Six years after the world first learned of COVID-19, the pandemic has faded into an unpleasant memory for many. For others, however, it never fully ended.
A long-term study by Hiroshima University found that whilst lingering symptoms became far less common after the Omicron variant arrived, some people have continued to experience health problems years after infection.
The results were published in PLOS One on May 8.
Recovery from COVID-19 does not always end when the infection clears. Some people suffer persistent symptoms for months, regardless of how mild or severe their initial illness was. Often referred to as long COVID or post-COVID-19 syndrome, the condition commonly includes extreme tiredness, cognitive difficulties known as brain fog, dizziness, and disrupted taste or smell, among other symptoms.
"Although reports suggested that long COVID became less common after Omicron emerged, few studies had compared long-term outcomes across multiple pandemic waves, examined newer Omicron sublineages, or followed patients for more than two years after infection," said Aya Sugiyama , lecturer at Hiroshima University's Graduate School of Biomedical and Health Sciences and lead author of the study.
To address these gaps, the researchers expanded an ongoing cohort study launched in 2020 in collaboration with medical institutions within Hiroshima Prefecture. The team 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 tracking 13 post-COVID symptoms and how long they lasted, allowing researchers to compare long-term recovery patterns across five pandemic periods — the original strain, Alpha, Delta, Omicron-2022 and 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.
At six months after infection, adults infected during the Delta period had the highest rate of lingering symptoms, with an estimated 47% still reporting symptoms. By comparison, prevalence of such symptoms dropped to 23% during the Omicron wave in 2022 and 21% during Omicron infections in 2024.
Children generally fared better than adults.
"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."
The study showed that recovery was not universal. Two years after infection, around 20% of adults infected before Omicron and 10% infected during Omicron periods still reported symptoms. Among children, persistent symptoms were much less common, affecting 4.1% of those infected during the Delta period and 1.9% during Omicron-2022.
The researchers also found that symptoms continuing beyond two years showed little additional improvement over time, suggesting recovery may plateau for some individuals, though improvement beyond the observation period remains possible.
The analysis further revealed that recovery speed differed by infection period and age. Symptoms resolved more slowly in people infected during the Delta period, whilst those infected during Omicron waves recovered faster. Younger age was strongly associated with quicker recovery; this was especially true with children aged 12 years and younger.
The findings confirm that even as the overall burden has declined since the height of the pandemic, long COVID has not disappeared, underscoring the continuing need for long-term monitoring and support.
"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 — making the trajectory of post-COVID symptoms visible and enabling appropriate care and support from the early stages of illness," Sugiyama said.
The research team also includes Toshiro Takafuta and Tomoki Sato at Hiroshima City Funairi Citizens Hospital; Kanon Abe at the National Center for Geriatrics and Gerontology; Yayoi Yoshinaga, Ko Ko, Tomoyuki Akita, Shingo Fukuma, and Junko Tanaka at Hiroshima University; and Masao Kuwabara at the Hiroshima Prefecture Center for Disease Control and Prevention.
This study was supported by the Japan Agency for Medical Research and Development (AMED) and the Hiroshima Prefecture Government-academia collaboration project funding.
