Causes of impaired oral vaccine efficacy in developing countries

Transplacental antibodies

Newborn infants passively acquire serum antibodies from their mothers via the placenta at titers comparable to those in maternal serum [35,36]. These antibodies are predominantly of class IgG (IgA does not cross the placenta [37]) and decay with a half-life of approximately 28 days [38]. Maternally derived antibodies have been shown to interfere with both oral and parenteral vaccines [21,39], and are in this respect distinct from several of the mechanisms considered below. Precise mechanisms of interference remain uncertain, but may involve the masking of vaccine-derived antigens from B- and T-cell receptors [40].

An impaired immune response in infants with elevated titers of passively acquired antibodies has consistently been demonstrated in field trials of OPV [21,41–43], with the greatest inhibitory effect typically observed for serotype 3 [41,43]. Among infants in Brazil who received trivalent OPV at 0, 6, 10 and 14 weeks of age, type 3 seroconversion was observed in 70% of individuals who had a baseline homotypic antibody titer of less than 1:64, but only 29% of those with a baseline titer of at least 1:256 [21]. Diminished shedding of vaccine polioviruses after immunization (which correlates strongly with immunogenicity) has been observed among nonbreastfed newborn infants with elevated titers of passively acquired antibodies. This suggests that the inhibitory effect of maternal antibodies on OPV immunogenicity reflects in part a reduction in vaccine virus replication [44,45], possibly mediated by the passive transfer of maternal IgG across the intestinal epithelium (termed transudation) [40]. Passively acquired serum antibodies have also been linked with a reduction in the immunogenicity of oral rotavirus vaccines, including Rotarix [36], RotaTeq [46] and Rotavac [47]. An increase in seroconversion rates by up to 25% has been observed following the administration of Rotarix at 10 and 14 weeks of age as opposed to the standard schedule of 6 and 10 weeks (when maternal antibodies will have had less time to decay) [48]. However, a recent study of Rotavac in north India concluded that maternal antibodies explained less than 10% of variation in post-vaccination antibody levels [49]. Oral cholera and typhoid vaccines are not licensed in infants and studies demonstrating inhibition by transplacental antibodies are therefore lacking.

The degree to which trends in maternal antibodies contribute to the geographic, age-related and seasonal patterns of OPV and rotavirus vaccine performance is less certain. Maternal exposure to natural infection, and consequent boosting of serum antibody titers, will be greatest in places and at times of year that transmission is most efficient, enhancing the potential influence of passively acquired antibodies in young infants. These trends generally appear to correlate with observed patterns of OPV and rotavirus vaccine immunogenicity, although cross-sectional surveys of poliovirus- and rotavirus-specific maternal antibody titers in different sites using standardized methods are currently lacking [50,51]. It is also worth noting that maternal infection with malaria or HIV is associated with a reduction in the transfer of antibodies across the placenta [52]. As such, the effect of maternal antibodies on vaccine immunogenicity may be diminished in populations with a high burden of these infections.

Histo blood group antigens

Among the potential genetic sources of variation in oral vaccine outcome, histo blood group antigens (HBGAs) are important candidates. HBGAs are ABH and Lewis glycans present on the surface of epithelial cells and in mucosal secretions. HBGA synthesis occurs via the sequential addition of monosaccharides to precursor disaccharides through the expression of genetically encoded glycosyltransferases such as FUT2 (secretor gene) and FUT3 (Lewis gene). Individuals with functional FUT2 are referred to as secretor-positive because HBGAs can be found in mucosal secretions; those with functional FUT3 are referred to as Lewis-positive. The fucosyltransferases are nonfunctional in individuals with mutations in FUT2 and FUT3, and such individuals are referred to as secretor-negative (or nonsecretors) and Lewis-negative, respectively.

A number of enteric pathogens and pathogen-derived toxins use HBGAs as attachment factors. These interactions, in turn, may give rise to marked associations between HBGA phenotype and enteric pathogen susceptibility [74]. For individual pathogens, HBGA interactions are often strain/genotype-specific. For example, secretors are significantly more susceptible than nonsecretors to norovirus genotype GII.4, but the same is not true of other norovirus genotypes such as GII.3 [75]. Rotaviruses are classified into G and P types based on the composition of the outer capsid proteins VP7 and VP4, respectively. Binding to HBGA occurs through the VP8* domain of protein VP4 [76,77]. To date, 47 VP4 (P) genotypes have been identified, of which P[4], P[6] and P[8] are most common. A recent systematic review based on studies in France, Vietnam, the USA and Burkina Faso documented a 27-fold increase in susceptibility to P[8] rotavirus in secretors versus nonsecretors, but showed no association between secretor status and susceptibility to P[4] or P[6] genotypes [75]. In Burkina Faso, no Lewis-negative secretors (expressing H type 1 antigen) were infected with P[8] strains, despite making up 25% of the study population [78], suggesting that the infectivity of P[8] strains may be dependent on both secretor and Lewis status. However, population-specific differences may exist; for example, P[8] infections were detected in both secretors and nonsecretors during a study in Tunisia [79].

In addition to being among the globally dominant rotavirus genotypes, P[8] strains are a component of Rotarix and RotaTeq. It is therefore plausible that genotype-specific HBGA binding patterns may contribute to geographic differences in vaccine efficacy. While nonsecretors make up just 20% of Caucasians, this phenotype may reach a prevalence of 30–50% in parts of Bangladesh, India and Pakistan [80–82], where the immunogenicity and efficacy of oral rotavirus vaccines is poor. Similarly, the Lewis-negative phenotype is significantly more common in parts of Africa (reaching >30% in some populations) than in Caucasians (4–6%) [83]. Thus, an impaired replicative capacity of P[8] strains combined with the high prevalence of P[6] genotypes in Lewis-negative individuals, as reported in Burkina Faso [78], would be consistent with observed discrepancies in rotavirus vaccine efficacy.

A key challenge for the coming years will be to determine the extent to which host genetic determinants of rotavirus diarrhea susceptibility apply to attenuated vaccine strains. Notably, during a recent study in Pakistan, Rotarix immunogenicity was associated with secretor status and ABO blood group – seroconversion was seen in 19% of nonsecretors, 30% of secretors with non-O blood groups and 51% of secretors with blood group O [84]. How this poorer immunogenicity impacts clinical protection is not yet clear and may be complex. For example, while nonsecretors may be less likely to respond to vaccination, they will also be less susceptible to natural infection with certain genotypes.

The effect of HBGAs on OPV immunogenicity has not been studied, although blood group O has been linked with an increased occurrence of paralytic poliomyelitis [85]. HBGA phenotype also appears to influence susceptibility to bacterial enteropathogens [86,87], including V. cholerae [88,89]. Nonsecretors are more likely to suffer from severe cholera than secretors [90]. Somewhat paradoxically, individuals with blood group O are less likely to be colonized by V. cholerae [89], but are more likely, upon colonization, to develop severe symptoms [88,89]. The extent to which HBGA phenotype influences cholera vaccine response remains uncertain. Several studies have documented increased immunogenicity or protective efficacy following administration of CVD 103-HgR to individuals with O versus non-O blood groups [11,91–93], though a recent study in North-American adults failed to replicate these findings [94]. Meanwhile, trials of oral-killed whole-cell vaccines have yielded conflicting results when comparing immunogenicity or efficacy according to ABO blood type, including observations of either improved or impaired response in individuals with blood group O (vs non-O) [32,95,96], or no difference according to blood group [97,98]. It is worth noting that regions in which oral cholera vaccine candidates have underperformed (e.g., Thailand and Indonesia) do not differ markedly in blood group O frequency from those in which a robust vaccine response is seen (e.g., North America) [11,99]. Nonetheless, HBGA phenotype (including Lewis and secretor status) should continue to be a consideration during cholera vaccine trials.

Another relevant genetic factor for future consideration is maternal secretor status. This significantly impacts the quantity and quality of oligosaccharides found in breast milk [100], potentially modifying any direct or indirect effects of breastfeeding on oral vaccine efficacy. Ultimately, the HBGA status of a mother–infant pair may play a role in shaping infant microbiota composition [101,102], mucosal immune development and pathogen receptor expression (in breast milk, mucus and on intestinal epithelial cells), thereby influencing both pathogen susceptibility and oral vaccine outcomes. However, further work is required to disentangle these pathways.

Other genetic associations

Genetic risk factors for oral vaccine failure may extend beyond HBGAs. Candidate gene and genome-wide association studies have helped to uncover genetic variants that influence the performance of vaccines targeting measles, rubella and hepatitis B, among others [103,104]. Variants within the HLA locus (encoding the major histocompatibility complex proteins involved in antigen presentation) have typically been among the top associations identified. A recent genome-wide association study among infants in Bangladesh identified several single-nucleotide polymorphisms that may contribute to variation in OPV response, including those associated with histone modification [105]. However, there is no reason to suspect that geographic variation in oral vaccine response is correlated with the distribution of these genetic polymorphisms. Analogous studies are presently lacking for oral vaccines other than OPV.

Enteric infections

Enteric infections have long been recognized as a major cause of morbidity and mortality among young infants, and this burden is greatest in the developing countries where oral vaccines are least immunogenic [2,124]. Enteric infections present at the time of vaccination or in the period leading up to vaccination might impede oral vaccines in several ways. Interference with live-attenuated viral vaccines by viral pathogens may arise via competition for cell entry or receptor binding, or (after entry) for cellular factors required during transcription, replication and assembly – a notion supported by the interference that occurs among the three vaccine poliovirus serotypes when co-administered [12]. Interference might also arise via the induction of nonspecific innate immunity. In particular, type I, II and III interferons induced by viral or bacterial pathogens may inhibit vaccine virus replication and immunogenicity through a number of mechanisms including direct antiviral activity, innate immune cell activation and regulation of adaptive immune responses [125,126]. Other immunomodulatory effects may be pertinent, such as the suppression of Th1 cytokine production by intestinal helminths [127,128], the promotion of mucosal tolerance via the induction of regulatory T cells or the functional reprogramming of innate immune cells that protects against subsequent colonization (trained immunity) [129].

The inhibitory effect of non-polio enteroviruses (NPEVs) on OPV is perhaps the most robust association to be demonstrated between a group of enteric pathogens and an oral vaccine. NPEVs show both geographic and seasonal patterns of incidence that coincide with impaired immunogenicity of OPV [22]. In a systematic review of OPV trials dating back to 1959, the presence of NPEVs at the time of vaccination (detected in stool samples in the absence of symptoms) was consistently associated with a reduction in the odds of seroconversion per dose of OPV, though it remains unclear whether this inhibitory effect varies among enterovirus species [130]. The association between the presence of NPEVs and OPV immunogenicity has since been confirmed by studies in south India [28] and Bangladesh [131] that used sensitive PCR-based methods for the detection of multiple enteric pathogens, including enteroviruses, at the time of OPV administration. In the latter study, enterovirus quantity at 10 weeks of age was also negatively correlated with the immunogenicity and efficacy of Rotarix (administered at 10 and 17 weeks) – an effect that was not observed for any other viral, bacterial or eukaryotic pathogen, or for Sabin polioviruses [131]. Thus, the negative repercussions of enterovirus exposure for oral vaccines may extend beyond OPV.

Given that OPV and oral rotavirus vaccines are regularly co-administered as part of routine immunization programs, the potential interference between these attenuated viruses has garnered considerable interest. Rotavirus replication in vitro is impaired by the presence of an enterovirus, but the converse is not true [132]. Consistent with this, vaccine co-administration does not appear to inhibit the immunogenicity of OPV [133]. The situation for rotavirus vaccines may be more complex – while the first dose of either Rotarix or RotaTeq has generally proven to be less immunogenic when administered with OPV than without, after a full course antibody titers are similar in these groups [134]. Nonetheless, among infants in Bangladesh who received Rotarix at 6 and 10 weeks of age, concomitant OPV administration during at least one of these doses was associated with a reduction in rotavirus seroconversion rate (47%) compared with staggered vaccine administration (≥1 day apart) at both doses (63%) [135]. Thus, it is possible that the continued use of OPV in developing countries (as opposed to the IPV-only schedules generally adopted in high-income countries) contributes to the geographic discrepancies observed in rotavirus vaccine performance.

Aside from the clear inhibitory potential of enteroviruses (either NPEVs or OPV), the relationship between other enteric pathogens and oral vaccine outcome remains equivocal. Concurrent diarrhea has consistently been linked with a reduction in the immunogenicity of OPV [21,130,136]. Several studies have also highlighted the potential inhibitory effect of bacterial or eukaryotic pathogens on OPV, often in the absence of gastrointestinal symptoms [131,137,138]. However, a 3-day course of azithromycin (a broad-spectrum macrolide antibiotic) did not alter the immunogenicity of a subsequent dose of type 3 monovalent OPV among 6–11 month-old infants in India despite reducing the prevalence of bacterial pathogens (including Campylobacter, Salmonella and several pathotypes of E. coli) [28].

Studies concerning the association between enteric infections and oral cholera vaccine outcome have generally focused on specific pathogens. An inhibitory association between Helicobacter pylori colonization and CVD 103-HgR response has been reported among Chilean children <5 years of age, but not in children aged 5–9 years [93]. Notably, this species is rarely documented among children <1 year of age, and is therefore unlikely to influence the immunogenicity of oral vaccines administered in early infancy [28,131]. During a placebo-controlled trial among 6–13 year-old children in Ecuador who were infected with Ascaris lumbricoides, two doses of albendazole (an anthelmintic) prompted a modest improvement in immunogenicity following a subsequent dose of CVD 103-HgR [92], though seroconversion rates in albendazole-treated children (∼30%) still fell considerably short of those observed in North-American adults (>90%) [139]. A subsequent trial in Ecuadorian teenagers revealed that A. lumbricoides inhibits Th1 cytokine responses (including IL-2 and IFN-γ) to CVD 103-HgR, and that albendazole treatment prior to vaccination partly reverses these effects [128]. However, deworming did not yield similar benefits for the immunogenicity of inactivated cholera vaccine among children in Bangladesh or Gabon [140,141].

Maternal helminth exposure may also be pertinent to infant vaccination outcomes. During a recent study in Ecuador in which OPV and Rotarix were administered according to routine schedules, infants exhibited higher plasma IgA levels for rotavirus and poliovirus if their mothers were infected with helminths in the last trimester of pregnancy [142]. Mechanisms underlying these effects are unclear, but may involve the transfer of helminth-induced cytokines (e.g., IL-10) across the placenta or in breast milk.

Pre-vaccination exposure

The transmission intensity of enteric pathogens varies considerably by region. If natural exposure precedes vaccination, this may induce systemic and/or local immunity that impairs vaccine immunogenicity. In the case of rotavirus, exposure to natural infection in the neonatal period is common in settings with a high transmission intensity, and appears to be greatest at times of year associated with impaired vaccine immunogenicity [23]. In a review of Rotarix trials, baseline rotavirus-specific IgA seropositivity (indicative of prior virus exposure) was rare in studies performed in Europe and North America but was observed in up to 26% of participants in higher-burden settings [174]. A reduction in vaccine immunogenicity among infants positive for rotavirus-specific IgA was recently observed during studies of Rotarix conducted in Zambia [23], Bangladesh [135] and South Africa [36], but not in India [175]. Pre-vaccination exposure seems to have a similar effect on the immunogenicity of oral cholera vaccines, including both live-attenuated [11,93,176] and inactivated vaccines [96,177,178]. Together, these findings suggest that the induction of immunity following a previous encounter with a pathogen may contribute to the impaired immunogenicity of oral vaccines in high-transmission settings. Although potentially relevant in the past, pre-vaccination exposure is unlikely to influence OPV outcomes today given that individuals in endemic settings are typically immunized at birth and regularly thereafter (and thus prior to potential wild-type virus exposure).

Post-vaccination exposure

The nature of post-vaccination exposure to the targeted pathogen may also be pertinent to geographic variation in the protective efficacy of oral vaccines. Oral rotavirus vaccines confer homotypic and heterotypic protection against a wide variety of rotavirus genotypes [179,180]. However, as demonstrated in a range of settings, this protection appears to be lower against fully heterotypic strains (i.e., differing from the vaccine at both G and P epitopes) [181,182]. Such exposures may be more frequent in low-income countries, where the circulation of unusual rotavirus strains is more common [183,184].

In addition to strain diversity, variation in transmission intensity may be a relevant factor. Following a trial of Ty21a in Indonesia, the authors suggested that an increase in the infectious inoculum (“more frequent inoculations of greater numbers of bacteria”) in this high-transmission setting may have contributed to the lower efficacy observed in comparison to earlier trials in Chile and Egypt [185]. Similarly, an increase in infectious load following extensive flooding was implicated during a large outbreak of paralytic poliomyelitis within a highly immunized population in South Africa between 1987 and 1988 [186]. These hypotheses rest on the assumption that for a given level of vaccine-induced immunity there can be a ‘breakthrough dose’ that is capable of causing infection. In support of this idea, increasing the viral titer of OPV has been shown to increase the likelihood of infection among previously vaccinated individuals [187,188]. Comparable data for cholera and rotavirus infection are lacking.