HMN 2026: How COVID’s lingering shadow faded after omicron—but not for everyone

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 has found that while 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, a lecturer at Hiroshima University’s Graduate School of Biomedical and Health Sciences and lead author of the study.

A wider look across waves

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.

Delta left the heaviest toll

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, the 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.

Recovery slowed with age

The analysis further revealed that recovery speed differed by infection period and age. Symptoms resolved more slowly in people infected during the delta period, while those infected during omicron waves recovered faster. Younger age was strongly associated with quicker recovery; this was especially true in children ages 12 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.

This content is provided for information purposes only.