Do most of us have slightly diminished brainpower as a result of getting covid-19? That is the implication of the largest study on this so far, involving more than 100,000 people, but its findings raise more questions than they answer.
A team in the UK invited 800,000 people in the country to take part in research on the cognitive effects of covid-19. Of those, around 113,000 completed a series of online tests at some point between August and December 2022, evaluating their memory, reasoning and attention.
After adjusting for factors such as age, the team compared the scores of the 67,000 people who were confirmed to have had covid-19 with the 46,000 who had not had a definite infection.
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As expected, they found that people who had symptoms for more than 12 weeks 鈥� which is how long covid is usually defined 鈥� or had gone to hospital for treatment did worse in the cognitive tests on average. This is in line with many previous studies showing that severe covid-19 can affect cognitive skills.
To the researchers鈥� surprise, however, even those whose symptoms lasted less than 12 weeks and who didn鈥檛 need hospital treatment did worse on average than the no-confirmed-infection group.
鈥淚 was not expecting that we would see any significant impact in the short-duration groups,鈥� says team member at Imperial College London. That said, some smaller studies have suggested that even mild cases of covid-19 can affect the brain. One found that , for instance.
Brain fog has been a widely reported symptom of covid-19, with the latest study finding measurable differences in the memory and executive abilities, such as spatial planning, of people who reported having it, says Hampshire.
The good news is the overall difference between those who had or did not have covid-19 was small 鈥� 0.2 standard deviations on average. In an IQ test, a difference of this size would equate to three points, says team member , also at Imperial College.
Elliott doesn鈥檛 think that anyone who experiences the effect on this scale will notice it, though this is just the average and some will be hit harder.
He also points out the biggest differences were found in people who got infected early on in the pandemic. One reason听for this is that many of those who got infected later were protected by vaccination, with people who had two or more doses scoring higher on the cognitive tests.
It is also possible that later variants of the virus had less effect on cognitive abilities, but the study cannot say if this is so. 鈥淲e can鈥檛 disentangle these things,鈥� says Elliott.
Does this mean that if you get covid-19 now, your cognitive abilities are less likely to be affected? Maybe, but it is also possible the protective effects of vaccination are declining now that many people are no longer getting boosters.
Another big question is whether people鈥檚 cognitive abilities recover over time or if the effect is permanent. And if it is permanent, will our abilities decline a bit every time we get covid-19? 鈥淲e don鈥檛 know what the long-term effects will be,鈥� says Elliott.
Most people with confirmed covid-19 in the study were tested by the researchers more than 12 weeks after infection, so the effect can certainly last many weeks. The team hopes to do follow-up studies of the participants so see if there is any cognitive recovery.
The researchers are also planning to examine 10,000 of the participants further, which might help reveal how the virus can affect our brains in this way. Hampshire thinks more than one mechanism is probably involved.
What is really needed, though, is a study where people are tested before getting covid-19 and afterwards at regular intervals to see how their cognitive abilities change. The big weakness of the latest study is people only did the tests once.
But it is, of course, no longer possible to do such a study because pretty much everyone on the planet has already been exposed to the virus that causes covid-19, meaning we may never know for sure exactly what it has done to our minds.
New England Journal of Medicine
