People who experience a long Covid-19 bout have measurable memory and cognitive deficits equivalent to a difference of around six IQ points, a study suggests.
The study, which assessed more than 140,000 people in summer 2022, found that Covid-19 can have an impact on cognitive skills and memory that lasts a year or more after infection. People with unresolved symptoms lasting longer than twelve weeks had more significant deficits in performance on tasks involving memory, reasoning and executive functions. Scientists said this showed that ‘brain fog’ had a quantifiable impact.
Prof. Adam Hampshire, a cognitive neuroscientist at Imperial College London and first author of the study, said: “It’s not at all clear what brain fog actually is. As a symptom it has been reported quite extensively, but what our study shows is that brain fog can correlate with objectively measurable deficits. That is a very important finding.”
Last year the Office for National Statistics estimated this about 2 million people in Britain were experiencing self-reported long Covid-19 epidemics. Previous Imperial College analysis found tens of thousands of people in England may have symptoms that last a year or more after infection.
The latest study recruited more than 140,000 participants from the original React cohort, which launched in April 2020 as one of the world’s largest and most comprehensive Covid surveillance studies. Between August and September 2022, participants were given online cognitive tests designed to test memory, attention, reasoning and other aspects of brain function.
Approximately 3.5% of the cohort had experienced symptoms lasting more than 12 weeks, and of these approximately two-thirds were still experiencing symptoms at the time of assessment.
The analysis found that there were small deficits that were still noticeable a year or more after infection, for those who were infected and no longer had symptoms. The difference in test scores between those who had been infected and those who had not was equivalent to about three IQ points if they had been given an IQ test.
For an individual, this magnitude of change is unlikely to be noticeable, scientists said, although some may have experienced more pronounced effects.
The patients with unresolved symptoms lasting longer than twelve weeks were found to have a greater deficit, equivalent to six IQ points.
Dr. Maxime Taquet, a psychiatrist and researcher at the University of Oxford who was not involved in the study, said: “Even if cognitive deficits post-Covid-19 are small in magnitude on average, a substantial minority of people have more significant deficits that are likely to affect their ability to to work and influence functioning. Given the scale of the pandemic and the number of people affected, this is particularly concerning.”
More encouragingly, those with longer-lasting symptoms that had resolved showed similar deficits as those who had experienced mild, short-term illness.
Prof. Paul Elliott, senior author and director of the React programme, from Imperial College London, said: “It is reassuring that people with persistent symptoms after the resolution of Covid-19 can expect their cognitive functions to experience some improvement to similar levels . as those who have experienced a short illness.”
There were greater differences for people with unresolved symptoms lasting twelve weeks or more (consistent with long Covid-19) and for those who had been in hospital for their illness, who had the most noticeable deficits and extended to a wider range of cognitive functions. The differences were also greater for those infected with one of the early variants of the virus, but it was not possible to say whether this was due to the introduction of vaccines and better treatments as the pandemic developed.
The findings are published in the New England Journal of Medicine.