Risk factors of influenza transmission in households

We quantify here the risk factors of influenza transmission in households from a prospective study conducted in the 1999-2000 winter season in France. Two hundred seventy-nine households were enrolled in the study, where a member-the index case-visited his/her general practitioner for influenza-like illness and tested positive for influenza A/H3N2. A standardized daily questionnaire allowed for identification of secondary cases of clinical influenza among the 543 contacts followed up for 15 days. Overall, 131 secondary cases occurred among the 543 household contacts (24.1%). There was an increased risk of clinical influenza in preschool contacts as compared with older contacts. There was also an increased risk in contacts exposed to preschool index cases and school-age index cases as compared with those exposed to adult index cases. No other factor was associated with transmission of the disease. (C) 2004 Elsevier B.V. All rights reserved.