Long COVID rates fell after omicron, Japan cohort study finds
Hiroshima University researchers found persistent symptoms became less common after omicron, though some adults still reported problems years later.
By Priya Raghavan · Science Reporter
3 min read
Long COVID became less common after the omicron variant emerged, but it did not disappear, according to a Hiroshima University study. The findings matter because a subset of adults continued to report symptoms more than two years after infection, even as the overall burden eased.
The study, published May 8 in PLOS One, followed people in Hiroshima Prefecture who had been diagnosed with COVID-19 between March 2020 and June 2024. Researchers examined how persistent symptoms differed by age and by the period in which people were infected.
Hiroshima University said the research team analyzed an ongoing cohort study developed with medical institutions in the prefecture. The study included 2,689 participants: 1,524 adults and 1,165 children.
Participants completed surveys on 13 post-COVID symptoms and how long they lasted, according to the university. The researchers compared outcomes across five pandemic periods: the original strain, alpha, delta, omicron in 2022 and omicron in 2024.
Long COVID, also called post-COVID-19 syndrome, can include severe fatigue, brain fog, dizziness, and changes in taste or smell, according to Hiroshima University. The condition can occur after either mild or severe initial illness.
Delta period showed the highest adult symptom rate
Adults infected during the delta period had the highest level of lingering symptoms at six months, with an estimated 47% still reporting symptoms, the study found. That share fell to 23% among adults infected during the 2022 omicron period and 21% among those infected during omicron in 2024.
Aya Sugiyama, a lecturer at Hiroshima University’s Graduate School of Biomedical and Health Sciences and the study’s lead author, said in a university statement that the long-term pattern of post-COVID symptoms differed significantly by infection period and age.
Children had lower rates than adults across the pandemic periods studied. Sugiyama said symptom prevalence among children stayed at about one-quarter to one-third of the adult level across all waves.
The study also found no children had disruptions to daily life lasting more than two years after infection, Sugiyama said, even when symptoms continued.
Some symptoms persisted beyond two years
Two years after infection, about 20% of adults infected before omicron and 10% infected during omicron periods still reported symptoms, according to the study. Among children, persistent symptoms were less common: 4.1% after delta-period infection and 1.9% after omicron-2022 infection.
The researchers reported that symptoms lasting beyond two years showed little further improvement during the observed period. Hiroshima University said later improvement remains possible, but the data suggest recovery may level off for some people.
Recovery speed also varied. Symptoms cleared more slowly among people infected during the delta period, while those infected in omicron periods recovered more quickly, according to the study. Younger age was linked to faster recovery, especially among children age 12 and under.
Sugiyama said previous reports had indicated long COVID became less common after omicron, but fewer studies had compared long-term outcomes across several pandemic waves, assessed newer omicron sublineages or followed patients for more than two years.
The Hiroshima University team said it plans to use the cohort to build a model for predicting persistent-symptom risk. Sugiyama said the researchers also aim to release a web-based tool showing symptom trajectories for people with similar characteristics, to support earlier care.
This story draws on original reporting from Medical Xpress.