Household transmission of H1N1 may be greater than previously estimated

**Papenburg J. Clin Infect Dis. 2010;51:1033-1041.**

A secondary attack rate of nearly 50% was observed in a prospective observational study of patients with 2009 influenza A (H1N1) and their household contacts.

Further understanding of household transmission of 2009 influenza A (H1N1) can aid in designing measures to curb spread of the infection, according to the researchers. The aim of the current study was to estimate secondary attack rates, the proportion of asymptomatic infections and risk factors for transmission within households based on active clinical follow-up and laboratory-confirmed cases.

The study was conducted from May to July 2009 in Quebec City, Canada.

There were 43 primary case patients and 119 contacts from 42 households in the final analysis. Clinical criteria and lab diagnostic tests — including serological and molecular methods — were used to identify secondary case patients. The mean age of primary case patients was 13 years (range, 0.1 to 46 years), and the mean age of household contacts was 27 years (range, 0.4 to 62 years; P<.001).

Secondary attack

Fifty-three laboratory-confirmed secondary case patients with 2009 influenza A (H1N1) were observed, resulting in a secondary attack rate of 45% (95% CI, 35.6–53.5). One or more confirmed secondary case patients were observed in 81% of the households.

The serial interval between onset of primary and secondary cases had a mean of 3.9 days (median interval, 3 days). Household contacts developed influenza-like illness — defined as fever and cough or sore throat — at a rate of 29% (95% CI, 20.5–36.7).

There were five (9.4%) secondary case patients who were asymptomatic, and there was one patient (1.9%) who reported only gastrointestinal symptoms.

Clinical influenza-like illness developed in 58% of children younger than 7 years and in 19% of adults 18 years or older (P<.01). Laboratory-confirmed influenza-like illness developed in 50% of children younger than 7 years and 19% of adults 18 years or older (P=.03).

Children younger than 7 years were not at a significantly greater risk for acquiring laboratory-confirmed H1N1 than adults 18 years or older (58% vs. 42%; P=.35).

Among household contacts vaccinated for the 2008-2009 season, the secondary attack rate for laboratory-confirmed seasonal influenza was 56%; among unvaccinated contacts, the secondary attack rate was 40% (P=.15).

Subcohort

Among 12 contacts exposed to a subcohort of primary case patients aged 7 to 17 years who had diarrhea and vomiting, 83% developed laboratory-confirmed 2009 influenza A (H1N1). This resulted in an RR of 2.2 (95% CI, 1.5-3.1) compared with contacts exposed to patients who did not have both symptoms. Diarrhea and vomiting were the only symptoms associated with a higher risk for H1N1 transmission, according to the results.

“Household transmission of [pandemic influenza A] H1N1 may be substantially greater than previously estimated, especially in association with clinical presentations that include gastrointestinal complaints,” the researchers concluded. “Approximately 10% of [pandemic influenza A] H1N1 infections acquired in the household may be asymptomatic.”

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