Molecular Characterization of Human Adenoviruses Before and After a Severe HAdV‐7 Military Outbreak in Finland in 2024

Background In 2024, a severe outbreak caused by human adenovirus type 7d (HAdV‐7) occurred in Finnish military garrisons. Molecular typing was implemented to support outbreak control. The aim of this study was to determine adenovirus types circulating in Finnish garrisons during 2013–2023 by retrospective surveillance and to review earlier findings to better understand factors contributing to the 2024 outbreak. Methods Adenovirus‐positive respiratory samples collected through routine surveillance (2013–2023) and enhanced surveillance (2024–2025) were typed using type‐specific PCR assays targeting types 3, 4, 7, 11, 14, 16, and 21. Whole‐genome sequencing was performed on 11 selected samples. Index case interviews were conducted among conscripts infected with HAdV‐7 in late 2023. Results During 2013–2023, among 497 typed samples, HAdV‐4 was most common ( n = 446; 89.7%), followed by HAdV‐3 ( n = 25; 5%). Other types detected were HAdV‐21 ( n = 11; 2.2%), HAdV‐7 ( n = 6; 1.2%), and HAdV‐11 ( n = 1; 0.2%). HAdV‐4 was predominant in 2013–2017 and HAdV‐3 in 2018–2019. Between February 2024 and June 2025, 1692 adenovirus‐positive respiratory samples were typed. The adenovirus types detected were HAdV‐4 ( n = 1133; 67%), HAdV‐7 ( n = 440; 26%), co‐detection of HAdV‐4 and ‐7 ( n = 70; 4.1%), and HAdV‐14 ( n = 1; 0.1%). Epidemiological investigation and whole‐genome sequencing data revealed that the adenovirus associated with the 2024 outbreak (HAdV‐7d) was already circulating in Finnish garrisons in 2023. Conclusion The severity of the 2024 outbreak may reflect limited prior circulation of HAdV‐7 in Finland. Although HAdV‐7 continued to circulate among conscripts in 2025, the severity of the epidemic subsided, possibly due to effective preventative measures guided by active surveillance.