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.
From the aSchool of Public Health, The University of Hong Kong, Pokfulam, Hong Kong Special Administrative Region, China; bPRESTO, Japan Science and Technology Agency, Saitama, Japan; cTheoretical Epidemiology, University of Utrecht, Utrecht, The Netherlands; and dDepartment of Microbiology, The University of Hong Kong, Hong Kong Special Administrative Region, China.
Submitted 22 February 2011; accepted 27 May 2011; posted 30 August 2011.
B.J.C. is currently at School of Public Health, The University of Hong Kong, Pokfulam, Hong Kong Special Administrative Region, China.
Supported by the Harvard Center for Communicable Disease Dynamics from the National Institute of General Medical Sciences (grant no. U54 GM088558), the Research Fund for the Control of Infectious Disease, Food and Health Bureau, Government of the Hong Kong SAR (grant no. PHE-2), and the Area of Excellence Scheme of the Hong Kong University Grants Committee (grant no. AoE/M-12/06). Supported by a Hong Kong PhD Fellowship from the Research Grants Council, Hong Kong (to B.K.), and JST PRESTO program (to H.N.).
B.J.C. has received study funding from MedImmune Inc., a manufacturer of influenza vaccines. The authors report no other conflicts of interest.
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Correspondence: Benjamin J. Cowling, School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Units 624–7, Cyberport 3, Pokfulam, Hong Kong. E-mail: firstname.lastname@example.org.