A study tracked 33,000 Scottish people who had tested positive for the virus and 63,000 who had never been diagnosed with Covid, checking symptoms at six-month intervals.Credit...Jeff J. Mitchell/Getty Images

A study of tens of thousands of people in Scotland found that one in 20 people who had been sick with Covid reported not recovering at all, and another four in 10 said they had not fully recovered from their infections many months later.

The authors of the study, published on Wednesday in the journal Nature Communications, tried to home in on the long-term risks of Covid by comparing the frequency of symptoms in people with and without previous Covid diagnoses.

People with previous symptomatic Covid infections reported certain persistent symptoms, such as breathlessness, palpitations and confusion or difficulty concentrating, at a rate roughly three times as high as uninfected people in surveys from six to 18 months later, the study found. Those patients also experienced elevated risks of more than 20 other symptoms relating to the heart, respiratory health, muscle aches, mental health and the sensory system.

The findings strengthened calls from scientists for more expansive care options for long Covid patients in the United States and elsewhere, while also offering some good news.

The study did not identify greater risks of long-term problems in people with asymptomatic coronavirus infections. It also found, in a much more limited subset of participants who had been given at least one dose of Covid vaccine before their infections, that vaccination appeared to help reduce if not eliminate the risk of some long Covid symptoms.

People with severe initial Covid cases were at higher risk of long-term problems, the study found.

“The beauty of this study is they have a control group, and they can isolate the proportion of symptomatology that is attributable to Covid infection,” said Dr. Ziyad Al-Aly, chief of research at the V.A. St. Louis Health Care System and a clinical epidemiologist at Washington University in St. Louis, who was not involved in the research.

“It also tracks with the broader idea that long Covid is truly a multisystem disorder,” Dr. Al-Aly said, one that resides “not only in the brain, not only in the heart — it’s all of the above.”