NEW YORK - 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-19 symptoms scored the equivalent of six IQ points lower than people who had never been infected with the coronavirus, according to the study, published on Feb 28 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 three IQ 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 Dr James Jackson, a neuropsychologist at Vanderbilt Medical Centre, 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 IQ scale, people who score 85 to 115 are considered of average intelligence.

The standard variation is about 15 points, so a shift of three points is not usually considered significant and a shift of even six points might 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 three 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 IQ 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 per cent, said they had never had Covid-19.

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-19 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-19 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.

The vast majority of the participants were white, and more than half were female. There were more people from affluent areas than from economically struggling areas.

The test, which the researchers developed and have used in previous studies of patients with brain injuries, Covid-19 and other conditions, consisted of eight tasks designed to evaluate skills such as spatial planning, verbal reasoning and word definitions, along with several aspects of memory.

The senior author of the study, Professor Paul Elliott, a public health researcher and specialist, said the assessment, called Cognitron, has been consistent with other cognitive scales.

He also said that, as an online test rather than an in-person or paper assessment, it is useful for studying large numbers of people.

Prof Elliott said that the team took the statistical results from the test and computed how they would translate to a standard IQ scale.

He noted that the study measured performance at only one point in people’s lives, and that the IQ computations were averages for the different groups, so they may not have reflected individual results.

Prof Elliott also said it was impossible to know if other stresses in people’s lives, not just Covid-19, had contributed to their test performances.

The lowest scores were generally seen in people who had infections earlier in the pandemic, before vaccines and antiviral treatments were available.

People who were vaccinated performed somewhat better than those who were not. People who were infected more than once scored only slightly lower than those who were infected a single time.

Most study participants, including those who had developed long Covid, had not been hospitalised for their infection, meaning they had a manageable or mild illness during that initial stage.

Still, as in other studies, outcomes were worse for people who had been hospitalised.

The 228 participants in the study who spent time in intensive care units scored the equivalent of nine IQ points lower than people who had not been infected.

Dr Ziyad Al-Aly, the chief of research and development at the Veterans Affairs St Louis Health Care System and a clinical health researcher at Washington University in St Louis, Missouri, who co-wrote an editorial about the study, said the findings raise many questions: How significant are the real-life effects of the small cognitive deficits identified? Do they raise people’s risk of dementia later in life? What other factors might affect Covid-19 patients’ cognitive scores?

Dr Jackson said that overall, the fact that cognitive scores showed some deficit in patients with lingering symptoms is concerning, but should also “validate many people with long Covid whose cognitive complaints have been dismissed”. NYTIMES