CDC Analysis of RSV Epidemic Provide Insight During Shifting Seasonality


A new report tracks how the epidemic periods of RSV shifted throughout recent years, addressing the disruption in consistency during the 2020-2021 season.

CDC Analysis of RSV Epidemic Provide Insight During Shifting Seasonality

Sarah Hamid, PhD

Credit: CDC

Circulation of the respiratory syncytial virus (RSV) during the 2020-21 season was historically low, and the virus started earlier and continued longer than usual during the 2021-22 season, according to a recent report from the Centers for Disease Control and Prevention (CDC). 1

This year, during the 2022-23 season, RSV circulation started later than in recent years but earlier than the prepandemic seasons, indicating a return to normal seasonality.

The typical timing of RSV epidemics in the US occur between October and April, but the COVID-19 pandemic disrupted this pattern. Infection of RSV causes thousands of hospitalizations every year, mostly among children under the age of 5 and adults 65 years or older.

The report stated the low prevalence of RSV observed during the pandemic was a result of nonpharmaceutical interventions implemented to inhibit the spread of COVID-19, and subsequently, the transmission of other respiratory viruses.

Examining the RSV Epidemic

Investigators from the CDC, led by Sarah Hamid, PhD, Epidemic Intelligence Service, CDC, analyzed the seasonality of RSV in the US from 2017-2023, which consisted of 3 seasons before the COVID-19 pandemic and 2 seasons during the pandemic.

More than 90% of the tests reported to the National Respiratory and Enteric Virus Surveillance System (NREVSS) by participating clinical and public health laboratories were PCR.

The RSV epidemics were identified based on a 3% test positivity threshold. Epidemic onset, offset, peak, and duration were examined for each season at the national level and by US Department of Health and Human Services (HHS) region.

Recent RSV Patterns of Transmission

The analysis showed that during the prepandemic seasons, RSV epidemics began in October, peaked in December, and ended in April. During the pandemic, the typical winter RSV epidemic did not occur in 2020-21.

However, the 2021-22 season onset was observed in May, peaked in July, and ended in January. The 2022-23 season followed, initiating in June and peaking in November, which was later than the 2021-22 season, but earlier than the prepandemic seasons.

“The consistent pattern of RSV circulation starting in Florida and the Southeast and later in regions to the north and west could help predict the timing of future epidemics in specific regions,” investigators wrote. “Monitoring RSV seasonality remains crucial for planning immunoprophylaxis and evaluating new immunization products.”

These findings have important implications for public health practice. Investigators emphasized that ongoing monitoring of RSV seasonality can help healthcare professionals plan for the timing of immunoprophylaxis and the evaluation of new immunization products.

Currently, there is no vaccine for RSV approved by the US Food and Drug Administration (FDA), however, Moderna has investigational mRNA vaccines in development for RSV and influenza.2

According to the report, clinicians should remain vigilant in diagnosing and treating RSV infections, particularly in children and older adults who are at higher risk for severe illness, highlighting the importance of continued monitoring and preparedness for future RSV seasons.


  1. Hamid S, Winn A, Parikh R, et al. Seasonality of Respiratory Syncytial Virus — United States, 2017–2023. MMWR Morb Mortal Wkly Rep 2023;72:355–361. DOI:
  2. Cosdon N. Moderna Gives COVID-19, RSV, Flu Shot Updates on their Vaccines Day. ContagionLive. April 11, 2023.
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