Flu shots reduce spread of infection within households

Influenza vaccination appears to mitigate the risk of secondary infections among unvaccinated household contacts, according to a study.

The study included 699 primary cases (median age 13 years, 54.5 percent female, 79.1 percent non-Hispanic White, 49.1 percent vaccinated) and their 1,581 household contacts (median age 31 years, 52.7 percent female, 81.2 percent non-Hispanic White, 50.1 percent vaccinated). Primary cases were defined as the first household members with laboratory-confirmed influenza.

Nasal swab samples were collected for up to 7 days, and all participants completed daily symptom diaries. The samples were tested using reverse transcription-polymerase chain reaction for influenza infection.

Of the household contacts, 356 (22.5 percent) had laboratory-confirmed influenza during follow-up, with 24 of them being asymptomatic (7 percent). The median serial interval was 3 days.

The overall chain binomial model-based secondary infection risk of influenza among household contacts who were unvaccinated was 18.8 percent (95 percent confidence interval [CI], 15.9–22.0). Children not more than 5 years of age had the highest risk of secondary infection (20.3 percent, 95 percent CI, 16.4–24.9). Secondary infection risk by influenza type was 20.3 percent for influenza A and 15.9 percent for influenza B.

The overall estimated vaccine effectiveness (VE) for preventing secondary infections among unvaccinated household contacts was 21.0 percent (95 percent CI, 1.4–36.7). Looking at influenza type, the estimated VE was 5.0 percent (95 percent CI, −22.3 to 26.3) against influenza A and 56.4 percent (95 percent CI, 30.1–72.8) against influenza B.