Self-reported data from nearly 100,000 people in the UK indicate that COVID-19 cases caused by the Delta variant were 2.4 times more likely to result in long COVID than those caused by Omicron. The research, published online yesterday (June 16) as a letter in The Lancet, claims to be the first large-scale study to examine the risk of developing long COVID from an Omicron infection.

See “Mechanisms of Long COVID Remain Unknown but Data Are Rolling In

The data came from the COVID Symptom Study app, which is run by King’s College London researchers and allows people to report symptoms and test results. In all, the researchers obtained data from 56,003 people who tested positive for COVID-19 during the Omicron wave (defined by the researchers as December 20, 2021–March 9, 2022) and 41,361 who tested positive during the Delta wave (June 1, 2021–November 27, 2021). Participants were considered to have long COVID if they reported symptoms such as fatigue, brain fog, or heart problems more than four weeks after the start of acute COVID-19.

There was not enough data available from unvaccinated individuals or children to include those populations in the study. But among the vaccinated adult app users, who skewed slightly female (55 percent of Omicron cases analyzed and 59 percent of Delta cases), 4.5 percent of Omicron cases resulted in lasting COVID symptoms, while 10.8 percent of Delta cases did. The difference in long COVID risk between the two variants was greatest among people who were vaccinated at least six months prior to infection, with Omicron associated with about one-quarter the long COVID rate of Delta. The findings line up fairly well with those from a UK Office for National Statistics bulletin released May 6, though in that smaller study, the two variants’ risk of long COVID was comparable in boosted individuals.

National Institutes of Health Medical Officer Andrea Lerner, who was not involved in the study, tells NBC News that the new result “is an important piece of data.” However, because researchers did not examine initial or chronic symptom severity, “what it doesn’t tell us is the clinical details about what they’re experiencing or how long they’re experiencing symptoms or effects,” she says.

Some experts also express concern that the public may interpret the findings to underestimate the potential for disability stemming from COVID-19. While a reduced likelihood of long COVID is always good news, Akiko Iwasaki, a Yale School of Medicine immunobiologist who was not involved in the new research, tells NPR that 5 percent of cases resulting in long COVID is still “scary.”

As coauthor and King’s College London researcher Claire Steves explains to the outlet: “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.”