The COVID-19 pandemic in the United States has created a change in the timing and intensity of re-emergent respiratory syncytial virus (RSV) epidemics, depending on the duration of mitigation measures and the extent of virus introduction from other regions. A simulation modeling study was conducted to explore a range of potential scenarios and to better understand the factors linked to RSV epidemics. Results of the analysis were published in JAMA Open Network.

The investigators sought to examine the association of various factors, such as mitigation strategies, duration of maternal-derived immunity, and the importance of external infections, with the dynamics of re-emergent RSV epidemics. They also evaluated how the age distribution of RSV infections and hospitalizations for the disease might change in the coming years.

RSV-specific hospitalization data from New York (2005 to 2014) and California (2003 to 2011) were obtained from the State Inpatient Databases of the Healthcare Cost and Utilization Project maintained by the Agency for Healthcare Resource and Quality. Comprehensive databases included all hospital discharge records from community hospitals in the participating states. The month of the hospitalization and age of the patient were included in all datasets.

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The simulation models were modified in order to predict the trajectory of RSV epidemics between 2020 and 2025 using different scenarios with fluctuating stringency of mitigation measures for patients with SARS-CoV-2 infections. Factors that might impact the re-emergence of RSV epidemics included introduction of the virus from out-of-state sources and decreased rates of transplacentally acquired immunity in infants. Further, models that used parameters fitted to similar inpatient datasets from the states of Florida and Colorado were utilized to characterize these relationships in populations with year-round RSV circulation and biennial RSV epidemics, respectively.

The primary study outcome was the estimated number of monthly RSV hospitalizations in the total population. Secondary study outcomes were the incidence of any RSV infection, age distribution of hospitalizations among children less than 5 years of age, and the incidence of RSV lower respiratory tract infections (LRTIs). Percentage changes in incidence were calculated by comparing the differences between the estimated incidence in the 2021 to 2022 RSV season with and without changes in the transmission rate associated with mitigation measures and introduction of the virus from external sources.

There were 5 simulation scenarios used: (1) constant low level of transmission; (2) sudden decrease plus gradual increase in transmission; (3) decrease in nonhousehold contacts; (4) importance of external infections; and (5) decrease in the curation of protective maternal immunity. The first 3 scenarios were related to the stringency of mitigation measures, whereas the last 2 scenarios covered factors associated with the re-emergence of RSV epidemics.

A simulated population of 19.45 million individuals were included in the current study. With the use of scenario 4, when virus introduction was derived from external sources, a rise in RSV hospitalizations was anticipated in the spring and summer of 2021. Higher rates of introductions of the virus from other locations were related to more intense spring and summer epidemics, which were expected to be followed by earlier-than-usual winter epidemics.

Among children 1 year of age, the estimated incidence of RSV hospitalizations was 707 per 100,000 children per year in the 2021 and the 2022 RSV seasons, compared with 355 per 100,000 children in a typical RSV season (ie, without a coexistent COVID-19 pandemic). Infants between 3 to 5 months of age, however, were anticipated to continue having the largest absolute incidence of LRTIs (ie, 30,075 LRTIs per 100,000 infants per year).

Additionally, infants less than 3 months of age were expected to continue to exhibit the largest incidence of RSV hospitalizations (ie, 3116 hospitalizations per 100,000 infants per year). With no virus importation, the risk for RSV infections across all age groups in the winters of 2021 and 2022 would be higher, because more susceptible individuals were spared from infections because of the lack of summer epidemics.

The researchers concluded that the findings from the present study suggest that pediatric departments need to be alert to large RSV outbreaks in the coming seasons, with the intensity of these epidemics based on the size of the spring and summer epidemics in that location. They recommend enhanced surveillance for both the administration of prophylaxis and the management of hospital capacity.

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


Zheng Z, Pitzer VE, Shapiro ED, Bont LJ, Weinberger DM. Estimation of the timing and intensity of reemergence of respiratory syncytial virus following the COVID-19 pandemic in the US. JAMA Netw Open. Published online December 1, 2021. doi:10.1001/jamanetworkopen.2021.41779