AS VACCINATIONS stem the tide of severe covid-19 cases in the rich world, attention is turning to the virus’s after-effects. Many people experience symptoms after recovery, such as fatigue, sometimes forcing changes in work and lifestyle. Among the least understood aspects of “long covid” are problems with memory and concentration, often known as “brain fog”.

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A joint Anglo-American research team set out to measure covid-19’s effect on mental ability, publishing their results in the Lancet EClinicalMedicine. They teamed up with the BBC, Britain’s national broadcaster, on the Great British Intelligence Test, an online assessment billed to the public as a way to “test your cognitive strengths”. Some 80,000 people signed up.

Participants completed several activities assessing cognitive function. They were asked to remember lists of numbers or locations on a chessboard to test their recall. A “Tower of Hanoi”-style game, in which stackable blocks are moved from one pin to another, measured planning capacity. Two tasks assessed verbal skills, while a Tetris-style game assessed problem-solving. The ability to manipulate information was tested by matching identical shapes that had been rotated. To prevent bias, questions about the virus, as well as about demographic factors such as age, ethnicity and socioeconomic status, were asked only after the test had ended.

After controlling for factors including age, sex, educational level, first language and income, people who reported having recovered from covid-19 were found to perform worse than those who thought they had never caught it. The more severe the symptoms, the worse they performed. Those who had been put on a ventilator fared poorest of all. The gap between their average performance and that of participants who reported never having had covid-19 was equivalent to seven IQ points. Those who had been put on a ventilator scored lower than people who reported having a learning disability or had previously had a stroke.

People ventilated for different respiratory diseases often face similar cognitive issues. But the performance of those with mild covid-19 symptoms suffered more than would be expected in the event of mild illness. (Additionally, patients who had actually tested positive for covid-19, rather than merely suspecting they had caught it, suffered from greater cognitive impairment; this is probably because some in the latter camp had misdiagnosed a common cold or throat infection.)

Scientists are still unsure how exactly covid-19 is linked to cognitive impairment. One preprint study compared brain scans done before the pandemic with scans of the same patients taken after they had tested positive. The researchers found that regions of the brain associated with memory, taste and smell had shrunk. There are several theories as to the cause. Some studies argue it is the body’s own immune response causing harm. Others show that the virus specifically attacks astrocytes, the brain’s “support cells”. A final camp claims the damage is caused by a lack of oxygen.

It remains unclear how long brain fog takes to clear, or if it does at all. That makes it difficult to estimate the lasting economic and social impact. And it reinforces worries that the health effects of covid-19 will linger long after the pandemic is tamed.