Risk Factors of Pandemic Influenza A/H1N1 in a Prospective Household Cohort in the General Population

Results From the Copanflu-France Cohort

Rosemary M. Delabre; Nathanael Lapidus; Nicolas Salez; Yohann Mansiaux; Xavier de Lamballerie; Fabrice Carrat


Influenza Resp Viruses. 2015;9(1):43-50. 

In This Article

Abstract and Introduction


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.


Households are useful epidemiological settings to study influenza infection as an estimated 30% of influenza infections are transmitted within the home.[1] Risk factors for infection by the novel influenza A/H1N1 pandemic virus (H1N1pdm09) have been studied in households since its identification in spring 2009, and findings have been summarized in a review.[2] These household studies, however, have largely focused on a limited number of determinants, which mainly rely on sociodemographic characteristics, contact behaviors, or efficacy of prevention measures.[3–7] Furthermore, identified risk factors may not be generalized to the general population as they were largely based on data from case-ascertained studies, in which households including an "index case" are recruited and followed up.

Influenza A/H1N1pdm09 has continued to circulate in conjunction with influenza A/H3N2 and influenza B since the 2009 pandemic. Identification of the determinants of H1N1pdm09 infection, which are not well understood in the context of cocirculation with other seasonal viruses, is important for guiding future public health measures. The CoPanFlu-France cohort, established to study the risk of influenza infection in households in the general population, presents the opportunity to identify the determinants of H1N1pdm09 infection using an large collection of data from both questionnaires and biological samples.[8]

Risk factors associated with high post-epidemic titer following the 2009–2010 pandemic season have previously been reported for this cohort.[9] Here, we present an analysis relying on the first prospective year of study data to identify factors associated with H1N1pdm09 infections over the 2010–2011 season, exploring a large panel of covariates possibly involved in H1N1pdm09 transmissions.