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Why the Respiratory Disease RSV Is Having an Off-Season Surge

But this summer, RSV cases are spiking, particularly in southern states. Around 2,000 confirmed cases were recorded across the U.S. during the week of July 10, 2021, compared to less than a dozen during July 25, 2020. The actual number of infections is likely higher since clinicians may not test sick children for RSV outside its regular season, the U.S. Centers for Disease Control and Prevention (CDC) said in a recent advisory.
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The spike is somewhat logical, even if the timing is unusual. When the pandemic hit, sending people inside and behind masks, respiratory illnesses like RSV circulated at “historically low levels,” the CDC said in a report published today. Now that people are easing up on COVID-19 precautions, they are also coming into contact with pathogens that existed but weren’t spreading much throughout the pandemic. RSV infections began to tick upward in April 2021, the CDC says.

In the U.S., RSV case counts are “incredibly high for the summer,” Antoon says, “but it’s about on par with what we see in the winter.” That suggests COVID-19 prevention delayed the regular RSV season. A similar RSV spike happened during Australia and South Africa’s summer seasons.

But what’s harder to explain is why RSV is circulating widely while some other respiratory viruses, like influenza, aren’t. (Though infection rates for parainfluenza, which causes croup in children, are also rising right now, he notes.)

RSV is quite transmissible, more so than some other viruses. But one reason for the surge may be that children who typically wouldn’t be susceptible to RSV are vulnerable this year.

Human immunity builds up over time. You’re likely to have the worst reaction to a pathogen the first time you see it. After that, your body knows what it’s up against and is better at fighting it off.

Typically, the CDC says, almost all children catch RSV in their first two years of life. But babies who were born during or shortly before the pandemic may not have encountered RSV as they usually would have, meaning they’re extra susceptible to it now.

“These viruses don’t disappear in the summer; they’re just much, much lower in frequency,” explains Dr. Richard Malley, a senior physician in pediatrics at Boston Children’s Hospital. Coming off a year when few children got RSV during its regular season, infections may spike “at times when they would normally not be present, presumably because a little bit of the immunity in the whole community was not reinforced by exposure,” he says.

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