Intrahousehold Transmission of Pandemic (H1N1) 2009 Virus, Victoria, Australia

Caroline van Gemert, Margaret Hellard, Emma S. McBryde, James Fielding, Tim Spelman, Nasra Higgins, Rosemary Lester, Hassan Vally,¹ and Isabel Bergeri
Author affiliations: Burnet Institute, Melbourne, Victoria, Australia (C. van Gemert, M. Hellard, E.S. McBryde, T. Spelman, I. Bergeri); Australian National University, Canberra, Australian Capital Territory, Australia (C. van Gemert, J. Fielding, H. Vally); Monash University, Melbourne (M. Hellard); Victorian Department of Health, Melbourne (E.S. McBryde, J. Fielding, N. Higgins, R. Lester); Royal Melbourne Hospital, Melbourne (E.S. McBryde); University of Melbourne, Melbourne (E.S. McBryde); and Victorian
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Abstract
To examine intrahousehold secondary transmission of pandemic (H1N1) 2009 virus in households in Victoria, Australia, we conducted a retrospective cross-sectional study in late 2009. We randomly selected case-patients reported during May–June 2009 and their household contacts. Information collected included household characteristics, use of prevention and control measures, and signs and symptoms. Secondary cases were defined as influenza-like illness in household contacts within the specified period. Secondary transmission was identified for 18 of 122 susceptible household contacts. To identify independent predictors of secondary transmission, we developed a model. Risk factors were concurrent quarantine with the household index case-patient, and a protective factor was antiviral prophylaxis. These findings show that timely provision of antiviral prophylaxis to household contacts, particularly when household members are concurrently quarantined during implementation of pandemic management strategies, delays or contains community transmission of pandemic (H1N1) 2009 virus.

Households play a major role in secondary transmission of pandemic influenza. Modeling estimates that household transmission has accounted for 25%–40% of all pandemic (H1N1) 2009 cases (1,2). Although understanding the effect of individual-level and household-level factors on secondary transmission of pandemic (H1N1) 2009 is paramount to informing population-level prevention strategies, few studies have evaluated household-level risk factors (3–8).
The Australian Health Management Plan for Pandemic Influenza (AHMPPI), revised in 2008, provides a framework for preparedness and response to pandemic influenza (9). The emergence and magnitude of pandemic (H1N1) 2009 in Melbourne, Australia (10–15), coupled with intensive follow-up and case identification data collected during the delay and contain phases of the AHMPPI (16), presented a unique opportunity to characterize intrahousehold transmission during a period of community transmission. Introduction of a suite of prevention and control measures in accordance with AHMPPI also provided an opportunity to measure the effects of these interventions on pandemic (H1N1) 2009 virus transmission.
We therefore conducted a retrospective cross-sectional study of index case-patients and their household contacts in Melbourne (population >3.5 million), Australia (17). We examined transmission of pandemic (H1N1) 2009 in households, identified possible risk factors for intrahousehold secondary transmission, and assessed the effects of prevention and control measures introduced to limit transmission.
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