Secondary transmission of pandemic Influenza A(H1N1) 2009 in families during May-June 2009 - Victoria, Australia

Background: The aim of this study was to describe transmission of Pandemic Influenza A(H1N1) 2009 in Victorian families, and to identify risk factors for intra-family secondary transmission.

Methods: We conducted a retrospective cross-sectional study of randomly-selected cases and their family contacts notified to the Victorian Department of Health during the Contain Phase of the Australian Health Management Plan for Pandemic Influenza (22 May-3 June 2009). Eligibility criteria included living in households of >2 related people in selected municipalities in Victoria. Information collected included household characteristics, use of prevention and control measures, and symptoms. Secondary cases were defined as family contacts who met the case definition for influenza like illness within the specified time period. We calculated the secondary attack rates (SAR) and developed unadjusted and adjusted logistic analysis models to investigate exposures associated with secondary transmission.

Results: We randomly selected 72 cases, of which 21 (29.2%) were not contactable, 12 (23.5%) declined participation and 3 (4.2%) were not eligible. Thirty-six cases and 131 family contacts were interviewed. Eighteen family contacts met the case definition of a secondary case, with an overall SAR of 13.7% (95% CI: 8.4%-20.8%). In adjusted analysis, increased odds of secondary transmission were associated with concurrent quarantine with the family index case (OR 1.2; 95% CI: 1.0-1.5), and reduced odds were associated with use of antiviral prophylaxis (OR 0.05, 95% CI: 0.0-0.4).

Discussion Our findings provide evidence for the effectiveness of antiviral prophylaxis to prevent secondary transmission, yet suggest that quarantine increases risk of secondary transmission within families. Pandemic management plans need to ensure antiviral prophylaxis of family contacts and quarantine strategies are implemented in tandem to reduce likelihood of secondary transmission within families.

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Author (s): 

Caroline van Gemert, I Bergeri, J Fielding, N Higgins, R Lester, H Vally, E McBryde, M Hellard

Presenter (s): 
Ms. Caroline van Gemert
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