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Infection with Covid-19 can cause lasting damage to cognition and memory, with “brain fog” detectable in long and short-term sufferers of the disease, according to new research.
But the study, one of the most detailed efforts to describe the impact of Covid on mental acuity, showed that more severe cases can improve and the danger has become less acute as the coronavirus has mutated.
The paper, published in the New England Journal of Medicine on Wednesday, suggests people whose longer-term symptoms had resolved performed just as well in cognitive tests as those who had recovered swiftly from the initial infection.
Four years after the emergence of the pandemic, the results show the importance of continued monitoring of the long-term impact of Covid-19 on brain functions, researchers said.
“It is reassuring that people with persistent symptoms after Covid-19, which had resolved, may expect to experience some improvement in their cognitive functions to similar levels as those who experienced short illness,” said Paul Elliott, an Imperial College London professor and senior author of the research.
The study recruited more than 140,000 people, representing a wide range of cases of Covid severity and duration, including uninfected individuals. Respondents were asked to carry out online cognitive tests to measure faculties including memory, reasoning, executive function, attention and impulsivity. The results were controlled for factors including age, demographics and pre-existing medical conditions.
The results showed Covid-19 infection was associated with cognitive deficits, with memory functions hardest hit. People found it more challenging to remember pictures of objects they had seen a few minutes previously, a problem the researchers suggested may reflect difficulties in forming new memories, rather than increased amnesia.
They also performed slightly worse than individuals who had not contracted Covid in tests focusing on spatial planning and verbal reasoning.
The biggest impact was observed on patients who had been hospitalised, had long-lasting symptoms or had been infected with earlier Covid variants. For cases of shorter duration, the effect was small — perhaps sufficiently so that individuals would not notice any impairment, the researchers said.
The researchers found a “small cognitive advantage” among participants who had received two or more vaccinations, while repeated Covid-19 infections appeared to have had only a “minimal” additional effect.
The problem of “brain fog” is commonly reported by sufferers of long Covid, defined as symptoms or conditions that last more than 12 weeks after initial infection with Covid-19. Long Covid is estimated to affect about 5 per cent of people who contract the virus and is becoming an increasing burden on health systems.
Health experts welcomed the study’s contribution to evolving knowledge of Covid’s residual effects. Michael Zandi, neurologist and researcher at UCL’s Queen Square Institute of Neurology, said the research pointed to “concussion-like mechanisms of attention as the main deficit”. Data suggesting that memory storage parts of the brain remained undamaged was “reassuring”, he added.
“The biological mechanisms underlying these findings are likely multiple, remain unclear and deserve detailed longitudinal study and therapeutic trials,” Zandi said.
The paper underlined the need to “understand what helps people recover” from Covid-19, said Claire Steves, professor of ageing and health, and Dr Nathan Cheetham, senior postdoctoral data scientist at King’s College London.
“Deficits in tasks focused on memory, reasoning, and executive function . . . affect [people’s] ability to perform daily activities like work and caring for others,” they said. “Thus, reasonable adjustments from employers, and understanding and support from family and friends, remains crucial for those who continue to live with Covid-19.”
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