Title: Double Strike - Co-infections with influenza viruses during the 2009/10 and 2010/11 influenza seasons in Pennsylvania

Background 2009 A/H1N1 and seasonal influenza viruses co-circulated during the 2009/10 and 2010/11 influenza seasons. Objectives We examined factors associated with influenza co-infections among rt-PCR flu cases reported in Pennsylvania during the 2009/10 and 2010/11 influenza seasons Methods Influenza data from the Pennsylvania Department of Health for the 2009/10 flu season were mined to identify rt-PCR influenza co-infections. Then hospital chart abstractions were done on all co-infected individual to see if recent vaccination history, compromised immunity or underlying medical conditions were associated with co-infection or severe illness.

Results 50106 lab positive flu cases were reported to the PA Department of Health between October 2008 and January 2011. Of these, 6809 (14%) cases were r-PCR positive influenza of which137 (2%) were dual infections of two influenza viruses. 130 co-detections were between influenza A and B viruses while 5 were between A/H3N2 and 2009A/H1N1. Co infections were commonly reported among recent flumist recipients and during the January-March period when high levels of flu A and B co circulated.

Conclusions

Although influenza co-infections are rare, we have shown that about 2% of individuals with influenza may be infected with two subtypes virues when several flu subtypes co circulate. This co-circulation may pose a risk for further reassortment of influenza viruses, which could result in novel or virulent strain or antiviral resistant influenza strains. Testing for multiple flu viruses at the same time may enhance influenza treatment and control efforts.