The work, published in the journal Nature, included 785 participants, ranging in age from 51 to 81. This is the first study in which volunteers’ brains were scanned before and after Covid-19.

The two brain scans were about three years apart.

With normal aging, people lose a small amount of gray matter each year.

But Covid-19 patients in the study lost more gray matter (from 0.2% to 2%) than normal people.

`To me, this is convincing evidence that there are changes in the brain in people with Covid-19 in general,` said Dr. Serena Spudich, Head of the Department of Neuroinfectious Diseases at Yale University School of Medicine, USA,

She was surprised to see that this condition is quite common, the amount of gray matter lost is more than estimated.

Gwenaëlle Douaud, professor of neurology, Oxford University, lead author of the study, said the number of patients needing hospitalization was 15, too few to draw conclusions.

Digital model shows the brain areas with the most gray matter loss in people with Covid-19.

People with Covid-19 also have cognitive decline, concentrate more than groups who have not been infected with the virus, and have reduced efficiency when doing complex tasks.

For example, the largest areas of gray matter loss involved the sense of smell, including the frontal cortex into the hippocampus.

Neurological experts say the study is valuable and unique, but also say the effects of the virus on the brain are unclear.

A limitation of the study is that scientists did not have information about the volunteers’ symptoms, such as cough, fever or loss of taste and smell.