Estimation of the Timing and Intensity of Reemergence of Respiratory Syncytial Virus Following the COVID-19 Pandemic in the US

Abstract

Importance: Respiratory syncytial virus (RSV) is a leading cause of hospitalizations in young children. RSV largely disappeared in 2020 owing to precautions taken because of the COVID-19 pandemic. Estimating the timing and intensity of the reemergence of RSV and the age groups affected is crucial for planning for the administration of prophylactic antibodies and anticipating hospital capacity.

Objective: To examine the association of different factors, including mitigation strategies, duration of maternal-derived immunity, and importation of external infections, with the dynamics of reemergent RSV epidemics.

Design, setting, and participants: This simulation modeling study used mathematical models to reproduce the annual epidemics of RSV before the COVID-19 pandemic in New York and California. These models were modified to project the trajectory of RSV epidemics from 2020 to 2025 under different scenarios with varying stringency of mitigation measures for SARS-CoV-2. Simulations also evaluated factors likely to affect the reemergence of RSV epidemics, including introduction of the virus from out-of-state sources and decreased transplacentally acquired immunity in infants. Models using parameters fitted to similar inpatient data sets from Colorado and Florida were used to illustrate these associations in populations with biennial RSV epidemics and year-round RSV circulation, respectively. Statistical analysis was performed from February to October 2021.

Main outcomes and measures: The primary outcome of this study was defined as the estimated number of RSV hospitalizations each month in the entire population. Secondary outcomes included the age distribution of hospitalizations among children less than 5 years of age, incidence of any RSV infection, and incidence of RSV lower respiratory tract infection.

Results: Among a simulated population of 19.45 million people, virus introduction from external sources was associated with the emergence of the spring and summer epidemic in 2021. There was a tradeoff between the intensity of the spring and summer epidemic in 2021 and the intensity of the epidemic in the subsequent winter. Among children 1 year of age, the estimated incidence of RSV hospitalizations was 707 per 100 000 children per year in the 2021 and 2022 RSV season, compared with 355 per 100 000 children per year in a typical RSV season.

Conclusions and relevance: This simulation modeling study found that virus introduction from external sources was associated with the spring and summer epidemics in 2021. These findings suggest that pediatric departments should be alert to large RSV outbreaks in the coming seasons, the intensity of which could depend on the size of the spring and summer epidemic in that location. Enhanced surveillance is recommended for both prophylaxis administration and hospital capacity management.

Figure 1.. Association of Importation of External Infections and Respiratory Syncytial Virus (RSV) Hospitalizations in New York, 2019–2025

The dotted blue line corresponds to the counterfactual scenario that seasonal RSV epidemics were not interrupted. The solid lines represent a range of background importations of external infections. The gray shaded area indicates absence of external infections related to travel restrictions beginning in April 2020 and the absence of RSV activity in other regions of the US until February 2021. We assumed the viral importation rate increased linearly back to prepandemic levels over 3 months. In this scenario, we assumed RSV transmission within the population dropped to 80% of prepandemic levels in March 2020 and gradually increased back to prepandemic levels over a 13-month period.

Figure 2.. The Mean Age of RSV Hospitalizations Among Children Aged Younger Than 5 Years

The background color represents the incidence of RSV hospitalization per 100 000 people per month in each age group in each month. Darker red colors indicate a higher incidence. The black line and values indicate the mean age of hospitalization (in years) varies with time.

Figure 3.. Age Distribution of Respiratory Syncytial Virus (RSV) Infections, Lower Respiratory Infections (LRIs), and Hospitalizations, 2021-2022 RSV Season

The figures show a comparison of RSV infections (A), RSV LRIs (B), and RSV hospitalizations (C) during a typical season (counterfactual incidence of RSV cases during the 2021-2022 RSV season if there was no COVID-19 pandemic and no mitigation measures in place) and the expected season (expected incidence of RSV cases during the 2021-2022 RSV season under the assumption that substantial virus importation was disrupted by mitigation measures between April 2020 and February 2021).

Figure 4.. The Association of Mitigation Measures and RSV Hospitalizations Without External Infections, New York, 2019–2025

All scenarios in this figure assumed virus circulated locally without external sources of viral importation, both before pandemic and during pandemic. The dotted blue line shows the counterfactual scenario that there is no COVID-19 pandemic and no mitigation measures in place. The solid lines show 3 scenarios of stringency of mitigation measures. Scenario 1 was characterized by constant low level of transmission (80% prepandemic level) from March 2020 to March 2021. Scenario 2 was characterized by sudden 20% decrease in RSV transmission in March 2020 followed by a linear increase back to prepandemic levels. Scenario 3 was characterized by an 82% decrease in nonhousehold contacts and 10% increase in household contacts between April 1 and July 1, 2020. The solid red rectangle on the top indicates a constant strict mitigation measure lasting for 13 months, from March 2020 to March 2021. The gradient red rectangle in the middle indicates a gradually relaxed mitigation measure that lasts for the same period of time (March 2020 to March 2021). The blue rectangle on the bottom indicates a 3-month stay-at-home order starting at the end of March 2020.