Co-infection with influenza A/H1N1 and A/H3N2 viruses in a patient with influenza-like illness during the winter/spring of 2008 in Shanghai, China

Co-infection with different influenza viruses occurs naturally and plays an important role in epidemiology and pathogenicity. To monitor the prevalence of influenza viruses in humans during seasonal influenza epidemics in Shanghai, China, and to analyze the genetic characteristics of the viruses, 365 nasopharyngeal swabs collected from patients with influenza-like illness between January and April 2008, were tested by a colloidal gold assay, viral isolation in Madin-Darby canine kidney (MDCK) cells, direct immunofluorescence assay and multiplex RT-PCR. The genetic characteristics of the viruses were analyzed by full-length PCR amplification of the HA segment. One case of co-infection with influenza A/H1N1 and A/H3N2 viruses was detected among the 7 cases of A/H1N1, 84 cases of A/H3N2 and 48 cases of influenza B virus during the winter/spring of 2008. All influenza A/H1N1 and A/H3N2 isolates were similar, including the co-infecting isolates. The present study demonstrates the possibility of natural co-infection with different types of influenza viruses in humans, which could provide the opportunity for the occurrence of viral genetic reassortment within the human respiratory tract.