Repeat infections with 2009 A/H1N1 influenza during three inflenza seasons, 2008/09 to 2010/11

Background There is limited literature on the occurrence of influenza repeat infections – re-infection with the same subtype of influenza within a period of 12 months. Objectives We examined the occurrence of influenza A/H1N1 re-infections among rt-PCR positive cases reported to the Pennsylvania Department of Health (PA DOH) during three influenza seasons, 2008/09 to 2010/11.

Methods All lab positive influenza cases reported to the PA DOH between October 1, 2008 to October 2011 were examined for repeat infections. Only r-PCR based tests were included in the final analysis.

Results A total of 103, 210 lab positive flu cases were reported during the study period. Of these, 128 (0.1%) individuals tested positive for influenza on two different occasions within a 12 month period. A majority (102, 80%)) of these individuals tested positive for influenza B followed by influenza type A. The majority of second infections occurred within 8 months (median 236 days, range 103 to 346). Twenty six (26) individuals tested positive for the same type of influenza twice. However, only 4 of these were confirmed by rt-PCR and they all involved 2009A/H1N1. A majority (115/87%) of repeat infections involved children <20 years old. None of these subsequent infections resulted in death or hospitalizations.

Conclusions Although rarely documented in our statewide surveillance, a small number of repeat influenza infections were identified. Most were caused by different influenza types, and occurred predominantly in children and teenagers.