Posted by Long Covid may lead to measurable cognitive decline, especially in the ability to remember, reason and plan, a large new study suggests.
Cognitive testing of nearly 113,000 people in England found that those with persistent post-Covid symptoms scored the equivalent of 6 I.Q. points lower than people who had never been infected with the coronavirus, according to the study, published Wednesday in The New England Journal of Medicine.
People who had been infected and no longer had symptoms also scored slightly lower than people who had never been infected, by the equivalent of 3 I.Q. points, even if they were ill for only a short time.
The differences in cognitive scores were relatively small, and neurological experts cautioned that the results did not imply that being infected with the coronavirus or developing long Covid caused profound deficits in thinking and function. But the experts said the findings are important because they provide numerical evidence for the brain fog, focus and memory problems that afflict many people with long Covid.
“These emerging and coalescing findings are generally highlighting that yes, there is cognitive impairment in long Covid survivors — it’s a real phenomenon,” said James C. Jackson, a neuropsychologist at Vanderbilt Medical Center, who was not involved in the study.
He and other experts noted that the results were consistent with smaller studies that have found signals of cognitive impairment.
The new study also found reasons for optimism, suggesting that if people’s long Covid symptoms ease, the related cognitive impairment might, too: People who had experienced long Covid symptoms for months and eventually recovered had cognitive scores similar to those who had experienced a quick recovery, the study found.
In a typical I.Q. scale, people who score 85 to 115 are considered of average intelligence. The standard variation is about 15 points, so a shift of 3 points is not usually considered significant and a shift of even 6 points may not be consequential, experts said.
“The issue is: Are people able to function in their routine capacity in whatever they are doing? And this is not really answered by 3 points more or less,” said Dr. Igor Koralnik, the chief of neuro-infectious diseases and global neurology at Northwestern Medicine in Chicago, who was not involved in the study.
He added: “The determination of X points on an I.Q. scale is less important than the people’s perception of their cognitive difficulties.”
Still, Dr. Jackson, the author of a book about long Covid called “Clearing the Fog,” said that while cognitive tests like the one in the study “identify relatively mild deficits,” even subtle difficulties can matter for some people. For example, he said, “if you’re an engineer and you have a slight decline in executive functioning, that’s a problem.”
The study, led by researchers at Imperial College London, involved 112,964 adults who completed an online cognitive assessment during the last five months of 2022. About 46,000 of them, or 41 percent, said they had never had Covid. Another 46,000 people who had been infected with the coronavirus said their illness had lasted less than four weeks.
About 3,200 people had post-Covid symptoms lasting four to 12 weeks after their infection, and about 3,900 people had symptoms beyond 12 weeks, including some that lasted a year or more. Of those, 2,580 people were still having post-Covid symptoms at the time they took the cognitive test.
The researchers noted that they relied on self-reported symptoms, rather than diagnoses of long Covid, and that the demands of taking a cognitive test might have meant that participants in the study were not the most seriously impaired.