The omicron variant of Covid-19 appears less likely than delta to lead to long Covid, according to British researchers.

But because omicron is much more contagious than previous variants, the enormous number of people who have been infected since it began spreading in the winter means there will still be many who are struggling with long-lasting symptoms, such as brain fog, headaches and debilitating fatigue.

The new research, which was published Thursday in The Lancet, is an observational analysis of people who signed up for a smartphone app-led project called the ZOE Covid Study. Users regularly report any Covid symptoms, vaccination status and other demographic information.

Since the app's launch in March 2020, approximately 4.7 million people, most of whom reside in the U.K., have signed on.

"For each successive wave of virus, we've been looking at whether the likelihood of going on to get long Covid is affected by different variants," said Claire Steves, an author of the new study and senior clinical lecturer at King's College London.

Steves and her team found no differences in long Covid prevalence when they compared the original strain of the virus to subsequent variants up through delta.

"We were really keen when omicron came in, especially because it went through our population really, very quickly," Steves said. "We wanted to find out as quick as possible what this meant for long Covid."

The researchers zeroed in on data from 41,361 adults who tested positive for Covid-19 between June 2021 and late November 2021 —when the delta variant was dominant — comparing them to 56,003 adults who tested positive after omicron took hold between December 2021 and March 2022, when more than 70% of cases in the U.K. were estimated to be omicron.

Long Covid is defined by the U.K. team as having new or ongoing symptoms at least one month after initial infection.

Nearly 11% of people who became infected during the delta period met the criteria. When researchers looked at people infected during the omicron wave, the percentage of long Covid patients fell to 4.5%.

All participants had been vaccinated prior to their infection. Some research has suggested that vaccines may offer little protection against long Covid.

When Steves and her team accounted for vaccination status, they continued to see a reduction in long Covid risk during the omicron wave. "That's a robust finding," Steves said.

The research does not, however, provide details on how long people were experiencing long Covid or the severity of the symptoms.

"This is an important piece of data," said Andrea Lerner, a medical officer at the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health. She is affiliated with a major study of long Covid called RECOVER, which aims to recruit at least 18,000 people.

"But what it doesn't tell us is the clinical details about what they're experiencing or how long they're experiencing symptoms or effects," Lerner said.

Rising cases of long Covid

There is no definitive data on how many people may have long Covid. Estimates generally settle on a range of 20% to 30% of all Covid infections.

Even if omicron is less likely to cause long-lasting symptoms, particularly for people who have been vaccinated, the actual number of people that are affected by long Covid isn't going down, Steves said.

"In fact, it's going up," she said.

Download the NBC News app for full coverage of the Covid-19 pandemic

Because the U.K. report tracked infections when the omicron version BA.1 was dominant, it's not clear yet whether the newer subvariants BA.2.12.1 and BA.4 and BA.5, which are gaining a foothold in the U.S., will lead to more or less long Covid symptoms.

Clinics specializing in long Covid in the U.S., which is currently averaging more than 100,000 cases a day, according to NBC News data, are seeing plenty of new patients who have recovered from omicron and are still experiencing a range of symptoms.

"We've seen patients who were infected in December, January and February in our clinic," said Dr. Upinder Singh, a professor of infectious diseases and division chief for infectious diseases at Stanford Medicine in California.

But while the ZOE app can gather user-driven data in real time, a backlog in long Covid appointments makes it difficult for physicians treating long Covid to know whether patients were infected during the delta or omicron waves.

"I don't have the numbers to know whether it's more or less than after delta, to be honest," Singh said. "We're scheduling so far out."

Dr. Greg Vanichkachorn, an occupational medicine specialist at the Mayo Clinic in Rochester, Minnesota, said that his team is seeing long Covid patients who were infected during the omicron wave, as well.

"While the omicron variant may have less risk for long Covid, the sheer number of people with omicron means that there can be a rapid increase in long Covid cases around the world," Vanichkachorn said.

"Long-haul Covid continues to be a public health concern that must be addressed," he said./Agencies