Around 1 in 6 unvaccinated individuals say they are still experiencing health effects of COVID-19 up to two years after infection, finds a study.

Most people who have COVID-19 recover soon after the initial phase of the disease, but others experience persistent health problems (known as long COVID), which can impact their quality of life and ability to work.

The findings, published by The BMJ, show that 17 per cent of participants did not return to normal health and 18 per cent reported COVID-related symptoms 24 months after initial infection.

Compared with people who did not have an infection, those with COVID had excess risks for both physical problems, such as altered taste or smell (9.8 per cent), malaise after exertion (9.4 per cent), and shortness of breath (7.8 per cent), and mental health issues, such as reduced concentration (8.3 per cent) and anxiety (4 per cent) at month six.

"Persisting health issues create significant challenges for affected individuals and pose an important burden on population health and healthcare services," said researchers Universities of Zurich, in Switzerland, and California-San Francisco, in the US.

They called for clinical trials "to establish effective interventions to reduce the burden of post-COVID-19 conditions."

In the study, researchers looked at patterns of recovery and symptom persistence over two years in 1,106 unvaccinated adults (average age 50) with a confirmed SARS-CoV-2 infection between August 6, 2020, and January 19, 2021, and 628 adults (average age 65) randomly selected from the general population who had not had the virus.

Participants provided information on 23 potential long COVID symptoms six, 12, 18, and 24 months after infection.

Overall, 55 per cent of participants reported returning to their normal health status less than a month after infection, and 18 per cent reported recovery within one to three months.

By six months, 23 per cent of participants reported that they had not yet recovered, reducing to 19 per cent at 12 months, and 17 per cent at 24 months.

The proportions of people still experiencing symptoms thought to be related to COVID-19 at the three-time points were similar but slightly higher, decreasing from 29 per cent at six months, to 20 per cent at 12 months, and to 18 per cent at 24 months.

People who reported symptoms at all follow-ups or reported worsened symptoms were more likely to be older and to have pre-existing health problems.

These are observational findings and the researchers acknowledge several limitations, including that they focused on only wild-type SARS-CoV-2 in an unvaccinated population and relied on self-reported health, which can be unreliable.

Nevertheless, this was a large population-based study with regular assessments of a range of health outcomes, and findings were similar after further analyses, strengthening the credibility of the estimates, they said.