Long COVID symptoms have declined since COVID-19 Omicron but can persist

A long-term Hiroshima University cohort study has found that post-COVID-19 disease symptoms became less common after the arrival of Omicron but a proportion of adults has continued to report fatigue, brain fog and other health problems two years after infection

Six years after the world first learned of coronavirus disease 2019 (COVID-19), the pandemic has receded into memory for many people. For others, infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has left lasting health problems.

A long-term study led by researchers at Hiroshima University, Japan, has found that persistent symptoms after COVID-19 became substantially less common after the Omicron variant emerged but did not disappear. The findings showed that some adults continued to experience health problems for years after infection, even though the overall burden of post-COVID illness declined after the early pandemic waves.

Recovery from COVID-19 does not always end when the period of the acute infection clears. Some people develop symptoms that persist or can recur for months, regardless of whether their original illness was mild, moderate or severe. The condition, commonly known as long COVID or post-COVID-19 syndrome, can include fatigue, cognitive impairment, dizziness, breathlessness, disrupted taste or smell, sleep disturbance and reduced ability to carry out everyday activities.

Although Omicron infections have generally carried a lower risk of severe acute illness than COVID-19’s earlier variants, uncertainty has remained about their long-term effects. The Hiroshima study compared symptom patterns across successive pandemic periods – including later Omicron sublineages – with follow-up extending beyond two years.

“Although reports suggested that long COVID became less common after Omicron emerged, few studies had compared long-term outcomes across multiple pandemic waves, examined more recent Omicron sublineages, or followed patients for more than two years after infection,” said Dr. Aya Sugiyama, of Hiroshima University’s Graduate School of Biomedical and Health Sciences and lead author of the study.

The researchers expanded upon a cohort study launched in 2020 with medical institutions across the Hiroshima Prefecture. They followed 2,689 people diagnosed with COVID-19 between March 2020 and June 2024, including 1,524 adults and 1,165 children.

Participants completed surveys that recorded 13 post-COVID symptoms and their duration. The researchers compared recovery across the five epidemic periods, from the original strain, Alpha, Delta, Omicron-2022 to Omicron-2024.

“We found that the long-term course of post-COVID-19 symptoms varies significantly depending on the period of infection and age,” Sugiyama said.

Adults infected during the Delta period had the highest rate of persistent symptoms six months after infection. The researchers estimated that 47 per cent still reported symptoms at that point. By comparison, prevalence fell to 23 per cent among adults infected during the Omicron wave in 2022 and 21 per cent among those infected during Omicron infections in 2024.

The findings suggested that the transition to Omicron substantially reduced the frequency of long-term symptoms without completely eliminating the risk. Recovery also differed between pandemic waves. Symptoms resolved more slowly after Delta infection, while adults infected during Omicron waves generally recovered more quickly.

Children experienced a much lower burden of post-COVID symptoms than adults, and younger age was strongly associated with faster recovery, particularly among those aged 12 years or younger.

“The prevalence among children remained about one-quarter to one-third that of adults throughout all epidemic waves,” Sugiyama said.

“Notably, no children in the study experienced disruptions to daily life for more than two years after infection, even when symptoms persisted,” she added.

However, recovery was not universal. Two years after infection, around 20 per cent of adults infected in the three waves before Omicron still reported symptoms, compared with around 10 per cent of adults infected during Omicron periods. Among children, persistent symptoms affected 4.1 per cent of those infected during the Delta period and 1.9 per cent of those infected during Omicron-2022. Symptoms that persisted beyond two years showed little further improvement during the study, suggesting recovery may plateau for some patients.

The findings support continued surveillance and clinical support for people with persistent symptoms.

“Building on the long-term cohort established in this study, we plan to develop a model for predicting the risk of persistent symptoms. We further aim to create and publicly release a web-based tool that uses this model to display, for individuals with similar characteristics, the proportion who experienced persistent symptoms at each time point – to make the trajectory of post-COVID symptoms visible and enable appropriate care and support from the early stages of illness,” Sugiyama explained.

For further reading please visit: 10.1371/journal.pone.0348954