Pandemic influenza A(H1N1)pdm09 improves vaccination routine in subsequent years: a cohort study from 2009 to 2011

In 2009 the pandemic influenza virus A(H1N1)pdm09 emerged with guidance that people at risk should be vaccinated. It is unclear how this event affected the underlying seasonal vaccination rate in subsequent years.

To investigate the association of pandemic influenza A(H1N1)pdm09 and seasonal flu vaccination status in 2009 with vaccination rates in 2010 and 2011.

Data were collected in 40 Dutch family practices on patients at risk for influenza during 2009-2011; data analysis was conducted in 2012.

A multilevel logistic regression model (n=41,843 patients) adjusted for practice and patient characteristics (age and gender, as well as those patient groups at risk), showed that people who were vaccinated against A(H1N1)pdm09 in 2009 were more likely to have been vaccinated in 2010 (OR 6.02; 95%CI 5.62-6.45, p<.0001). This likelihood was even more for people who were vaccinated against seasonal flu in 2009 (OR 13.83; 95%CI 12.93-14.78, p<.0001). A second analysis on the uptake rate in 2011 (n=39,468 patients) showed that the influence of the vaccination state in 2009 declined after two years, but the diminishing effect was smaller for people vaccinated against A(H1N1)pdm09 than for seasonal flu (OR 5.50; 95%CI 5.13-5.90, p<.0001; OR 10.98; 95%CI 10.26-11.75, p<.0001, respectively).

Being vaccinated against A(H1N1)pdm09 and seasonal influenza in the pandemic year 2009 enhanced the probability of vaccination in the next year and this was still effective in 2011. This suggests that peoples' vaccination routines were not changed by the rumor around the outbreak of A(H1N1)pdm09, but rather confirmed underlying behavior.