Researchers and clinicians are growing increasingly concerned about the possibility of COVID-19 having a persistent impact on health that extends far beyond recovery from acute illness. In particular, anecdotal reports of former patients experiencing depression, “brain fog,” and difficulty finding the right words in conversation even months later appear to point to a potential long-term impact on cognition. A recent large-scale study offers evidence that COVID-19 promotes long-term cognitive deficits, even among those who experience the mildest form of the disease.

The study recruited a representative cohort of 81,337 UK participants (average age, 46 years) to take part in a citizen science project known as the Great British Intelligence Test, a widely promoted free opportunity for people to identify their cognitive strengths. Participants performed a series of clinically validated online tasks assessing their emotional processing abilities, as well as cognitive skills such as attention, problem solving, and working memory. Scores from this latter group of tests were pooled to extract a composite score of global cognitive ability for each participant.

After completing the cognitive tasks, participants were asked to provide some details about their experiences with COVID-19 – information that the researchers went on to use to build a predictive model of their global cognitive and emotional processing abilities.

Their analysis revealed that clinically confirmed cases of COVID-19 infection were associated with significant cognitive deficits even nine months following infection (the longest period recorded in the study). This was particularly evident in complex tasks that required participants to reason, plan ahead, and solve problems using analogies, although the researchers also observed a general slowing of reaction times across all tasks.

The scale of this cognitive impairment depended on the severity of respiratory symptoms. People who had severe cases of COVID-19 that required hospitalization and ventilation exhibited the greatest cognitive decline, surpassing the average decline observed in former stroke victims, as well as the mean cognitive difference between groups that differed by 10 years of age.

Perhaps most surprisingly, the researchers found a significant degree of cognitive decline even in mild cases of COVID-19 who remained at home for the duration of their illness. These individuals performed below what would have been expected if they had not contracted the infection, roughly equivalent to 3.5 IQ points in a classic intelligence test.

This study offers evidence that COVID-19 infection might have long-term health implications far beyond respiratory symptoms and that recovery may be associated with tangible impairments in certain aspects of higher cognitive function. It remains to be explored whether these deficits remain in the longer term.

• Link to full study.