Transmission of pandemic influenza A (H1N1) 2009 within households: Edmonton, Canada.

Background: In April 2009, a novel influenza A, subtype H1N1, now referred to as the Pandemic (H1N1) 2009 virus (pH1N1), began circulating in countries around the world. Describing the transmission characteristics of this novel influenza A virus is important to predict current, and future, disease spread. The Public Health response to the first wave of pH1N1 in Alberta focused on the identification and management of secondary cases within households.

Objectives: The purpose of this study was to describe transmission characteristics of pH1N1 in households in Edmonton, the capital city of Alberta, during the first wave, and to identify the serial interval and secondary attack rate (SAR) in this setting.

Study Design: This is a prospective analysis of pH1N1 household transmission within 87 urban Canadian households between April 30 and June 9, 2009; with each household having at least one laboratory-confirmed case. The secondary attack rate was calculated in the 262 household contacts using a weighted average by number of susceptible individuals in each household. The serial interval, or time to illness in secondary cases, was analyzed using survival analysis with a Weibull model.

Results: Within the 87 households, 42 (48.3%) had no secondary cases develop; 25 (28.7%) had one secondary case develop; and 20 (22.9%) had more than one secondary case develop. The secondary attack rate did not decrease with increasing household size and households with two members exhibited the lowest secondary attack rate at 14.1%. Children under the age of 19, and individuals with an underlying medical condition, were at increased risk (p<0.05) of becoming a secondary case. The overall secondary attack rate among the 262 susceptible household contacts was 30.2% (95% CI: 12.6-52.2). The median serial interval for pH1N1 transmission was 3.4 days (95% CI: 2.9-3.9).

Conclusions: The identified transmission characteristics of pH1N1 among Canadian households differ slightly from other previously reported North American estimates, but are in keeping with historical transmission rates of pandemic influenza viruses.