Covid-19 significantly increases the risk of developing dementia, psychosis and brain fog two years after infection, according to the first extensive study of the disease’s persistent neurological and psychiatric effects.
Long Covid, often defined as symptoms experienced for 12 weeks or more after a Covid diagnosis, encompasses a range of symptoms from fatigue and breathlessness to problems related to the brain, such as a lack of mental clarity. Scientists have estimated that it could affect more than 100mn people worldwide.
Researchers at Oxford university analysed the electronic health records of 1.25mn people diagnosed with Covid and a matched control group consisting of an equal number of patients who had other respiratory infections. The data, covering 14 brain disorders, were provided by the US-based TriNetX global health research network.
The impact of Covid was most marked in people aged 65 and over, among whom 4.5 per cent developed dementia over the subsequent two years, compared with 3.3 per cent of the control group. For psychotic disorders, the figures were 0.85 per cent in Covid patients and 0.6 per cent in controls.
The most significant increased risk among younger adults aged 18 to 64 was cognitive deficit, sometimes called brain fog. It affected 6.4 per cent of people who had Covid during the previous two years and 5.5 per cent of controls.
“The results have important implications for patients and health services as they suggest that new cases of neurological conditions linked to Covid infection are likely to occur for a considerable time after the pandemic has subsided,” said Paul Harrison, senior author of the study published in The Lancet Psychiatry.
But he added: “We are not talking about a tsunami of neurological and psychiatric illness.”
Max Taquet, who led the analysis, said two findings stood out in the study’s 185,000 patients aged under 18. Children were twice as likely to develop epilepsy or seizures, he said — 2.6 per cent developed the condition after Covid, against 1.3 per cent after another respiratory infection.
Secondly, Taquet added: “Children are at three-fold increased risk of psychotic disorder, even though this is rare — 18 in 10,000 in the two years after Covid.”
But the study also showed that the impact of Covid was sometimes shortlived. “Excess risks of depression and anxiety after Covid disappear within two to three months, with no overall excess of cases over the two years,” Taquet said.
The researchers attempted to distinguish between the effects of different Sars-Cov-2 variants, although there is inevitably less long-term data for strains that emerged more recently, particularly Omicron.
“The emergence of the Delta variant was associated with an increase in risk for several conditions,” said Taquet. “With Omicron as the dominant variant, although we see much milder symptoms directly after infection, similar rates of neurological and psychiatric diagnoses are observed as with Delta,” he added.
Taquet said the findings suggested that the burden on healthcare systems may continue even with variants that are less severe in other respects.
Looking at long Covid more broadly, the Oxford scientists stressed that a great deal of research was still needed on all aspects of the condition, from definition and prevalence to biological causes and possible treatments.
“My sense is that we know much less than we think we know about long Covid,” said Harrison. “There are many more questions than answers,” agreed Taquet.