People who have recovered from COVID-19 are more likely to score lower on intelligence tests, new research has found.

Scientists tested 81,337 people between January and December last year as part of the Great British Intelligence Test, including almost 13,000 who had been infected with the virus.

Those who had been on ventilators saw the biggest deficit – equivalent to a seven-point drop.

After controlling for factors including age, sex, first language and education level, researchers found those who had contracted COVID saw the greatest underperformance on tasks requiring reasoning, planning and problem-solving compared to those who had not had the virus.

The study said: “These results accord with reports of long-COVID, where ‘brain fog’, trouble concentrating and difficulty finding the correct words are common.”

The observed deficit in performance was “not insubstantial”, with the drop seen in people placed on a ventilator larger than in those who had previously suffered a stroke.

One possibility was the observed cognitive deficits related to ongoing symptoms of the virus, such as a high temperature or respiratory problems, with 4.8% of participants who were ill reporting residual symptoms.

However, the scientists behind the study urged caution in drawing conclusions without brain imaging data but said the results should act as a “clarion call” for further research into the issue.

The report continued: “Speculatively, we believe there are likely to be multiple contributing factors.

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“For example, previous studies in hospitalised patients with respiratory disease not only demonstrate objective and subjective cognitive deficits but suggest these remain for some at 5-year follow-up.”

Only 275 participants completed the intelligence test both before and after contracting COVID. This limits the ability to draw firm conclusions about cause and effect.

“The large and socioeconomically diverse nature of the cohort enabled us to include many potentially confounding variables in our analysis, which goes some way to mitigating the possibility that observed differences were present prior to illness,” the study said.

“Premorbid estimates also indicate that those who were ill were likely to have had somewhat higher as opposed to lower cognitive ability pre-illness.”

However, it concluded that a follow up of the cohort should “further confirm the cognitive impact of COVID-19 infection”.

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The study – Cognitive Deficits In People Who Have Recovered From COVID-19 – involved researchers from Imperial College London, Kings College and the Universities of Cambridge, Southampton and Chicago, and was published in The Lancet.

Previous research from UCL found patients with long COVID reported more than 200 symptoms affecting 10 organ systems.

The most common symptoms identified were visual hallucinations, itchy skin, menstrual cycle changes, sexual dysfunction, bladder control issues, diarrhoea, heart palpitations and tinnitus.

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