In a recent study posted to the medRxiv* preprint server, researchers assessed respiratory syncytial virus (RSV) transmission after the coronavirus disease 2019 (COVID-19) pandemic within the Netherlands.

Study: Yr-round RSV Transmission within the Netherlands Following the COVID-19 Pandemic – A Prospective Nationwide Observational and Modeling Study. Image Credit: ART-ur/Shutterstock

Background

Before the COVID-19 pandemic, the Netherlands experienced RSV epidemics annually during winter. After the adoption of COVID-19-associated non-pharmaceutical measures (NPIs), RSV was almost absent through the winter of 2020/21. After NPIs were relaxed, diverse patterns of RSV pandemic resurgence were present in several nations. Multiple studies exhibit that the reopening of faculties is linked with a rise in RSV activity.

Nonetheless, little research has been performed to evaluate the effect of upper RSV susceptibility amongst children in addition to adults in consequence of low RSV exposure observed through the COVID-19 pandemic on resurgence patterns.

In regards to the study

In the current study, researchers characterised the peculiar endemic pattern of RSV and the effect of waning immunity on the severity, timing, and duration of the re-emergence of the RSV pandemic.

The team launched a prospective, countrywide monitoring program wherein data are gathered in real-time on hospitalizations of youngsters as a result of RSV in 46 hospitals within the Netherlands. In 10 hospitals having RSV testing as an ordinary of care, clinical patient data were obtained between 3 May 2021 (week 18) and 4 September 2022 (week 35). Data segregated by age were collected between October 2018 and June 2022. All children aged between zero and two years hospitalized as a result of RSV-bronchiolitis at participating hospitals comprised the study population.

Potential variations within the patient age distribution through the pre-COVID-19, post-COVID-19 summer outbreak, and post-COVID-19 endemic periods were evaluated. RSV-positive admissions were manually chosen from patient files by employing diagnosis treatment combination (DBC) codes resembling 3210 for RSV-bronchiolitis, 3208 for lower respiratory tract infection, and 3104 for upper respiratory tract infection. The Mann-Whitney U test was employed to guage whether subgroup differences were statistically significant.

The team simulated RSV transmission between July 2018 and June 2025 to investigate the various parameters that influence the expected trajectories of RSV hospitalizations. Moreover, the estimated trajectories were qualitatively in comparison with the observed re-emergence of the RSV epidemic. The next 4 scenarios were analyzed: (1) moderate level of NPIs together with low virus importation levels, with no waning immunity; (2) moderate NPIs levels together with low virus importation levels and waning immunity; (3) moderate level of NPIs and high virus importation levels, with waning immunity; and (4) strict NPIs with high virus importation levels and waning immunity.

Results

Starting on 24 May 2021 (week 21), an RSV summer outbreak was reported. Through the highest week of RSV-bronchiolitis admissions (19 July, week 29), 240 individuals were admitted. Subsequently, continued RSV transmission was found, with weekly RSV-related admissions settling at 50 patients. Moreover, the team noted that in winters before COVID-19, the variety of patients aged lower than six months was higher.

Through the pre-COVID-19 winter seasons of 2018/2019 and 2019/2020, the median age was 69 days. Through the summer epidemic, the median age rose to 161 days. Through the endemic period between September 2021 and August 2022, the median age reduced to 137 days, even though it remained higher than that noted during pre-COVID-19 winters.

A model that predicted moderate NPIs with low virus importation and diminishing population immunity against RSV as a result of low RSV circulation most closely approximated the Dutch situation, which had a serious summer epidemic followed by persistent RSV transmission. On this case, the variety of RSV hospitalizations amongst children over a 12 months was anticipated to be greater through the summer epidemic than through the winter. Through the subsequent endemic period, this fraction progressively declined.

Alternative cases that assumed RSV immunity wouldn’t wane failed to supply a summer outbreak in 2021 that was more severe than prior winter epidemics. RSV activity in France and Germany followed a model that anticipated strict NPIs, a high virus importation level, and decreasing immunity following a protracted duration of low viral exposure.

Conclusion

The study findings demonstrated a unique pattern of persistent RSV transmission within the Netherlands after an out-of-season RSV outbreak and the relief of COVID-19-related NPIs. Unlike adjoining nations, this pattern could also be partially explained by “immunity debt,” diminishing population immunity following a lengthy period of low RSV exposure. The researchers imagine that constant assessment of RSV seasonality utilizing hospital-based data is important in predicting future RSV epidemics.

*Essential notice

medRxiv publishes preliminary scientific reports that should not peer-reviewed and, due to this fact, mustn’t be thought to be conclusive, guide clinical practice/health-related behavior, or treated as established information.