Novel Statistical Approach for Identification of Household Secondary Attack Rate During Pandemic H1N1 in Los Angeles County

Background: Household secondary attack rate (SAR) estimates for symptomatic pandemic influenza A (H1N1) 2009 (pH1N1) were lower than expected given the speed of spread within communities worldwide. Methods: Our work demonstrates a novel statistical method accounting for confounding by unobserved immunity and asymptomatic infection. We provide the first SAR for pH1N1 infection estimated without serological confirmation. We estimate age, sex, and antiviral prophylaxis effects for transmission. Results: Ninety-nine symptomatic pH1N1 cases occurred among the 299 members of 58 households during the Spring of 2009. The SAR for symptomatic pH1N1 was 7% (95% confidence interval [CI]: 5%-10%), whereas the SAR for pH1N1 infection was 37% (95% CI: 18-62). Children (<18 years) were 2.5-times (95% CI: 1.2-5.0) more susceptible than adults (≥18 years) to symptomatic pH1N1. The age-group odds ratio for pH1N1 infection was 4.4 (95% CI: 1.4-13.4). Prophylactic antiviral efficacy was 42% for symptomatic pH1N1 and 21% for infection. Conclusions: The SAR for symptomatic pH1N1 is confounded by unobserved immunity and asymptomatic infection. Age is an important determinant of susceptibility. Our novel method extracts additional information from data routinely-collected for outbreak investigations, so we advocate its application in other transmission investigations. Further prospective cohort studies of influenza transmission are needed.