Risk factors of pandemic influenza A/H1N1 in a prospective household cohort in the general population: results from the CoPanFlu-France cohort

BACKGROUND: The CoPanFlu-France household cohort was set up in 2009 to identify risk factors of infection by the pandemic A/H1N1 (H1N1pdm09) virus in the general population. OBJECTIVES: To investigate the determinants of infection during the 2010–2011 season, the first complete influenza season of study follow-up for this cohort. PATIENTS/METHODS: Pre- and post-epidemic blood samples were collected for all subjects, and nasal swabs were obtained in all subjects from households where an influenza-like illness was reported. Cases were defined as either a fourfold increase in the serological titer or a laboratory-confirmed H1N1pdm09 on a nasal swab, with either RT-PCR or multiplex PCR. Risk factors for H1N1pdm09 infections were explored, without any pre-specified hypothesis, among 167 individual, collective and environmental covariates via generalized estimating equations modeling. We adopted a multimodel selection procedure to control for model selection uncertainty. RESULTS: This analysis is based on a sample size of 1121 subjects. The final multivariable model identified one risk factor (history of asthma, OR = 2·17; 95% CI: 1·02–4·62) and three protective factors: pre-epidemic serological titer (OR = 0·51 per doubling of the titer; 95% CI: 0·39–0·67), green tea consumption a minimum of two times a week (OR = 0·39; 95% CI: 0·18–0·84), and proportion of subjects in the household always covering their mouth while coughing/sneezing (OR = 0·93 per 10% increase; 95% CI: 0·86–1·00). CONCLUSION: This exploratory study provides further support of previously reported risk factors and highlights the importance of collective protective behaviors in the household. Further analyses will be conducted to explore these findings.