Six years after the world first learned about COVID-19, the pandemic has faded into an unpleasant memory for many. But some people aren’t quite finished.
A long-term study by Hiroshima University found that after the emergence of the Omicron variant, symptoms were far less common, but some people continued to experience health problems years after infection.
The result is Pro Swan May 8th.
Recovery from COVID-19 does not necessarily end when the infection subsides. Regardless of whether the initial illness was mild or severe, some people may suffer from symptoms that last for several months. This condition is often referred to as long-term COVID-19 or post-COVID-19 syndrome, and commonly includes symptoms such as extreme fatigue, cognitive impairment known as brain fog, dizziness, and loss of taste or smell.
The report suggests that long-term COVID-19 infections became less common after the emergence of Omicron, but few studies compared long-term outcomes across multiple pandemic waves, examined new Omicron sublineages, or followed patients for more than two years after infection. ”
Aya Sugiyama, Lecturer, Graduate School of Biomedical and Health Sciences, Hiroshima University, Lead author of the study
To address these gaps, researchers expanded an ongoing cohort study that began in 2020 in collaboration with medical institutions in Hiroshima Prefecture. The research team followed 2,689 people (including 1,524 adults and 1,165 children) who were diagnosed with novel coronavirus disease (COVID-19) from March 2020 to June 2024.
Participants completed a survey that tracked 13 post-COVID-19 symptoms and their duration, allowing researchers to compare long-term recovery patterns across five pandemic periods for the original strain, Alpha, Delta, Omicron-2022, and Omicron-2024.
Professor Sugiyama said, “We found that the long-term course of symptoms after infection with the new coronavirus varies greatly depending on the time of infection and age.”
Six months after infection, adults infected during the delta phase have the highest rate of remaining symptoms, with an estimated 47% still reporting symptoms. By comparison, the prevalence of such symptoms decreased to 23% in the 2022 Omicron wave and 21% in the 2024 Omicron infection.
In general, children performed better than adults.
“The prevalence in children remained about one-quarter to one-third of that in adults throughout the entire wave of the epidemic,” Sugiyama said. “Notably, children whose daily activities were impaired for more than two years after infection, even if symptoms persisted, were not included in the study.”
This study showed that recovery is not universal. Two years after infection, approximately 20% of adults infected before Omicron and 10% of adults infected during Omicron still reported symptoms. In children, symptoms are much less persistent, affecting 4.1% of infected people during the Delta period and 1.9% during Omicron-2022.
The researchers also found that if symptoms persisted for more than two years, there was little further improvement over time, suggesting that while improvement may occur beyond the observation period, recovery may plateau in some people.
The analysis also revealed that the speed of recovery differs depending on the time of infection and age. Symptoms recovered more slowly for those infected during the delta period, and faster for those infected during the omicron wave. Younger age was strongly associated with faster recovery. This is especially true for children under 12 years of age.
The findings confirm that long-term COVID-19 infections are not going away, despite a decline in the overall burden compared to the height of the pandemic, and highlight the continued need for long-term monitoring and support.
“Based on the longitudinal cohort established in this study, we plan to develop a model to predict the risk of persistent symptoms. Furthermore, using this model, we aim to create and publish a web-based tool that displays the proportion of individuals with similar characteristics who experienced persistent symptoms at each time point. This will allow us to visualize the trajectory of symptoms after COVID-19 and enable appropriate care and support from the early stages of the disease,” said Sugiyama.
The research team also includes Toshiro Takagata and Tomoki Sato of Hiroshima City Funairi Municipal Hospital. Kanon Abe of the National Center for Geriatrics and Gerontology. Yayoi Yoshinaga, Takahiro, Tomoyuki Akita, Shingo Fukuma, and Junko Tanaka from Hiroshima University; Mr. Masao Kuwabara of the Hiroshima Prefectural Disease Prevention and Control Center.
This research was supported by the Japan Agency for Medical Research and Development (AMED) and the Hiroshima Prefecture Industry-Academia Collaboration Project Fund.
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Reference magazines:
Sugiyama. A., others. (2026) Differences in the long-term course of symptoms after COVID-19 infection in adults and children over the epidemic period: a retrospective cohort study in Japan, 2020-2024. Prosuonand. DOI: 10.1371/journal.pone.0348954. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0348954
