Flu Vaccine Guards Household Contacts of Infected People

TOPLINE:

About one in five people who live in the same household as an individual infected with the influenza virus develop secondary infections within a 7-day follow-up period, with children facing the highest risk. Vaccination lowers the risk of contracting the infection among household contacts.

METHODOLOGY:

• Researchers conducted a prospective cohort study of data between 2017 and 2020 to determine the estimated effectiveness of influenza vaccines in preventing secondary infections in household contacts.
• Overall, 699 people were primary contacts, or the first in a household to get infected (median age, 13 years; 54.5% women); there were 1581 household contacts (median age, 31 years; 52.7% women), and both groups were followed for 7 days.
• Participants collected daily symptom diaries and nasal swabs during the follow-up period.
• Participants also submitted their history of influenza vaccination; 50.1% of household contacts had received a shot at least 14 days before the first case of disease onset in the household.
• The risk for secondary infection and vaccine effectiveness in preventing infection among household contacts was estimated overall and by virus type, subtype, and lineage.

TAKEAWAY:

• Nearly half (48.2%) of primary cases were from children and teens between ages 5 and 17 years.
• Overall, 22% household contacts had laboratory-confirmed influenza during follow-up, of which 7% were asymptomatic.
• The overall risk for secondary infection among unvaccinated household contacts was 18.8%, with the highest risk observed among children younger than age 5 years (29.9%).
• The overall effectiveness of influenza vaccines in preventing laboratory-confirmed infections among household contacts was 21% (95% CI, 1.4%-36.7%).
• The vaccine demonstrated specific protection against influenza B infection (56.4%; 95% CI, 30.1%-72.8%), particularly among those between ages 5 and 17 years.

IN PRACTICE:

“Although complementary preventive strategies to prevent influenza in household settings may be considered, seasonal influenza vaccination is the primary strategy recommended for prevention of influenza illness and its complications,” the authors wrote.

SOURCE:

The study was led by Carlos G. Grijalva, MD, MPH, of Vanderbilt University Medical Center in Nashville, Tennessee, and was published online on November 21, 2024, in JAMA Network Open.

LIMITATIONS:

The recruitment of infected individuals from clinical testing pools may have limited the generalizability of the risk for secondary infection in households in which the primary case had a milder or asymptomatic infection. The study was unable to assess the effectiveness of specific vaccine formulations, such as those receiving high doses. The stratification of estimates by influenza subtypes and lineages was challenging due to small cell sizes.

DISCLOSURES:

This study was supported by grants from the US Centers for Disease Control and Prevention (CDC) and authors reported support from grants from the National Institute Of Allergy And Infectious Diseases. Some authors reported contracts, receiving personal fees and grants from the CDC and various pharmaceutical companies such as Merck and Sanofi.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.