Atypical respiratory syncytial virus epidemics may continue, and testing patients for multiple respiratory pathogens when indicated might be necessary.
The respiratory syncytial virus (RSV) season started later in 2023 compared to the 2021 and 2022 seasons but still earlier than pre-pandemic seasons, suggesting a return toward pre-pandemic seasonality, according to a CDC report.
RSV infections cause an estimated 58,000 to 80,000 hospitalizations among children younger than 5 years of age each year, and between 60,000 and 160,000 hospitalizations among adults aged 65 years or older. These epidemics typically follow seasonal patterns, peaking in December or January.
The COVID-19 pandemic significantly disrupted the usual seasonality of RSV in the United States; however, with historically low circulation during the 2020 and 2021 season, and earlier and longer circulation during the 2021 and 2022 season. Monitoring these shifts in seasonality can help guide decisions around immunoprophylaxis and new immunization products, according to the report.
To define RSV seasonality in the United States before and during the COVID-19 pandemic, investigators analyzed polymerase chain reaction (PCR) test results between July 2017 and February 2023. Seasonal RSV endemics were defined as the weeks during which the percentage of PCR test results positive for RSV was 3% or greater.
Nationally, investigators found that pre-pandemic seasons of RSV between 2017 and 2020 began in October, peaked in December, and ended in April. During 2020 and 2021, however, that typical epidemic did not occur.
Interestingly, the 2021 and 2022 season began in May, peaked in July, and ended in January. Additionally, the 2022 and 2023 season started in June and peaked in November—later than the 2021 and 2022 season, but still markedly earlier than pre-pandemic seasons. In both pre-pandemic and pandemic periods, RSV epidemics began earlier in Florida and the Southeast, and later in regions further north and west.
Despite the earlier initiation and longer duration of the 2021 and 2022 epidemic season (33 weeks compared to a median of 27 weeks between 2017 and 2020), researchers did note that the peak percentage of RSV-positive PCR results (15%) was comparable with that during pre-pandemic seasons. During the 2022 and 2023 surveillance year, the proportion of positive PCR results peaked in November and the peak was higher (19%) than that during pre-pandemic seasons.
During the 2020 and 2021 RSV season, disruption of the usual circulation period was largely attributed to non-pharmaceutical interventions as a result of the COVID-19 pandemic, such as school closures and masking. This disruption also led to an accumulation of people susceptible to RSV, resulting in large epidemics and atypical seasonality.
Despite these findings, the investigators said clinicians should be aware that atypical RSV epidemics may continue, and testing patients for multiple respiratory pathogens when indicated might be necessary. With new vaccines and prevention products progressing through the FDA approval process—including vaccines for older adults, maternal vaccines, and long-acting RSV immunoprophylaxis for infants and children—the investigators also said policymakers should consider RSV seasonality when making recommendations about the administration of new immunizations.
Importantly, the investigators did note several key limitations. The report is based on voluntary reporting from the National Respiratory and Enteric Virus Surveillance System, and regional differences in testing and diagnostics could have affected the baseline percentage of positive test results and trends. Additionally, there is no standard method for characterizing seasonality, and seasonal attributes vary based on the method used. Finally, the analysis describes regional and national trends, and locally available data may better reflect circulation patterns.
Hamid S, Winn A, Parikh R, Jones J, et al. Seasonality of Respiratory Syncytial Virus—United States, 2017-2023. Centers for Disease Control and Prevention. April 7, 2023. Accessed April 14, 2023. https://www.cdc.gov/mmwr/volumes/72/wr/mm7214a1.htm?s_cid=mm7214a1_w