Trials of candidate pandemic vaccines against avian influenza A (H5N1) strains had shown that without adjuvant, doses of up to 90 µg hemagglutinin are required to elicit satisfactory immune responses, whereas with the use of adjuvants the dose of antigen required was much lower. To attain satisfactory antibody levels against H5N1, two-dose vaccination schedules were needed, irrespective of adjuvant content.8-10
The hemagglutinin of the influenza virus causing the 2009 pandemic was derived from the 1918 influenza A strain that entered swine around that time.14
The H1N1 strains circulating in humans between 1918 and 1957, and again between 1977 to the present day, have drifted considerably so that seasonal influenza H1N1 strains and the H1N1 pandemic strain in 2009 had little antigenic similarity.14,15
Moreover, prior seasonal influenza vaccination induced little or no antibody cross-reactivity to the 2009 H1N1 pandemic strain.16
It was originally hypothesized that for vaccines against the 2009 H1N1 pandemic virus, similarly to H5N1 and consistent with the two-dose vaccination schedule recommended for naïve young children receiving a seasonal influenza vaccine for the first time, a two-dose vaccination schedule would be needed. This hypothesis turned out to be incorrect, and a finding of our study was that a single vaccination induced robust antibody responses in children as young as three years. In the present study, 94% of children 3–8 y old and 100% of those aged 9–17 seroconverted (HI antibodies) after a single dose of non-adjuvanted vaccine. Other non-adjuvanted pandemic H1N1 2009 vaccines evaluated in children aged 3 y or above indicated similar17
or lower immunogenicity.18-20
In the situations of lower immunogenicity, it is difficult to assess whether the observations reflect true differences in immunogenicity across vaccines from different manufacturers. Other factors, such as the dose of HA actually included in the vaccine or the laboratory and method used for assessing HI responses, may have also impacted the immunogenicity results. Nevertheless, in our study as well as in the others, in children less than 3 y of age, a second dose of the non-adjuvanted H1N1 vaccine was required, as is the case for seasonal vaccination.
Another key observation of this study was the differential adjuvant effect seen in the different age groups. Previous reports have indicated that in adult subjects, adjuvanted and non-adjuvanted vaccines display similar immunogenicity. In one study reported in 2010, 98% of adults 18 to 60 y seroconverted after a single dose of 5.25µg AS03 adjuvanted vaccine and 95% after a single 21 µg dose of a non-adjuvanted vaccine.21
Another study conducted in young adults concluded that adjuvanted and non-adjuvanted vaccines showed similar immunogenicity. In this report, 21 d after a single injection of 7.5 µg HA without adjuvant or 7.5 µg HA with the MF59 adjuvant, respectively 72% and 73% of the subjects seroconverted.22
In contrast, our data indicate that the effect of the adjuvant increases with decreasing age. This was illustrated by the comparison of HI and NT titers yielded by adjuvanted vs. non-adjuvanted vaccines, and by the fact that a single administration of the AF03 adjuvanted vaccine (but not the non-adjuvanted vaccine) induced seroconversion in most of the children less than 3 y of age. Interestingly, another study that compared adjuvanted and non-adjuvanted vaccines from different vaccine manufacturers in children aged 6 mo to 12 y reached a similar conclusion.23
Therefore, it may be concluded that an adjuvant gave a significant advantage for the immune response in young children but that for children over the age of 3 y a non-adjuvanted pandemic H1N1 2009 vaccine would be sufficient.
Immunological priming by exposure to one or several influenza strains with common epitopes provides a potential, partial explanation for the observed response to vaccination that was more typical of the response to a seasonal vaccination than to a pandemic vaccination. We detected a clear age-dependent presence of pre-existing antibodies against the 2009 H1N1 strain, by both HI and NT methods. This was not directly related to recent seasonal vaccination, as approximately one third of 3–8 y olds had been vaccinated with the 2008/09 seasonal vaccine, yet only one child in this age group had detectable cross-reactive antibodies. While it is also possible that some of the individuals without a detectable antibody response at baseline had nevertheless been primed through past exposure, it appears more likely that the magnitude of the immune response to vaccination may also be explained by the inherent high immunogenicity of the H1N1 strain. The fact that high titers and seroprotection rates were reached after vaccination without adjuvant in children from 3 y of age supports the high intrinsic immunogenicity of the H1N1 vaccine strain. While the adjuvant did further increase antibody responses, particularly in young children, the principal advantage of the AF03 adjuvant with the pandemic H1N1 2009 vaccine strain was its ability to provide antigen dose-sparing. The level of dose-sparing achieved in this study with AF03 adjuvanted vaccine appeared similar to that of an AS03 adjuvanted vaccine used in pediatric populations, with an antigen dose as low as 3.75µg in children from 3 y of age and 1.9µg in children less than 3 y inducing seroconversion in more than 95% of the children after first vaccination.24,25
The immunogenicity data appeared somewhat higher than reported for an MF59 adjuvanted vaccine.26
However, due to the lack of immunological assay standardization for influenza, it is difficult to compare the immunogenicity of vaccines in the absence of a head-to-head trial.
There were no safety concerns in either study, with no unexpected trends in adverse events. As expected, the adjuvanted vaccine was more reactogenic than the non-adjuvanted vaccine, and reaction rates were comparable with a previous report of an AF03-adjuvanted H5N1 vaccine candidate.8
These safety data can also be considered as comparable to the results reported for other adjuvanted vaccines used in pediatric populations. In the trial of an AS03 adjuvanted vaccine conducted in children less than 3 y, somewhat higher rates of local and systemic reactions (including fever) were reported than in this study.24
In another trial of an MF59 adjuvanted vaccine in children aged 3 to 8 y, local and systemic reactions appeared somewhat less frequent, even in the groups of subjects that received the full adult dose of adjuvant as in our study.26
Erythema was reported as the most common local symptom after injection while in our study pain occurred more often. Such comparisons, however, have to be taken with caution due to differences across studies in the methods used for monitoring safety and reactogenicity, and because the trials were conducted in different populations, in different countries, which can affect the reporting of symptoms.
In summary, an inactivated monovalent vaccine against the influenza A (H1N1) 2009 strain elicited robust immune responses in children after a single injection, even without adjuvant. In a pandemic setting, a dose-sparing vaccine such as the AF03-adjuvanted 3.8 µg candidate is thought to have public health benefit over a conventional non-adjuvanted vaccine because more people could be immunised from each manufactured batch of vaccine. However, in the 2009 H1N1 pandemic this benefit was not required since the demand for vaccine was less than had been expected. A non-adjuvanted 15µg vaccine, produced and formulated according to the conventional process that has been used to produce seasonal influenza vaccines for several decades has the advantage of being a very well characterized product. Still, the availability of an adjuvanted vaccine provides an advantage for the rapid immunization of young children. However, the amount of adjuvant used in the vaccine should be adjusted to a level that is both effective and minimally reactogenic.