This study examines safety and immunogenicity in various 2-dose regimens of TIV and LAIV influenza vaccines in 56 children 12–35 months of age. There were 3 reasons we undertook the study. First, LAIV appears to be more efficacious in children than inactivated vaccine; however, wheezing noted after LAIV in children <24 months of age suggests that inactivated vaccines might be a safer alternative for the initial immunization dose [2
]. Therefore, children <24 months old might benefit most from TIV priming followed by LAIV boosting, but this heterologous prime/boosting strategy has not been evaluated previously in this age group. Second, patient preference and/or vaccine availability often leads to some children receiving a mixed schedule of LAIV and TIV; the current study investigated the safety and immunogenicity of such an approach. Third, we were interested in assessing the cellular immune responses induced by the 2 types of vaccines in young children to explore the possibility that cellular immune responses might be an important factor involved in the enhanced protection in children associated with LAIV vaccination.
We observed no safety issues in this small trial; however, larger trials are needed to confirm that the TIV-LAIV combination vaccine strategy in children <24 months of age is safe and effective. In addition, children receiving TIV priming followed by LAIV boosting developed humoral and cellular immune responses comparable to the responses detected in the other homologous or heterologous prime/boosted groups. Furthermore, LAIV priming provided significant protection against LAIV shedding when LAIV was given as the booster vaccine. Although TIV priming showed a trend of protection against LAIV shedding, these results did not achieve statistical significance. These results are consistent with those of clinical trials demonstrating that LAIV is more efficacious than TIV in children, providing further support that LAIV should be included in prime/boost regimens given to children. Large safety studies will be required to confirm this schedule in children 6–23 months old. A phase 3 comparison of TIV priming followed by LAIV boosting with homologous TIV/TIV prime/boosting would be important to perform in children <24 months of age.
The 4 vaccine regimens tested here induced similar humoral immune responses directed against H1N1, H3N2, and influenza B seasonal viral strains. These results indicate that all combinations of TIV and LAIV vaccinations should be effective against well-matched seasonal influenza strains. In addition, our results suggest that all combinations of TIV/LAIV prime/boosting vaccine schedules are likely to be safe and successful in children >6 months of age whenever there is a good match between HA expressed by the vaccines and circulating infectious viral strains. Previously, LAIV was demonstrated to induce cross-reactive antibody against a drift variant [4
]—a response that likely contributes to improved protection [2
In contrast, only LAIV vaccination was shown to induce influenza-specific T-cell responses relevant for cell-mediated immune protection. These results are consistent with those of previous studies indicating that LAIV induces better T-cell responses than TIV in children >5 years old and adults [10
], but they are unique in showing that LAIV but not TIV induced CD4+
, and γδTCR+
T-cell responses capable of both proliferation and effector function in children 6–35 months of age. LAIV induced vigorous cellular immune responses regardless of whether it was given before or after TIV; however, subjects who received only TIV did not develop detectable memory T-cell responses in peripheral blood. These unique effects of LAIV are likely to be important for the high efficacy of a single dose of LAIV as shown in clinical studies [6
]. In addition, these enhanced cellular immune responses may help to explain why LAIV was shown in a head-to-head trial in children 6–59 months of age to be significantly more protective than TIV against culture-confirmed influenza; LAIV was 55% more protective for both antigenically well-matched and drifted viruses [2
]. Studies in elderly individuals also have shown that LAIV induces better heterosubtypic immunity than TIV, in terms of both humoral and cellular immune responses [15
]. Furthermore, many animal studies have indicated that influenza-specific T cells can provide broadly heterosubtypic protective immunity [16
]. Current data also indicate that T-cell responses can provide heterosubtypic influenza-specific protective immunity [22
]. We now demonstrate that LAIV induces conventional αβ T-cell responses that are reactive with highly conserved influenza-specific peptide pools relevant for universal influenza heterosubtypic protective immunity. Furthermore, these T cells that are reactive with highly conserved influenza-specific peptides can inhibit viral replication in human cells (). All of these results indicate that LAIV may be able to induce better protection against antigenically drifted influenza strains and even major pandemic strains.
αβ T cells can provide helper effects for immunity and direct inhibitory effects on viral replication. CD4+
αβ T cells capable of recognizing highly conserved influenza epitopes are relevant for broadly heterosubtypic protective immunity. If T cells are specific for conserved influenza peptides presented by common HLA alleles, they could be broadly protective in highly diverse human populations. We have recently shown that γδ T cells induced by BCG vaccination [26
] and/or live viral vectors (vaccinia [27
] and canarypox [28
]) develop memory responses [26
], undergo antigen-specific focusing [29
], and can potently inhibit intracellular pathogen replication [26
]. One report of γδ T cells having inhibitory effects on influenza viral replication in human monocytes has been published, although the antigen specificity of these inhibitory effects was unclear [31
]. The induction of γδ T cells by LAIV also may provide additional protective effects against influenza.
Influenza-specific T-cell responses induced by LAIV were correlated with each other but not with humoral immune responses (). Both influenza-specific humoral and cellular immune responses were induced in subjects with detectable LAIV shedding after the priming vaccination. It is expected that LAIV replication is important for immunogenicity and also the induction of protective immunity. In addition, our combined findings that influenza-specific T-cell and B-cell responses did not correlate, but both could be induced by prolonged LAIV shedding and are likely to be important for protective immunity, may explain differences in vaccinated persons with similar protective resistance against influenza but variable influenza-specific HAI antibody titers.
In summary, our results indicate that combinations of TIV and/or LAIV are safe and effective in inducing protective antibody responses against matching seasonal strains of influenza. Despite these similarities, striking differences in cellular immune responses were seen. Only LAIV induced T-cell responses that are potentially important for protection against both matching seasonal and heterosubtypically diverse strains of influenza. TIV priming followed by LAIV boosting may be the best prime/boost regimen for use in children <24 months old. Larger clinical trials are needed to confirm the safety and efficacy of this approach. Furthermore, LAIV vaccination may induce more broadly heterosubtypic influenza-specific protective immune responses.