Waning immunity to the respiratory syncytial virus (RSV) due to the COVID-19-pandemic may have resulted in a unique pediatric RSV transmission pattern in the Netherlands, where an initial summer 2021 pediatric RSV outbreak was followed by year-round transmission, according to study findings published in the Journal of Infectious Diseases.

Dutch researchers sought to assess the endemic pattern of RSV in children between May 2021 and August 2022 and the effects of waning population immunity on the re-emergent RSV epidemics, with use of prospective surveillance data from the Netherlands and simulation models.

Data were obtained from the prospective SPREAD study in which real-time data are collected on RSV-related pediatric hospitalizations from 46 hospitals in the Netherlands.

Study authors analyzed potential differences in the age distribution of children hospitalized for RSV during the pre-COVID-19 period (2018/2019 and 2019/2020 winter season [October through April]), the COVID-19 summer outbreak (May to August 2021), and the COVID-19 endemic phase (September 2021 to August 2022).

We showed year-round RSV transmission in the Netherlands after an initial 2021 summer outbreak. The pattern was unprecedented and distinct from neighboring countries.

A summer outbreak of RSV occurred beginning on May 24, 2021 (week 21). In the peak week of July 19 (week 29), 240 children were admitted with RSV-bronchiolitis. Afterward, continuous RSV transmission was observed, and RSV-related pediatric admissions stabilized at about 50 patients per week.

Age-stratified data of 1,269 pediatric patients less than 2 years of age admitted with RSV infection were available for 2018 to 2022 in 10 hospitals. The proportion of children less than 6 months of age with RSV was greater in the pre-COVID-19 winter seasons, with a median age in the seasons 2018/2019 and 2019/2020 of 69 days old (interquartile range [IQR], 34-175).

Participants’ median age increased to 161 days old in the summer outbreak (IQR, 55-364; P <.001). In the endemic phase of September 2021 to August 2022, the median age decreased again to 132 days old (IQR, 54-253), and it was still greater, compared with the pre-COVID winter seasons (P <.001).

A sensitivity analysis excluded pediatric patients from the only academic hospital, which included a large number of patients (20% pre-COVID-19 vs 8% in the summer outbreak and 8.5% in the endemic phase). No changes were observed in the differences among groups.

A simulation created by the researchers that included moderate nonpharmaceutical interventions (NPIs), low virus importation, and waning population immunity against RSV due to low RSV circulation resulted in a model comparable to the situation occurring in the Netherlands, in which a large summer outbreak was followed by continuous RSV transmission. In this simulation, the proportion of RSV hospitalizations among children older than 1-year of age was anticipated to be increased in the summer outbreak, compared with a typical winter season. This proportion was reduced over time in the subsequent endemic phase.

Other simulations that assumed no waning RSV immunity did not produce a summer outbreak in 2021 that was more intense, compared with previous winter epidemics.

Study limitations include the potential for underestimation of RSV-related hospitalizations, nonstandardized data collection between hospitals, and the short observation period.

“We showed year-round RSV transmission in the Netherlands after an initial 2021 summer outbreak. The pattern was unprecedented and distinct from neighboring countries,” said the study authors. “Our results suggest that the observed continuous RSV transmission pattern could be associated with waning immunity due to the period of very low RSV circulation during the COVID-19 pandemic,” the researchers concluded.

Disclosure: Some of the study authors declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of authors’ disclosures.

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