Household Transmission of Influenza A Viruses in 2021-2022

Abstract

ImportanceInfluenza 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.ObjectiveTo 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 ParticipantsThis 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.ExposuresHousehold contacts living with a primary case.Main Outcomes and MeasuresRelative 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.ResultsDuring 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 RelevanceAmong 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.

Increased population susceptibility to seasonal influenza during the COVID‐19 pandemic in China and the United States

“…Apart from the study countries, similar changes have been reported in other regions, such as Australia and Denmark. 12,20 Sudden outbreaks and increased susceptibility are generally associated with genetic predisposition, 21 immune status, 22 age, 23 underlying medical history, 24 viral evolution, 25 environment, 18 and behavior. 26 However, the natural environment, biology of the population, and characteristics of the influenza virus did not undergo any short-term abrupt changes in China and the United States, and therefore, the changed epidemiology is inextricably linked to changes in social behavior patterns after the emergence of COVID-19.…”

“…Furthermore, most previous studies of influenza transmission dynamics were conducted over a short period, and do not include the period after lifting of COVID‐19 PHSMs 5,15–18 . China and the United States implemented PHSMs of different intensities and durations to control COVID‐19, and the effects of differences in COVID‐19 PHSMs on influenza susceptibility in the two countries have not previously been quantified.…”

“…14 Furthermore, most previous studies of influenza transmission dynamics were conducted over a short period, and do not include the period after lifting of COVID-19 PHSMs. 5,[15][16][17][18]…”

Influenza virus shedding and symptoms: Dynamics and implications from a multi-season household transmission study

“…part 56). The study has been described in detail elsewhere [9, 22]; information relevant for the current analysis is summarized below.…”

“…Individual shedding and infection progression can be followed through routine PCR testing of household members and recording of symptoms that arise [7,8]. Such information can be used to characterize patterns of virus shedding from asymptomatic and symptomatic individuals, and to estimate household secondary attack rates and influenza vaccine effectiveness [9][10][11][12][13]. Individual-level data can also be modeled to explore potential differences by age or vaccination status that may impact the effectiveness of isolation [14][15][16][17][18].…”