Background: The household secondary attack proportion (SAP) is commonly used to measure the transmissibility of an infectious disease.
Methods: We analyzed the final outbreak size distributions of pandemic A(H1N1), seasonal A(H1N1), and A(H3N2) infections identified in paired sera collected from members of 117 Hong Kong households in April and in August–October 2009.
Results: The estimated community probability of infection overall was higher for children than adults; the probability was similar for pandemic A(H1N1) and seasonal A(H3N2) influenza. The household SAP for pandemic A(H1N1) was higher in children than in adults, whereas for seasonal A(H3N2), it was similar in children and adults. The estimated SAPs were similar for seasonal A(H3N2) and pandemic A(H1N1) after excluding persons with higher baseline antibody titers from analysis.
Conclusions: Pandemic and seasonal influenza A viruses had similar age-specific transmissibility in a cohort of initially uninfected households, after adjustment for baseline immunity.