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

Frederikke Kristensen Lomholt; Hanne-Dorthe Emborg; Sarah Kristine Nørgaard; Jens Nielsen; Charlotte Munkstrup; Karina Lauenborg Møller; Jesper Schak Krog; Ramona Trebbien; Lasse Skafte Vestergaard

doi:10.1093/ofid/ofae069

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

Open Forum Infect Dis. 2024 Feb 5;11(3):ofae069. doi: 10.1093/ofid/ofae069. eCollection 2024 Mar.

Authors

Frederikke Kristensen Lomholt¹, Hanne-Dorthe Emborg¹, Sarah Kristine Nørgaard¹, Jens Nielsen¹, Charlotte Munkstrup¹, Karina Lauenborg Møller², Jesper Schak Krog³, Ramona Trebbien³, Lasse Skafte Vestergaard¹

Affiliations

¹ Department of Infections Disease Epidemiology and Prevention, Statens Serum Institut, Copenhagen, Denmark.
² Division of Infectious Disease Preparedness, Statens Serum Institut, Copenhagen, Denmark.
³ Department of Virus and Microbiological Special Diagnostics, Statens Serum Institut, Copenhagen, Denmark.

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.

Keywords: epidemiology; out-of-season epidemic; register-based study; respiratory syncytial virus (RSV); surveillance.