Covid-19 may have a small but lasting impact on people’s thinking and memory skills more than a year after infection, a study suggests.

More than 140,000 people in the UK took online tests to measure the changes in different aspects of their brain function including concentration, reasoning, memory, spatial planning and other skills, between August and December 2022.

Results, published in the New England Journal of Medicine, revealed “small deficits” in the cognitive performance of people who had recovered from Covid-19, when compared with those who had not had the disease.

This also included people who had Covid-19 symptoms for more than 12 weeks after infection – or long Covid – that had eventually resolved, the researchers said.

Deficits in brain function were found to be greater for people admitted to hospital with Covid-19 or infected with earlier coronavirus variants, such as the original “wild type” virus or alpha.

These deficits were still detectable a year or more after infection, even in people who recovered quickly, the researchers said.

Reassuringly, they added, people who had longer-lasting Covid-19 symptoms that had resolved by the time they did the tests had deficits that were similar in size to those who had shorter recovery times.

Professor Paul Elliott, senior study author and director of the React programme, from the School of Public Health at Imperial College London, said: “It is reassuring that people with persistent symptoms after Covid-19, that had resolved, may expect to experience some improvement in their cognitive functions to similar levels as those who experienced short illness.

“Furthermore, the cognitive impact of Covid-19 appears to have reduced since the early stages of the pandemic, with fewer people having persistent illness, and cognition being less affected amongst those that were infected during the time when Omicron was the dominant strain.

“However, given the large numbers of people who were infected, it will be important to continue to monitor the long-term clinical and cognitive consequences of the Covid-19 pandemic.”

Although not a medical term, “brain fog” is used to describe a range of symptoms including poor concentration, thinking more slowly than usual, feeling confused, forgetfulness and mental fatigue following Covid-19 infection.

To understand more about this widely-reported Covid-19 symptom, people taking part in the React Long Covid study were given eight online tasks to detect subtle changes in memory, reasoning, executive function (mental processes involved in planning and juggling tasks), attention and impulsivity.

They found Covid-19 infection was associated with deficits in a majority of areas of brain function.

The researchers said this was most noticeable in memory, such as the ability to remember pictures of objects that were viewed moments earlier.

The team speculates this may be due to problems forming new memories rather than accelerated forgetting.

People also showed small deficits in some tasks that required spatial planning or verbal reasoning, the researchers said.

First author Professor Adam Hampshire, from the Department of Brain Sciences at Imperial College London, said: “The potential long-term effects of Covid-19 on cognitive function have been a concern for the public, healthcare professionals, and policymakers, but until now it has been difficult to objectively measure them in a large population sample.

“By using our online platform to measure multiple aspects of cognition and memory at large scale, we were able to detect small but measurable deficits in cognitive task performance.”

Commenting on the study, Dr Michael Zandi, neurologist and researcher at UCL’s Queen Square Institute of Neurology, said: “This is a large-scale online study of over 100,000 individuals, with some caveats, e.g. ascertainment bias and the nature of dealing with computer testing.

“This study aligns findings in hospitalised and non-hospitalised individuals and points to concussion-like mechanisms of attention as the main deficit, with some reassuring data against damage to memory storage parts of the brain.

“The biological mechanisms underlying these findings are likely multiple, remain unclear and deserve detailed longitudinal study and therapeutic trials.”