A significant outbreak of respiratory human adenovirus infections among children aged 3−6 years in Hokkaido, Japan, in 2023

Abstract Human adenovirus (HAdV) infections present diverse clinical manifestations upon infecting individuals, with respiratory infections predominating in children. We surveyed pediatric hospitalizations due to respiratory HAdV infections across 18 hospitals in Hokkaido Prefecture, Japan, from July 2019 to March 2024, recording 473 admissions. While hospitalizations remained below five cases per week from July 2019 to September 2023, a notable surge occurred in late October 2023, with weekly admissions peaking at 15−20 cases from November to December. There were dramatic shifts in the age distribution of hospitalized patients: during 2019−2021, 1‐year‐old infants and children aged 3−6 years represented 51.4%−54.8% and 4.1%−13.3%, respectively; however, in 2023−2024, while 1‐year‐old infants represented 19.0%−20.1%, the proportion of children aged 3−6 years increased to 46.2%−50.0%. Understanding the emergence of significant outbreaks of respiratory HAdV infections and the substantial changes in the age distribution of hospitalized cases necessitates further investigation into the circulating types of HAdV in Hokkaido Prefecture and changes in children's neutralizing antibody titers against HAdV.