The omicron variant is much less likely than delta to cause long COVID, according to the first large-scale study published about the long-term risks posed by omicron.

But almost 5% of people who catch omicron still experience fatigue, brain fog, headaches, heart problems or other health issues at least a month after getting infected, the study found.

While some researchers found the results reassuring, others say the findings are alarming, given that so many people caught omicron and apparently remain at risk even if they're vaccinated.

"That's scary," says Dr. Akiko Iwasaki, an immunobiologist at Yale School of Medicine who studies long COVID but was not involved in the new research.

"People assume that because omicron is milder that, you know, 'Let's just get infected and get it over with,''' Iwasaki says.

The findings, published Thursday in The Lancet, come from researchers at King's College London who have been tracking thousands of people who test positive for the coronavirus to determine the risk of long COVID from different variants.

"The basic question that we're trying to answer is: 'Is long COVID as common ... in the delta period [as it is] in the omicron period?'" says Dr. Claire Steves, who helped conduct the research. "'What's the risk of going on to get long COVID, given the different variants?'"

The researchers compared 56,003 people who caught omicron from Dec. 20, 2021, through March 9, 2022, with 41,361 people who had caught delta between June 1, 2021, and Nov. 27, 2021, and kept track of their symptoms using a special app.

Those who caught omicron were about half as likely as those who got delta to still be experiencing health problems a month later, the researchers found.

"Thankfully, with the omicron variant, the risk of going on to get long COVID is substantially reduced compared to the delta variant," Steves told NPR in an interview. "That's great news, isn't it?"

It's especially good news because omicron is so contagious that it has infected an enormous number of people incredibly quickly. If the risk had been the same as delta or higher, the number of people ending up with long COVID would have exploded.

The findings are consistent with a smaller analysis released recently by the British government.

But lower risk does not mean people shouldn't worry about long COVID because of omicron, Steves and others agree. The chance of getting long COVID from omicron is 4.4%, compared with almost 10.8% from delta, according to the study.

"The caveat is that the omicron variant has spread very rapidly through our populations, and therefore a very much larger number of people have been affected. So the overall absolute number of people who are set to go on to get long COVID, sadly, is set to rise," Steves says. "So it's certainly not a time for us to reduce services for long COVID."

But for any individual person, the findings do indicate that the risk is sharply lower of both getting seriously ill and of developing persistent symptoms.

The study did not address why omicron might pose less of a risk for long COVID. But Steves and others say it makes sense that omicron less frequently leads to persistent symptoms because it doesn't tend to make people as sick as delta.

"Because of that lesser severity of disease, and also because it seems to be a bit more superficial in terms of the disease ... it's less affecting us in terms of severity of our immune response," Steves says. "And therefore that's leading to less likelihood of long COVID."

Other researchers say these findings need to be confirmed by additional research.

"They just looked at anybody who reported any symptoms over this app. They didn't actually evaluate these patients in a clinic anywhere or collect objective data about them," says Dr. Michael Sneller, who studies long COVID at the National Institutes of Health.

But Sneller says it wouldn't surprise him if omicron is less likely to cause long COVID since it does seem to cause less severe illness.

Some researchers say they hope the findings will correct the misconception that people don't have to worry about long COVID from omicron.

"We're saying, you know: 'You can take off your masks in airplanes. You don't need to be vaccinated anymore to enter a restaurant.' All of these policy decisions are going to increase the likelihood that people get infected with COVID, while there's still a 5% chance of severe chronic illness," says Dr. David Putrino, who treats long COVID at Mount Sinai in New York City. "That's short-sighted and going to create a lot of long-term disability that did not need to exist."

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