Owen Simwale, MPH1

Background

Infection with an influenza strain confers immunity to that strain for 6 to 12 months. Therefore, re-infections with the same strain of influenza within a short period of time are rare. There is limited literature on factors associated with such infections, particularly during the 2009 A/H1N1 pandemic.

Objectives

We examined the occurrence of and factors associated with human re-infections of influenza viruses during the 2009/10 and 2010/11 flu seasons.

Methods

Data on all rt-PCR positive influenza viruses subtyped in Pennsylvania between April 2009 and December 2011 were examined for repeat infections (detection of 2009/H1N1 in the same person within 12 months). Only rt-PCR based tests were included in the analysis.

Results

There were 50,106 lab positive flu cases reported to the PA Department of Health between April 2009 and December 2010. Of these, 6,809 (14%) cases were rt-PCR positive influenza. Eight of the rt-PCR positive cases had repeat infections of the 2009 pH1N1 within 12 months. Three cases with repeat infections had underlying medical conditions none involving immunosuppression. Repeat infections were more common in unvaccinated adults and in people with underlying medical conditions. Initial infections were more severe than secondary infections.

Conclusions

Although influenza repeat infections are rare, we have shown that they occur when influenza subtypes co-circulate at high levels. Repeat infections in vaccinated individuals suggest that other host, viral or environmental factors may limit host immunity from past infections. Further analysis of repeat infections may help elucidate the role of human and viral factors in influenza infection.