Examining the seasonal shift of RSV

Contemporary PEDS Journal, December 2021, Volume 38, Issue 12

The jury is out on whether testing or exposure better explains the unseasonable surge of respiratory syncytial virus.

In the United States, respiratory syncytial virus (RSV) season typically runs from fall until spring. But, like many aspects of the world of infectious disease, RSV is not behaving in its typical way, and the virus surged this past summer.

Why this happened is up for debate, but the leading theories include both increased testing and decreased exposure to respiratory viruses thanks to months of taking COVID-19 precautions. Whatever its cause, the surge made an unforgettable impact on pediatricians providing direct care.

Steven M. Selbst, MD, attending physician, Pediatric Emergency Medicine, Nemours/Alfred I DuPont Hospital for Children, Wilmington, Delaware and a member of the Contemporary Pediatrics® Editorial Advisory Board said that in his 24 years in emergency medicine, he never saw a busier summer virus season than in 2021—and RSV was a key player. “It was nonstop, all day every day. Every bed in the hospital was full,” recalled Selbst.

More testing may be a factor but is not solely behind the uptick in cases, according to Selbst. “I cannot fully explain why we are seeing so many infants with RSV,” he said. “I’m not convinced we know why this ‘winter virus’ became so common this summer and fall. We were admitting many infants with RSV because they were ill, so I do not believe it is just a matter of more testing.”

When COVID-19 emerged in late 2019, one unexpected benefit was that social distancing and use of face masks helped stem the spread of all kinds of respiratory diseases. The intended target of these interventions was SARS-CoV-2, but they also reduced transmission of other droplet-borne viruses such as influenza and RSV.1

Normally, RSV causes around 58,000 hospitalizations and 100 to 500 deaths in children age younger than 5 years each year, according to the Centers for Disease Control and Prevention (CDC).2 In 2020, however, at the height of the COVID-19 pandemic, positive RSV rates dropped by 97.4% from seasonal averages even though testing rates remained about the same.3

This situation shifted in 2021, as new COVID-19 infections waned in the early months. Many areas of the country relaxed masking and social distancing rules. RSV numbers declined sharply in April 2020, remaining low throughout the next RSV season, but that all changed by the spring of 2021. Cases of RSV started to climb in March, around the time pediatricians would normally see a drop-off.

By June and July 2021, the national average of RSV cases exceeded the peak number for 2019.4 This unseasonal jump was unprecedented but not entirely unexpected. A 2020 report published in PNAS predicted that masking and other COVID-19–prevention measures could lead to a resurgence in viruses like influenza and RSV when precautions were lifted.5 This already had been observed in Australia and other parts of the world in late 2020.

Some experts have suggested that increased testing contributed to the unseasonal spike in RSV cases: Many schools require negative respiratory testing as a COVID-19 safety measure before a child with any respiratory illness returns to school. Similarly, because children were not yet candidates for COVID-19 vaccination in summer 2021, multiplex testing provided the only way to screen for pediatric cases—and the results also revealed RSV cases that otherwise might have been chalked up to a summer cold or other virus. Most facilities use this combined testing, so increased testing for COVID-19 could have led to incidental findings of influenza and RSV.

Meanwhile, because data suggest that testing rates for RSV have remained about the same,3 it appears other factors contributed to the unseasonal surge.

The CDC warned of an increased risk of RSV-type illnesses because of decreased exposure respiratory viruses during the pandemic. The agency recommended that clinicians avoid wearing blinders because of COVID-19 and be on the lookout for other viruses as a cause of respiratory symptoms.6 Rana F. Hamdy, MD, MPH, MSCE, an infectious disease specialist and director of the Antimicrobial Stewardship Program at Children’s National Hospital in Washington, DC, and also a member of the Editorial Advisory Board at Contemporary Pediatrics®, said she agrees with the CDC assessment, suggesting that lower susceptibility to the RSV virus may play a big role.

“Because there was no RSV in the October 2020 to April 2021 season, younger kids who have never been exposed to it before did not get exposed that season. Therefore, there is a larger cohort of RSV-naive children who are now susceptible,” said Hamdy. “And the first-time infection is always be more severe than subsequent infections.”

Only time will tell if the seasonal shift of RSV will stick or is another consequence of the COVID-19 pandemic. Until then, infectious disease experts urge vigilance when it comes to protecting young children and other vulnerable populations against RSV.


1. Agha R, Avner JR. Delayed seasons RSV surge observed during the COVID-19 pandemic. Pediatrics. 2021;148(3):e2021052089. doi:10.1542/peds.2021-052089

2. Respiratory syncytial virus infection (RSV): trends and surveillance. Centers for Disease Control and Prevention. Updated December 18, 2020. Accessed November 4, 2021. https://www.cdc.gov/rsv/research/us-surveillance.html

3. Fox B, Barkley E, Rasmussen D, Teriakidis A. RSV cases dropped by 97% during the pandemic. Epic Health Research Network. June 7, 2021. Accessed November 2, 2021. https://ehrn.org/articles/rsv-cases-dropped-by-97-during-the-pandemic

4. RSV national trends. Centers for Disease Control and Prevention. Updated August 25, 2021. Accessed November 2, 2021. https://www.cdc.gov/surveillance/nrevss/rsv/natl-trend.html

5. Baker RE, Park SW, Yang W, Vecchi GA, Metcalf CJE, Grenfell BT. The impact of COVID-19 nonpharmaceutical interventions on the future dynamics of endemic infections. Proc Natl Acad Sci U S A. 2020;117(48):30547-30553. doi:10.1073/pnas.2013182117

6. Increased interseasonal respiratory syncytial virus (RSV) activity in parts of the southern United States. Centers for Disease Control and Prevention. Updated June 10, 2021. Accessed November 2, 2021. https://emergency.cdc.gov/han/2021/han00443.asp

7. Interim guidance for use of palivizumab prophylaxis to prevent hospitalization from severe respiratory syncytial virus infection during the current atypical interseasonal RSV spread. American Academy of Pediatrics. Updated September 23, 2021. Accessed November 1, 2021. https://www.aap.org/en/pages/2019-novel-coronavirus-covid-19-infections/clinical-guidance/interim-guidance-for-use-of-palivizumab-prophylaxis-to-prevent-hospitalization/