Statistical Analysis

Analyses were restricted to households affected by influenza A(H3N2) viruses, to contacts who provided nasal swab specimens on at least 4 days, and to household contacts with complete data for covariates of interest. Within a household, the person(s) with influenza virus infection who had the first illness onset was considered the primary case; individuals could be co-primary cases if they shared the same illness onset date with another person in the household. Only contacts from households with a single primary case were included in analyses. Analyses focused on people who were household contacts of the primary case. Characteristics of participants were compared across study periods, using 2-sided Fisher exact tests where P values were reported (P values <.05 were considered significant).

The risk of laboratory-confirmed influenza A(H3N2) virus infection among household contacts was compared by study period (2021-2022 vs prepandemic) using modified Poisson regression and generalized estimating equations to account for household clustering. Estimated relative risks of infection were adjusted for household density, age of the contact, current season influenza vaccination in the contact, and time spent in the same room as the primary case the day before enrollment.

Planned subgroup analyses were conducted by fitting 3 adjusted models with an interaction between study period and age of the household contact, influenza vaccination status of the household contact, or interaction with the primary case (defined by the amount of time the household contact spent in the same room as the primary case after the primary case’s illness onset). Because some sites were not represented across all study periods, a prespecified sensitivity analysis was performed using data from Tennessee, which enrolled participants in both study periods. Additional sensitivity analyses were conducted by modeling household infection risk by year (with 2018-2019 season as the reference). Analyses were conducted using R (version 4.0.4).¹⁰