RESUMO
BACKGROUND: Quantitative knowledge of the transmissibility of influenza is crucial to its prevention and control. OBJECTIVES: To quantify the transmission of influenza A (H1N1) and seasonal influenza in household contacts of patients with influenza diagnosed in a large university hospital. PATIENTS/METHODS: A prospective study was conducted between September and October 2009 in which all confirmed cases of influenza diagnosed at King Khalid University Hospital were included. All household contacts were followed by telephone calls every other day for 12 days. They were asked about the development of influenza symptoms in addition to their age and nationality. RESULTS: Overall, 432 household contacts of 69 influenza A (H1N1) cases and 417 contacts of 91 seasonal influenza cases were included. Suspected influenza was diagnosed in 16·9% and 14·4% of household contacts of H1N1 and seasonal influenza patients, respectively. Household reproduction numbers were 1·06 (0·84-1·28) for H1N1 and 0·66 (0·51-0·81) for seasonal influenza. Children in households were more susceptible than were adults (22·2% versus 13·7%, respectively). Evidence of coughing in the index case tripled the risk of infection in households afflicted with the H1N1 influenza [relative risk (RR) = 3·28, CI = 1·24-8·69], while evidence of a runny nose doubled it (RR = 1·89, CI = 1·19-2·92). CONCLUSIONS: Communicability of influenza in households in Riyadh is comparable to that in other countries. Children are more susceptible to influenza infection. The presence of a cough or runny nose in the index cases increases the risk of infection.