# Household Transmission of COVID-19 and Influenza During the Early Omicron Era: An Insurance Claims Database Study in Japan

## TL;DR

This study examines how COVID-19 and influenza spread within households in Japan, finding that children are key drivers of transmission.

## Contribution

The study quantifies transmission rates of COVID-19 and influenza within households, emphasizing children's role in spreading both diseases.

## Key findings

- Children under 12 years old had the highest transmission rates for both COVID-19 and influenza.
- Transmission from and to children was disproportionately high for both diseases.
- Adults showed increased transmission rates for COVID-19, suggesting a need for targeted interventions.

## Abstract

Recurrent outbreaks of coronavirus disease 2019 (COVID-19) in Japan highlight the continued need for effective prevention, particularly household transmission. This study clarifies the epidemiological profile of household transmission of COVID-19 and influenza by quantifying transmission rates according to demographic and relational characteristics. An anonymized health insurance claims database was used to identify index patients diagnosed with COVID-19 or influenza between November 2021 and August 2023. Transmission events were defined as cases in which household members were diagnosed with the same infection in the week following the index patient’s diagnosis (Day 1). Analyses were stratified by the age of the index patients and age-based transmission patterns between index patients and household members. A total of 1,001,509 index patients with COVID-19 and 207,090 with influenza met the inclusion criteria. Transmission rates were highest among index patients aged <12 years (COVID-19, 40.17%; influenza, 33.51%). Transmission from children was disproportionately high, while age-combination analysis demonstrated elevated transmission both from and to children for both diseases. COVID-19 exhibited increased transmission among adults. As the primary drivers of household transmission for both COVID-19 and influenza, preventive strategies should be tailored to children. COVID-19 interventions must also address transmission among adults to strengthen household-level infection control.