How COVID-19 Upended RSV Season in the United States, 2017–2023

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This report covers RSV seasons through February 2023. Several RSV prevention products that it describes as nearing licensure have since become available.

Respiratory syncytial virus (RSV) is a major cause of hospital care at both ends of life. Each year in the United States it causes an estimated 58,000–80,000 hospitalisations among children younger than 5 and 60,000–160,000 among adults 65 and older. Its epidemics usually follow a seasonal pattern, peaking in December or January. The COVID-19 pandemic threw that pattern off during 2020–2022.

How seasons were measured

CDC analysed PCR test results reported to the National Respiratory and Enteric Virus Surveillance System (NREVSS) from July 2017 to February 2023. Each week, participating clinical and public health laboratories voluntarily report how many RSV PCR tests they ran and how many were positive. The analysis kept to PCR tests, which made up more than 90% of those reported, and to laboratories that reported consistently.

An RSV epidemic was defined as the weeks in which at least 3% of PCR tests were positive:

onset — the first of 2 consecutive weeks at or above 3%;
offset, or end — the last of 2 consecutive weeks at or above 3%;
peak — the week with the highest share of positive tests;
duration — the number of weeks from onset to offset, inclusive.

The 3% threshold was chosen because, applied as data arrive, it captured a high share of each year's RSV detections within epidemics of moderate length. Surveillance years were set by the low points in RSV circulation: early July to late June before the pandemic, and early March to late February for 2021–22 and 2022–23. Seasons were described nationally and for each U.S. Department of Health and Human Services (HHS) region. Alaska, Florida and Hawaii were left out of the regional analysis because their RSV patterns differ from the rest of their regions; Florida is reported on its own, while too few laboratories in Alaska and Hawaii reported consistently to describe them.

Five epidemics, and a missing one

Between July 2017 and February 2023 there were five distinct RSV epidemics: three before the pandemic (2017–18, 2018–19 and 2019–20) and two during it (2021–22 and 2022–23). In the 2020–21 surveillance year there was none by the 3% definition. The number of tests rose substantially during the pandemic.

Line chart of the weekly percentage of PCR tests positive for RSV in the United States, July 2017–February 2023.

Percentage of PCR tests positive for RSV, United States, July 2017–February 2023. Credit: Centers for Disease Control and Prevention.

Season	Onset	Peak	End	Length
Before the pandemic (2017–2020)	October	December	March–April	median 27 weeks
2020–21	no epidemic	—	—	—
2021–22	May, 21 weeks earlier than before the pandemic	July, at 15% positive — similar to earlier seasons	January 2022	33 weeks
2022–23	June	November, at 19% positive — above the 13%–16% of earlier seasons	January	32 weeks

The pattern across the country

Before and during the pandemic, RSV epidemics began earliest in Florida and the Southeast and later further north and west.

Before the pandemic. Florida's seasons began, at the median, in August, peaked in November and ran until March, a median of 30 weeks. In the 10 HHS regions (without Alaska, Florida and Hawaii), median onset ranged from September in Region 4 to December in Region 8; median peaks from November in Region 6 to February in Regions 8 and 9; and median offsets from March in Region 5 to May in Region 7. Epidemics were shortest in Region 10 (median 21 weeks) and longest in Region 4 (median 27 weeks). In 2019–20, as the pandemic began, epidemics ended 2–6 weeks earlier than in the two years before.
2021–22. Across the regions and Florida, epidemics began a median of 20 weeks earlier than before the pandemic (between 13 and 25 weeks earlier), with onsets ranging from March in Florida to August in Region 10. Peaks came earlier too, from July in Region 6 to December in Region 10, and epidemics ended between November (Region 4) and February (Region 9) — around when pre-pandemic peaks usually came. Epidemics ran a median of 6 weeks longer than before, from 21 weeks in Region 2 to 38 weeks in Florida.
2022–23. Florida and Regions 3, 4 and 6 had early onsets, between April and June, but positivity levelled off before climbing again in September. Elsewhere, epidemics began between August and October. Peaks ran from October in Region 4 to November in Regions 2, 8, 9 and 10, and epidemics ended between December and February.

Three maps of RSV epidemic onset by HHS region and in Florida, for the pre-pandemic seasons, 2021–22 and 2022–23.

RSV epidemic onset by HHS region and in Florida, July 2017–February 2023. Credit: Centers for Disease Control and Prevention.

Why it happened, and what next

Measures such as school closures and masking cut the spread of respiratory viruses during the pandemic. After they began in March 2020, the 2019–20 RSV epidemic ended early, and through 2020 RSV circulated at historically low levels. Meanwhile, more and more people who had not been infected built up, and when the measures eased in late spring 2021, RSV returned early and stayed late. The 2022–23 epidemic started later than 2021–22 but still earlier than before the pandemic — a move back towards winter peaks — and its higher peak suggests more intense circulation. The steady south-to-north, east-to-west order of onsets could help predict when epidemics will arrive in particular regions.

Whether the return to the old pattern would continue was uncertain. CDC monitors RSV year-round, including through active, population-based surveillance of RSV hospitalisations and outpatient visits. The authors advised:

clinicians to be aware that out-of-season RSV epidemics might continue, and to test for multiple respiratory pathogens when appropriate;
policy makers to take RSV seasonality into account in timing studies and use of new prevention products — vaccines for older adults, maternal vaccines, and long-acting immunoprophylaxis for infants and children.

Limits

Reporting to NREVSS is voluntary, and laboratories that report consistently may not reflect local or state circulation. Testing differs between regions and changed over time — including more panel testing during the pandemic — which could shift the baseline and the measured onset, offset and duration. There is no standard way to define a season; an earlier U.S. description used a more sensitive method that can only be applied after the fact and gives longer seasons, whereas the 3% threshold works in near real time. And national and regional trends may not match what local data and thresholds would show.

Sources

Based on "Seasonality of Respiratory Syncytial Virus — United States, 2017–2023," by Sarah Hamid and colleagues (CDC; Goldbelt C6), Morbidity and Mortality Weekly Report 72(14), published by the Centers for Disease Control and Prevention; rewritten in hubnx's own words.