Resurgence of Respiratory Syncytial Virus in the Summer of 2021 in Denmark—a Large out-of-season Epidemic Affecting Older Children

• Published in: Open forum infectious diseases 2024-03, Vol.11 (3), p.ofae069-ofae069
• Main Authors: Lomholt, Frederikke Kristensen, Emborg, Hanne-Dorthe, Nørgaard, Sarah Kristine, Nielsen, Jens, Munkstrup, Charlotte, Møller, Karina Lauenborg, Krog, Jesper Schak, Trebbien, Ramona, Vestergaard, Lasse Skafte
• Format: Article
• Language: English
• ISSN: 2328-8957; 2328-8957
• DOI: 10.1093/ofid/ofae069

Abstract Background When coronavirus disease 2019 (COVID-19) restrictions were lifted in Denmark in the spring of 2021, a surge in respiratory syncytial virus (RSV) cases followed, causing a large out-of-season epidemic. This study aims to investigate the summer epidemic compared with 3 typical pre-COVID-19 RSV winter seasons using Danish registers to identify RSV cases, RSV-related admissions, and use of intensive care treatment. Methods Incidence rates (IR) per 1000 person-years for RSV cases, RSV-related admissions, and intensive care treatment were calculated with 95% confidence interval (CI) for each season, stratified by age groups and incidence rate ratios (IRR) with 95% CI were calculated to compare the summer epidemic with the winter season for 2019-2020. Results In the summer epidemic, the IR of RSV cases and admissions exceeded previous winter seasons for all age groups. The highest increases in IRs were seen among children aged 2 to 3 years and 4 to 5 years. The IRR of cases were 4.6 (95% CI, 4.1-5.2) and 3.3 (2.6-4.2) and the IRR of admissions were 3.3 (2.7-4.2) and 3.8 (2.3-6.5) in the 2 age groups, respectively, when compared with the winter season 2019-2020. Conclusions Likely because of immunity debt following COVID-19 restrictions, the summer epidemic was significantly larger than previous winter seasons, most markedly among children aged 2 to 3 and 4 to 5 years but had a similar disease severity spectrum.