Original Investigation 

January 26, 2023

Household Transmission of Influenza A Viruses in 2021-2022

Melissa A. Rolfes, H. Keipp Talbot, Huong Q. McLean, et al

JAMA. Published online January 26, 2023. doi:10.1001/jama.2023.0064

Key Points

Question  What is the risk of influenza A(H3N2) virus infection among household contacts of patients diagnosed with influenza during the 2021-2022 season compared with influenza seasons before the COVID-19 pandemic?

Findings  In this study of household transmission of influenza A(H3N2) viruses, which included 84 households from the 2021-2022 season and 152 households from 2 seasons prior to the COVID-19 pandemic, 50.0% of household contacts were infected during the 2021-2022 season vs 20.1% during prior seasons (adjusted relative risk, 2.31).

Meaning  Among cohorts in 5 US states, there was a significantly increased risk of household transmission of influenza A(H3N2) in 2021-2022 compared with prior seasons.

Abstract

Importance  Influenza virus infections declined globally during the COVID-19 pandemic. Loss of natural immunity from lower rates of influenza infection and documented antigenic changes in circulating viruses may have resulted in increased susceptibility to influenza virus infection during the 2021-2022 influenza season.

Objective  To compare the risk of influenza virus infection among household contacts of patients with influenza during the 2021-2022 influenza season with risk of influenza virus infection among household contacts during influenza seasons before the COVID-19 pandemic in the US.

Design, Setting, and Participants  This prospective study of influenza transmission enrolled households in 2 states before the COVID-19 pandemic (2017-2020) and in 4 US states during the 2021-2022 influenza season. Primary cases were individuals with the earliest laboratory-confirmed influenza A(H3N2) virus infection in a household. Household contacts were people living with the primary cases who self-collected nasal swabs daily for influenza molecular testing and completed symptom diaries daily for 5 to 10 days after enrollment.

Exposures  Household contacts living with a primary case.

Main Outcomes and Measures  Relative risk of laboratory-confirmed influenza A(H3N2) virus infection in household contacts during the 2021-2022 season compared with prepandemic seasons. Risk estimates were adjusted for age, vaccination status, frequency of interaction with the primary case, and household density. Subgroup analyses by age, vaccination status, and frequency of interaction with the primary case were also conducted.

Results  During the prepandemic seasons, 152 primary cases (median age, 13 years; 3.9% Black; 52.0% female) and 353 household contacts (median age, 33 years; 2.8% Black; 54.1% female) were included and during the 2021-2022 influenza season, 84 primary cases (median age, 10 years; 13.1% Black; 52.4% female) and 186 household contacts (median age, 28.5 years; 14.0% Black; 63.4% female) were included in the analysis. During the prepandemic influenza seasons, 20.1% (71/353) of household contacts were infected with influenza A(H3N2) viruses compared with 50.0% (93/186) of household contacts in 2021-2022. The adjusted relative risk of A(H3N2) virus infection in 2021-2022 was 2.31 (95% CI, 1.86-2.86) compared with prepandemic seasons.

Conclusions and Relevance  Among cohorts in 5 US states, there was a significantly increased risk of household transmission of influenza A(H3N2) in 2021-2022 compared with prepandemic seasons. Additional research is needed to understand reasons for this association.