctional and phenotypically distinctive CD8(+) T cell responses. J Exp Med 2007;(204): 1405–16. [36] Pulendran B, Ahmed R. Translating innate immunity into immunological memory: implications for vaccine development. Cell 2006;124:849–63. [37] Zhu J, Martinez J, Huang X, Yang Y. Innate immunity against vaccinia virus is mediated by TLR2 and requires TLR-independent production of IFN-beta. Blood 2007;109:619–25. [38] Hammarlund E, Lewis MW, Hansen SG, Strelow LI, Nelson JA, Sexton GJ, et al. Duration of antiviral immunity after smallpox vaccination. Nat Med 2003;9:1131–7. [39] Carson MM, Spady DW, Beeler JA, Krezolek MP, Audet S, Pabst HF. Follow-up of infants given measles vaccine at 6 months of age: antibody and CMI responses to MMRII at 15 months of age and antibody levels at 27 months of age. Vaccine 2005;23:3247–55. [40] Ward BJ, Boulianne N, Ratnam S, Guiot MC, Couillard M, De Serres G. Cellular immunity in measles vaccine failure: demonstration of measles antigen-specific lymphoproliferative responses despite limited serum antibody production after revaccination. J Infect Dis 1995;172:1591–5. [41] Permar SR, Klumpp SA, Mansfield KG, Carville AA, Gorgone DA, Lifton MA, et al. Limited contribution of humoral immunity to the clearance of measles viremia in rhesus monkeys. J Infect Dis 2004;190:998–1005. [42] Aaby P, Bukh J, Leerhoy J, Lisse IM, Mordhorst CH, Pedersen IR. Vaccinated children get milder measles infection: a community study from Guinea-Bissau. J Infect Dis 1986;154:858–63. [43] Gans H, DeHovitz R, Forghani B, Beeler J, Maldonado Y, Arvin AM. Measles and mumps vaccination as a model to investigate the developing immune system: passive and active immunity during the first year of life. Vaccine 2003;21:3398–405.

[44] Gans HA, Arvin AM, Galinus J, Logan L, DeHovitz R, Maldonado Y. Deficiency of the humoral immune response to measles vaccine in infants immunized at age 6 months. JAMA 1998;280:527–32. [45] Moon SS, Wang Y, Shane AL, Nguyen T, Ray P, Dennehy P, et al. Inhibitory effect of breast milk on infectivity of live oral rotavirus vaccines. Pediatr Infect Dis J 2010;29:919–23. [46] Gans HA, Yasukawa LL, Alderson A, Rinki M, DeHovitz R, Beeler J, et al. Humoral and cell-mediated immune responses to an early 2-dose measles vaccination regimen in the United States. J Infect Dis 2004;190:83–90. [47] Nair N, Gans H, Lew-Yasukawa L, Long-Wagar AC, Arvin A, Griffin DE. Agedependent differences in IgG isotype and avidity induced by measles vaccine received during the first year of life. J Infect Dis 2007;196:1339–45. [48] Jelonek MT, Maskrey JL, Steimer KS, Potts BJ, Higgins KW, Keller MA. Maternal monoclonal antibody to the V3 loop alters specificity of the response to a human immunodeficiency virus vaccine. J Infect Dis 1996;174:866–9. [49] Panpitpat C, Thisyakorn U, Chotpitayasunondh T, Furer E, Que JU, Hasler T, et al. Elevated levels of maternal anti-tetanus toxin antibodies do not suppress the immune response to a Haemophilus influenzae type b polyribosylphosphatetetanus toxoid conjugate vaccine. Bull World Health Organ 2000;78: 364–71. [50] Siegrist C, Barrios C, Martinex X, Brandt C, Cordova M, Kovarik J, et al. Influence of maternal antibodies on vaccine responses: inhibition of antibody but not T cell responses allows successful early prime-boost strategies in mice. Eur J Immunol 1998;28:4138–48. [51] Weeratna RD, Brazolot Millan CL, McCluskie MJ, Siegrist CA, Davis HL. Priming of immune responses to hepatitis B surface antigen in young mice immunized in the presence of maternally derived antibodies. FEMS Immunol Med Microbiol 2001;30:241–7. [52] Kim D, Huey D, Oglesbee M, Niewiesk S. Insights into the regulatory mechanism controlling the inhibition of vaccine-induced seroconversion by maternal antibodies. Blood 2011;117:6143–51. [53] Johnson CE, Nalin DR, Chui LW, Whitwell J, Marusyk RG, Kumar ML. Measles vaccine immunogenicity in 6- versus 15-month-old infants born to mothers in the measles vaccine era. Pediatrics 1994;93:939–44. [54] Markowitz LE, Sepulveda J, Diaz-Ortega JL, Valdespino JL, Albrecht P, Zell ER, et al. Immunization of six-month-old infants with different doses of Edmonston-Zagreb and Schwarz measles vaccines [published erratum appears in N Engl J Med 1990 Mar 22;322(12):863]. N Engl J Med 1990;322: 580–7. [55] Ndumbe PM, Gilchrist SA, Pabst H, Sama MT, Mbede J. Comparison of Edmonston-Zagreb, Connaught and Schwarz measles vaccines in Cameroonian infants aged 3–8 months. Vaccine 1995;13:276–80. [56] Tidjani O, Grunitsky B, Guerin N, Levy-Bruhl D, Lecam N, Xuereff C, et al. Serological effects of Edmonston-Zagreb, Schwarz, and AIK-C measles vaccine strains given at ages 4–5 or 8–10 months. Lancet 1989;2:1357–60. [57] Gans H, Yasukawa L, Rinki M, DeHovitz R, Forghani B, Beeler J, et al. Immune responses to measles and mumps vaccination of infants at 6, 9, and 12 months. J Infect Dis 2001;184:817–26. [58] Linnemann Jr CC, Dine MS, Roselle GA, Askey PA. Measles immunity after revaccination: results in children vaccinated before 10 months of age. Pediatrics 1982;69:332–5. [59] Hutchins SS, Dezayas A, Le Blond K, Heath J, Bellini W, Audet S, et al. Evaluation of an early two-dose measles vaccination schedule. Am J Epidemiol 2001;154:1064–71. [60] Stetler HC, Orenstein WA, Bernier RH, Herrmann KL, Sirotkin B, Hopfensperger D, et al. Impact of revaccinating children who initially received measles vaccine before 10 months of age. Pediatrics 1986;77:471–6. [61] Aaby P, Samb B, Simondon F, Seck AM, Knudsen K, Whittle H. Non-specific beneficial effect of measles immunisation: analysis of mortality studies from developing countries. BMJ 1995;311:481–5. [62] Ceyhan M, Kanra G, Erdem G, Kanra B. Immunogenicity and efficacy of one dose measles-mumps-rubella (MMR) vaccine at twelve months of age as compared to monovalent measles vaccination at nine months followed by MMR revaccination at fifteen months of age. Vaccine 2001;19:4473–8. [63] Garly ML, Bale C, Martins CL, Monteiro M, George E, Kidd M, et al. Measles antibody responses after early two dose trials in Guinea-Bissau with Edmonston-Zagreb and Schwarz standard-titre measles vaccine: better antibody increase from booster dose of the Edmonston-Zagreb vaccine. Vaccine 2001;19:1951–9. [64] John S, Lalitha G, George K, Joseph A. Serological response to early measles vaccination. J Trop Pediatr 2004;50:175–7. [65] Gans HA, Maldonado Y, Yasukawa LL, Beeler J, Audet S, Rinki MM, et al. IL12, IFN-gamma, and T cell proliferation to measles in immunized infants. J Immunol 1999;162:5569–75. [66] Wong-Chew RM, Islas-Romero R, Garcia-Garcia Mde L, Beeler JA, Audet S, Santos-Preciado JI, et al. Induction of cellular and humoral immunity after aerosol or subcutaneous administration of Edmonston-Zagreb measles vaccine as a primary dose to 12-month-old children. J Infect Dis 2004;189: 254–7. [67] Wong-Chew RM, Islas-Romero R, Garcia-Garcia Mde L, Beeler JA, Audet S, Santos-Preciado JI, et al. Immunogenicity of aerosol measles vaccine given as the primary measles immunization to nine-month-old Mexican children. Vaccine 2006;24:683–90. [68] Wong-Chew RM, Garcia-Leon ML, Espinosa-Torres Torrija B, Hernandez-Perez B, Cardiel-Marmolejo LE, Beeler JA, et al. Increasing the time of exposure to aerosol measles vaccine elicits an immune response equivalent to that seen in

H.A. Gans / Vaccine 31 (2013) 2531–2537

[69]

[70] [71]

[72] [73]

[74]

[75]

[76]

[77]

[78]

[79]

[80]

[81]

[82]

[83]

[84]

9-month-old Mexican children given the same dose subcutaneously. J Infect Dis 2011;204:426–32. Bellanti JA, Zeligs BJ, Mendez-Inocencio J, Garcia-Garcia ML, Islas-Romero R, Omidvar B, et al. Immunologic studies of specific mucosal and systemic immune responses in Mexican school children after booster aerosol or subcutaneous immunization with measles vaccine. Vaccine 2004;22:1214–20. Gray J. Rotavirus vaccines: safety, efficacy and public health impact. J Intern Med 2011;270:206–14. Vesikari T, Karvonen A, Ferrante SA, Kuter BJ, Ciarlet M. Sustained efficacy of the pentavalent rotavirus vaccine, RV5, up to 3.1 years following the last dose of vaccine. Pediatr Infect Dis J 2010;29:957–63. Ward R. Mechanisms of protection against rotavirus infection and disease. Pediatr Infect Dis J 2009;28:S57–9. Glass RI, Bhan MK, Ray P, Bahl R, Parashar UD, Greenberg H, et al. Development of candidate rotavirus vaccines derived from neonatal strains in India. J Infect Dis 2005;192(Suppl. 1):S30–5. Offit PA, Hoffenberg EJ, Pia ES, Panackal PA, Hill NL. Rotavirus-specific helper T cell responses in newborns, infants, children, and adults. J Infect Dis 1992;165:1107–11. Dennehy PH, Vesikari T, Matson DO, Itzler RF, Dallas MJ, Goveia MG, et al. Efficacy of the pentavalent rotavirus vaccine, RotaTeq(R) (RV5), between doses of a 3-dose series and with less than 3 doses (incomplete regimen). Hum Vaccin 2011;7:563–8. Vesikari T, Karvonen A, Forrest BD, Hoshino Y, Chanock RM, Kapikian AZ. Neonatal administration of rhesus rotavirus tetravalent vaccine. Pediatr Infect Dis J 2006;25:118–22. Bishop RF, Barnes GL, Cipriani E, Lund JS. Clinical immunity after neonatal rotavirus infection. A prospective longitudinal study in young children. N Engl J Med 1983;309:72–6. el-Madhun AS, Cox RJ, Soreide A, Olofsson J, Haaheim LR. Systemic and mucosal immune responses in young children and adults after parenteral influenza vaccination. J Infect Dis 1998;178:933–9. Johnson Jr PR, Feldman S, Thompson JM, Mahoney JD, Wright PF. Comparison of long-term systemic and secretory antibody responses in children given live, attenuated, or inactivated influenza A vaccine. J Med Virol 1985;17: 325–35. Goveia MG, DiNubile MJ, Dallas MJ, Heaton PM, Kuter BJ. Efficacy of pentavalent human-bovine (WC3) reassortant rotavirus vaccine based on breastfeeding frequency. Pediatr Infect Dis J 2008;27:656–8. Rennels MB. Influence of breast-feeding and oral poliovirus vaccine on the immunogenicity and efficacy of rotavirus vaccines. J Infect Dis 1996;174(Suppl. 1):S107–11. Chan J, Nirwati H, Triasih R, Bogdanovic-Sakran N, Soenarto Y, Hakimi M, et al. Maternal antibodies to rotavirus: could they interfere with live rotavirus vaccines in developing countries? Vaccine 2011;29:1242–7. Steele AD, De Vos B, Tumbo J, Reynders J, Scholtz F, Bos P, et al. Coadministration study in South African infants of a live-attenuated oral human rotavirus vaccine (RIX4414) and poliovirus vaccines. Vaccine 2010;28:6542–8. Patel M, Shane AL, Parashar UD, Jiang B, Gentsch JR, Glass RI. Oral rotavirus vaccines: how well will they work where they are needed most? J Infect Dis 2009;200(Suppl. 1):S39–48.

2537

[85] Aggarwal R, Sentz J, Miller MA. Role of zinc administration in prevention of childhood diarrhea and respiratory illnesses: a meta-analysis. Pediatrics 2007;119:1120–30. [86] Grassly NC, Jafari H, Bahl S, Durrani S, Wenger J, Sutter RW, et al. Mucosal immunity after vaccination with monovalent and trivalent oral poliovirus vaccine in India. J Infect Dis 2009;200:794–801. [87] Ing DJ, Glass RI, Woods PA, Simonetti M, Pallansch MA, Wilcox WD, et al. Immunogenicity of tetravalent rhesus rotavirus vaccine administered with buffer and oral polio vaccine. Am J Dis Child 1991;145:892–7. [88] Myaux JA, Unicomb L, Besser RE, Modlin JF, Uzma A, Islam AM, et al. Effect of diarrhea on the humoral response to oral polio vaccination. Pediatr Infect Dis J 1996;15:204–9. [89] Posey DL, Linkins RW, Oliveria MJ, Monteiro D, Patriarca PA. The effect of diarrhea on oral poliovirus vaccine failure in Brazil. J Infect Dis 1997;175(Suppl. 1):S258–63. [90] Ota MO, Vekemans J, Schlegel-Haueter SE, Fielding K, Sanneh M, Kidd M, et al. Influence of Mycobacterium bovis bacillus Calmette-Guerin on antibody and cytokine responses to human neonatal vaccination. J Immunol 2002;168:919–25. [91] Rhorer J, Ambrose CS, Dickinson S, Hamilton H, Oleka NA, Malinoski FJ, et al. Efficacy of live attenuated influenza vaccine in children: a meta-analysis of nine randomized clinical trials. Vaccine 2009;27:1101–10. [92] Fleming DM, Crovari P, Wahn U, Klemola T, Schlesinger Y, Langussis A, et al. Comparison of the efficacy and safety of live attenuated cold-adapted influenza vaccine, trivalent, with trivalent inactivated influenza virus vaccine in children and adolescents with asthma. Pediatr Infect Dis J 2006;25: 860–9. [93] Negri E, Colombo C, Giordano L, Groth N, Apolone G, La Vecchia C. Influenza vaccine in healthy children: a meta-analysis. Vaccine 2005;23:2851–61. [94] Forrest BD, Pride MW, Dunning AJ, Capeding MR, Chotpitayasunondh T, Tam JS, et al. Correlation of cellular immune responses with protection against culture-confirmed influenza virus in young children. Clin Vaccine Immunol 2008;15:1042–53. [95] Vesikari T, Fleming DM, Aristegui JF, Vertruyen A, Ashkenazi S, Rappaport R, et al. Safety, efficacy, and effectiveness of cold-adapted influenza vaccinetrivalent against community-acquired, culture-confirmed influenza in young children attending day care. Pediatrics 2006;118:2298–312. [96] Belshe RB, Gruber WC, Mendelman PM, Cho I, Reisinger K, Block SL, et al. Efficacy of vaccination with live attenuated, cold-adapted, trivalent, intranasal influenza virus vaccine against a variant (A/Sydney) not contained in the vaccine. J Pediatr 2000;136:168–75. [97] Belshe RB, Gruber WC, Mendelman PM, Mehta HB, Mahmood K, Reisinger K, et al. Correlates of immune protection induced by live, attenuated, cold-adapted, trivalent, intranasal influenza virus vaccine. J Infect Dis 2000;181:1133–7. [98] Belshe RB, Edwards KM, Vesikari T, Black SV, Walker RE, Hultquist M, et al. Live attenuated versus inactivated influenza vaccine in infants and young children. N Engl J Med 2007;356:685–96. [99] Wright PF, Karron RA, Belshe RB, Thompson J, Crowe Jr JE, Boyce TG, et al. Evaluation of a live, cold-passaged, temperature-sensitive, respiratory syncytial virus vaccine candidate in infancy. J Infect Dis 2000;182:1331–42.