This study demonstrates that a single dose of a monovalent MF59-adjuvanted influenza virus vaccine with influenza A virus (H1N1) 2009 produced an antibody response in 346 of 435 persons (79.5%). In addition, it was shown that a second vaccination had little or no additional effect on the antibody titers in persons under 50 years of age. However, significant increases in the proportion of persons with protective level HI titers were observed in older persons following booster vaccination. Finally, a statistically significant correlation was observed between increasing age and more rapid decline in HI titer over time.
The response to the first dose of the pandemic influenza virus vaccine was sufficient to fulfill the European licensure criteria for immunogenicity of influenza virus vaccines, in line with results of previous studies (7
). The advice to provide an additional second vaccine dose was a matter of debate in our country and elsewhere but was recommended based on the concern that risk groups might have a less favorable response to a single vaccine dose, as had been described for seasonal influenza virus vaccines (14
). Indeed, we found a clear age-specific effect in response to vaccination, despite the presence of a strong adjuvant.
We found that age at time of vaccination itself had a negative effect on the HI titer. In the literature, conflicting results of the effect of age on the immune response to influenza virus vaccine have been described (3
). In a quantitative review, Goodwin et al. described that the antibody response in the elderly (>65 years) is considerably lower than that in younger adults (12
). In contrast, Remarque et al. described that lower HI titers were not caused by age or aging of the immune system but probably by differences in priming histories and concomitant diseases (26
Our data indicate that this may be the case, as responses were lower and persisted for shorter times in persons with a history of seasonal influenza virus vaccination prior to pandemic influenza virus vaccination. This effect has repeatedly been observed in clinical and animal studies on trivalent seasonal influenza virus vaccines (5
). The mechanism behind it remains to be discovered, but it is tempting to speculate about the observation of Bodewes et al., who found a T cell-mediated protective effect of natural influenza virus infection to subsequent infection with heterologous strains in ferrets (4
). This heterosubtypic immunity supposedly is suppressed by regular vaccination, thus influencing the impact of a subsequent infection with an antigenically distinct influenza virus strain (5
Studies during the pandemic remained inconclusive, but some suggested increased severity of pandemic influenza in persons with prior seasonal influenza virus exposures. Skowronski et al. reported an increased risk of medically attended influenza virus H1N1 (2009) illness after prior vaccination with trivalent seasonal influenza virus vaccine 2008-2009 (29
), but these findings have never been corroborated by others. Obviously, an alternative explanation may be that such observations are confounded by the fact that vaccination is often limited to older persons and persons with comorbidities, who therefore possibly have impaired immune functions (6
In any case, in our study, the clear increase in proportion of persons over 50 years with an adequate response to vaccination following a booster vaccination showed that the decision to provide a 2-dose schedule was advantageous for this age group. Moreover, our data suggest that immune responses after vaccination need to be evaluated more carefully, taking vaccination history into account.
A limitation of our study was that vaccination of the health care workers in our hospitals commenced in week 46 of 2009, which coincided with the pandemic influenza peak in the Netherlands. However, in this study, we found baseline protective antibodies (HI titers of ≥40) in only 4.4% of the study population, compared with 4 to 31% found in other studies, which suggests that the majority of study participants had not encountered pandemic influenza before vaccination (13
). In addition, if people were naturally infected during the study period, we expected those effects to be randomly spread over the groups.
Previous studies reported the presence of some level of cross-reactive antibodies in persons born before 1957. Such cross-reactive antibodies may be present in these persons because the H1N1 subtype viruses before 1957 were more similar to the H1N1 (2009) than the H1N1 viruses that reemerged in the 1970s. We detected no difference in preexisting baseline titers to the 2009 H1N1 strain between individuals born before or after 1957, and no stronger immunologic response in HI titer was seen in this group (15
). This might be due to the small number of persons born before 1957 in this study.
A larger proportion of persons who were vaccinated annually than those who were unvaccinated had preexisting antibodies to the 2009 H1N1 strain. This finding resembles those of other studies, and the most likely explanation is that seasonal influenza virus vaccination induced antibodies that cross-react with 2009 H1N1 virus to a certain extent (15
Contrary to other studies, we did not find a difference in antibody response between men and women. Data from clinical trials of seasonal influenza virus vaccines reveal pronounced differences between antibody responses in men and women. HI titers are consistently higher in women than men, suggesting that women may be better protected against influenza disease (16
). Although the mechanisms underlying sex-specific differences in immune development are poorly understood, women have higher absolute CD4+
lymphocyte count and production of TH1 cytokines after immunization, as well as more sustained responses to antigenic challenge (9
). The absence of this difference in antibody response between men and women may be due to the use of MF59 as an adjuvant in the vaccine, which is known to enhance the immune response (2
In conclusion, in our population the first vaccination with a monovalent MF59-adjuvanted influenza virus H1N1 (2009) vaccine resulted in a titer of ≥40 in 79.5% of the participants, and 71.9% of the participants had persisting antibodies for 6 months. Overall, a second dose of the vaccine conferred little additional benefit, but in some age groups (50 to 60 years) a significant increase of HI titer was seen. Increasing age had an unfavorable effect on the postvaccination HI antibody titers of the participants. For each additional year of age, a 2.4% reduction in GMT was found.
There was a reduced HI antibody response, although not significant, in adults who annually received seasonal influenza virus vaccination compared with that in adults who never or occasionally received seasonal influenza virus vaccination, suggesting a possible unfavorable effect on immunogenicity for adults who annually received seasonal influenza virus vaccination. The mechanism underlying this negative effect of previous vaccination is unclear and remains to be elucidated.