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 from Switzerland published by The BMJ today.

The findings show that 17% of participants did not return to normal health and 18% reported covid-19 related symptoms 24 months after initial infection.

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 quality of life and ability to work.

Previous studies on longer term outcomes after covid-19 infection have reported a wide range of estimates (22-75% at 12-24 months) preventing researchers from making any firm conclusions about long term treatment and support.

To address some of this uncertainty, researchers looked at patterns of recovery and symptom persistence over two years in adults from the Zurich SARS-CoV-2 Cohort, an ongoing study of individuals with confirmed SARS-CoV-2 infection.

Their findings are based on 1,106 unvaccinated adults (average age 50) with a confirmed SARS-CoV-2 infection between 6 August 2020 and 19 January 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. Other potentially influential factors including age, sex, education, employment, and pre-existing health problems, were also taken into account.

Overall, 55% of participants reported returning to their normal health status less than a month after infection, and 18% reported recovery within one to three months. By six months, 23% of participants reported that they had not yet recovered, reducing to 19% at 12 months, and 17% at 24 months.

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

Compared with people who did not have an infection, those with covid-19 had excess risks for both physical problems, such as altered taste or smell (9.8%), malaise after exertion (9.4%), and shortness of breath (7.8%), and mental health issues, such as reduced concentration (8.3%) and anxiety (4%) at month six.

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.

"Persisting health issues create significant challenges for affected individuals and pose an important burden on population health and healthcare services," write the authors, and they call for clinical trials "to establish effective interventions to reduce the burden of post-covid-19 condition."

Understanding the trajectory of symptom burden and recovery from long covid is crucial for policy making, treatment decisions, and care coordination, says a researcher at the University of Southern California in a linked editorial.

The design of future treatments, clinical trials, and policy interventions "will depend on robust studies based on high quality population level data," he writes.

"Additionally, in view of the complexity of symptom trajectories and the unique disease burden experienced by each individual patient with long covid, patients should be more closely involved in the design and conduct of these studies going forward."