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Eric Caumes (Infectious disease specialist)
Almost 5 years after the appearance of Covid, we are still learning a little more about its effects. Two studies released a few days apart confirm the impact of the virus on cognitive functions, regardless of the age of the patients.
Is there a link between catching Covid and a weakening of one’s cognitive functions? Two recently published studies allow us to know a little more about the effects of an infection.
Two studies carried out on people of different ages
A first study, published in the scientific journal Nature Medicine on September 23, followed 351 patients aged 54 on average, one year after their hospitalization for a severe form of Covid. They were compared to 2,927 people called “controls. “We tested their cognitive abilities, took blood samples and performed brain scans,” says Greta Wood, lead author of this study on X.
The second, this time published in The Lancet early October, focused on “34 volunteers” aged 18 to 30 who received a dose of SARS-CoV-2 and eighteen of whom developed an infection. “Volunteers performed daily physiological measurements and computerized cognitive tasks to examine differences between ‘infected’ individuals and those ‘inoculated’, but not infected. the study indicates. Their memory and attention were notably evaluated.
Reduced cognitive abilities at all ages
What do these two studies teach us? The one carried out on seniors is rather clear: “One year after Covid-19, the cognitive deficits measured were equivalent to aging by 20 years” of the brain, announces Greta Wood. That is to say that a 50-year-old woman demonstrated cognitive abilities corresponding to those “of a person in their 70s”.
In addition to the tests, the team also detected the presence of markers of brain damage in the blood. Brain scans, for their part, revealed a “reduction in the volume of gray matter” in the anterior cingulate cortex, a region “which plays on cognition, attention and emotion”. Fortunately, a recovery trend is also detected.
On the younger side, the results are less drastic. Nevertheless, a difference in cognitive abilities emerged with the control people. “Infected volunteers showed statistically lower cognitive test scores than uninfected volunteers. the study indicates. “The reduction in the precision of memory and executive function are the main factors contributing to the decrease in scores after infection”write the authors, specifying however that the observation remains “slight”.
The reason is not yet known
If these two studies confirm others already highlighting this effect (such as that of Inserm, in 2023), the causes of these attacks still remain.“blurry” todaywrite the researchers in The Lancet. What is the mechanism at play? How are cognitive abilities impacted by the virus? In an article published on October 4, The World mentions several hypotheses: “prolonged cerebral inflammation, destruction or blockage of microvessels, viral persistence at very low level in the brain, autoimmune process”… Further studies will need to be carried out to find out more precisely.
These studies also contain biases, according to an infectious disease specialist
If the studies carried out on Covid are gradually trying to clarify the consequences, Professor Eric Caumes, infectious disease specialist contacted by Doctissimo, does not jump for joy upon reading these last two which he considers incomplete.
“For the Nature paper, there is a methodological problem, in my opinion, since there is no control group, so we cannot really draw conclusions without placing these participants in front of other hospitalized people for other respiratory infections. Furthermore, some people studied had encephalopathies. However, we already know that Covid causes encephalitis. It is therefore not very surprising to note that patients have not yet recovered. The good news is that there is recovery!”
The second study does not escape its skepticism either.
“It is more interesting, because there is a control group there. But we are talking about a group that is too small (18 patients on one side and 16 who are not) to draw conclusions. Moreover, these are people specially inoculated with the virus, which poses a big ethical problem for a disease which has no treatment And which poses a cognitive bias: these participants knew that they were going to be sick, and expected to encounter problems. and to be studied.
Conditions which, according to him, do not provide sufficient insight at this stage of the research.