Chronological trend of social contact patterns in Japan after the emergence of COVID-19

Background: The social contact data for Japan as of 2022 showed a substantially decreased number of contacts compared with before the COVID-19 pandemic. However, it is unclear whether social contact continues to be depressed following the end of countermeasures against the pandemic. There is also scarce evidence regarding the influence of influenza-like illnesses (ILIs) on social contacts in Japan. Therefore, this study examined whether the reduction in contact frequency during the pandemic was temporary or persists today and assessed the impact of ILIs on social mixing patterns.

Methods: We conducted online questionnaire surveys of individuals who experienced symptoms of ILIs periodically from 2022 to 2024 to compare the number of contacts per day during and after their illnesses. Contact matrices were obtained from the survey data. The impacts of the timing of the survey and the ILIs were examined using negative binomial regression analysis.

Results: Contact patterns were generally age-assortative, and the average contact numbers gradually increased from March 2022 to June 2024. Most recently, the median number of contacts per day during illness was 3 (interquartile range [IQR] 2-7) and then rose to 4.5 (IQR 2-11) after recovery. The earlier survey and ILIs showed a negative association with the frequency of social contacts.

Conclusion: The frequency of social contacts in Japan tended to rise compared with that in 2021 but has not yet reached pre-pandemic levels. Individuals tended to decrease their contacts when they had symptoms caused by ILIs.