Intense co-circulation of non-influenza respiratory viruses during the first wave of pandemic influenza pH1N1/2009: a cohort study in Reunion Island.

OBJECTIVE: The aim of the present study was to weigh up, at the community level, the respective roles played by pandemic Influenza (pH1N1) virus and co-circulating human Non-Influenza Respiratory Viruses (NIRVs) during the first wave of the 2009 pH1N1 pandemic. METHODS: A population-based prospective cohort study was conducted in Reunion Island during the austral winter 2009 (weeks 30-44) that allowed identification of 125 households with at least one member who developed symptoms of Influenza-like illness (ILI). Three consecutive nasal swabs were collected from each household member (443 individuals) on day 0, 3 and 8 post-ILI report and tested for pH1N1 and 15 NIRVs by RT-PCR. RESULTS: Two successive waves of viral infections were identified: a first wave (W33-37) when pH1N1 was dominant and co-circulated with NIRVs, sharply interrupted by a second wave (W38-44), almost exclusively composed of NIRVs, mainly human Rhinoviruses (hRV) and Coronaviruses (hCoV). Data suggest that some interference may occur between NIRVs and pH1N1 when they co-circulate within the same household, where NIRVs were more likely to infect pH1N1 negative individuals than pH1N1 positive peers (relative risk: 3.13, 95% CI: 1.80-5.46, P