Changes in Viral Dynamics Following the Legal Relaxation of COVID-19 Mitigation Measures in Japan From Children to Adults: A Single Center Study, 2020-2023

IntroductionRespiratory infections are an ongoing global health challenge. The COVID-19 pandemic triggered global nonpharmacological measures that reshaped public health. In Japan, the shift from legal to individual discretion in pandemic management started on May 8, 2023. However, it still unknown how the relaxation of measures affects respiratory pathogens across age groups.MethodsWe collected 16,946 samples from 13,526 patients between February 2020 and September 2023, analyzing the circulating respiratory pathogen dynamics using FilmArray respiratory panel.ResultsOur analysis revealed significant increases in the positivity rates of respiratory pathogens across multiple age groups after relaxation. The pathogens including adenovirus, Bordetella pertussis, parainfluenza 2 and parainfluenza 4 showed increased positivity predominantly in children aged under 10 years. Conversely, some pathogens including human metapneumovirus, rhinovirus/enterovirus, and respiratory virus (RSV) increased in broad range of age groups. SARS-CoV-2 positivity rates decreased in children under 10 years but increased in those aged over 60 years.DiscussionAge-stratified analysis reveals a dynamic pattern of circulating pathogen in each age group after relaxation measures. This study provides essential epidemiologic data that can guide strategies to protect different age groups and effectively respond to respiratory infections in post-COVID-19 era.