Open Access Highly Accessed Research article

Assessing secondary attack rates among household contacts at the beginning of the influenza A (H1N1) pandemic in Ontario, Canada, April-June 2009: A prospective, observational study

Rachel Savage1*, Michael Whelan2, Ian Johnson111, Elizabeth Rea3, Marie LaFreniere1, Laura C Rosella111, Freda Lam4, Tina Badiani2, Anne-Luise Winter1, Deborah J Carr5, Crystal Frenette6, Maureen Horn6, Kathleen Dooling6, Monali Varia6, Anne-Marie Holt7, Vidya Sunil7, Catherine Grift8, Eleanor Paget8, Michael King9, John Barbaro10 and Natasha S Crowcroft111

Author Affiliations

1 Ontario Agency for Health Protection and Promotion, Toronto, Ontario, Canada

2 Ontario Ministry of Health and Long-Term Care, Toronto, Ontario, Canada

3 Toronto Public Health, Toronto, Ontario, Canada

4 York Region Health Department, Newmarket, Ontario, Canada

5 Oxford County Public Health and Emergency Services, Woodstock, Ontario, Canada

6 Peel Public Health, Mississauga, Ontario, Canada

7 Haliburton, Kawartha, Pine Ridge District Health Unit, Port Hope, Ontario, Canada

8 Middlesex-London Health Unit, London, Ontario, Canada

9 Sudbury & District Health Unit, Sudbury, Ontario, Canada

10 Simcoe Muskoka District Health Unit, Barrie, Ontario, Canada

11 Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada

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BMC Public Health 2011, 11:234  doi:10.1186/1471-2458-11-234

Published: 14 April 2011



Understanding transmission dynamics of the pandemic influenza A (H1N1) virus in various exposure settings and determining whether transmissibility differed from seasonal influenza viruses was a priority for decision making on mitigation strategies at the beginning of the pandemic. The objective of this study was to estimate household secondary attack rates for pandemic influenza in a susceptible population where control measures had yet to be implemented.


All Ontario local health units were invited to participate; seven health units volunteered. For all laboratory-confirmed cases reported between April 24 and June 18, 2009, participating health units performed contact tracing to detect secondary cases among household contacts. In total, 87 cases and 266 household contacts were included in this study. Secondary cases were defined as any household member with new onset of acute respiratory illness (fever or two or more respiratory symptoms) or influenza-like illness (fever plus one additional respiratory symptom). Attack rates were estimated using both case definitions.


Secondary attack rates were estimated at 10.3% (95% CI 6.8-14.7) for secondary cases with influenza-like illness and 20.2% (95% CI 15.4-25.6) for secondary cases with acute respiratory illness. For both case definitions, attack rates were significantly higher in children under 16 years than adults (25.4% and 42.4% compared to 7.6% and 17.2%). The median time between symptom onset in the primary case and the secondary case was estimated at 3.0 days.


Secondary attack rates for pandemic influenza A (H1N1) were comparable to seasonal influenza estimates suggesting similarities in transmission. High secondary attack rates in children provide additional support for increased susceptibility to infection.