Most patients with COVID-19 who have lingering symptoms at 12 months are likely to still have symptoms at 18 months, new data suggest.

The findings are drawn from a large study of 33,281 people in Scotland who tested positive for the coronavirus. Most of the results are in line with those from earlier, smaller studies.

Among a subset of 197 survivors of symptomatic SARS-CoV-2 infections who completed surveys at 12 months and 18 months, most reported lingering symptoms at both time points, researchers reported in Nature Communications.

Rates of no recovery at 12 months were 11% with 51% partial recovery and 39% complete recovery. Rates at 18 months were 11% no recovery, 51% partial and 39% complete.

Asymptomatic infections were not associated with long COVID. But among the 31,486 people with symptomatic infections, nearly half reported incomplete recovery at six to 18 months.

A total of 3,744 participants with symptomatic infections completed questionnaires twice over the following year. At six months, 8% reported no recovery, 47% reported partial recovery, and 45% reported complete recovery. Those rates had barely changed at 12 months, with 8% reporting no recovery, 46% partial recovery and 46% complete recovery.

One in 20 patients with a symptomatic infection reported no recovery at the most recent follow-up, researchers said.

"Our study is important because it adds to our understanding of long COVID in the general population, not just in those people who need to be admitted to hospital with COVID-19," study leader Jill Pell of the University of Glasgow said in a statement.

Long COVID was more likely in patients who had been hospitalized and in those who were older, female, socioeconomically disadvantaged, and with pre-existing health conditions. The most common lingering symptoms included breathlessness, chest pain, palpitations, and confusion and "brain fog."

Vaccination before infection appeared to protect against some long-term symptoms, the researchers also found.

The researchers also surveyed nearly 63,000 individuals with only negative COVID tests, to distinguish between health problems that are due to COVID-19 and health problems that would be expected in the general population.