Health problems and brain fog can persist for years in people hospitalised by Covid early in the pandemic, with some patients developing more severe and even new symptoms after 12 months, researchers say.

They found that while many people with long Covid improved over time, a substantial proportion still had cognitive problems two to three years later and saw symptoms of depression, anxiety and fatigue worsen rather than subside.

The scientists studied long Covid in 475 people who were hospitalised with the virus before vaccines were available, to learn whether brain fog, fatigue and mental health problems persisted or emerged beyond the first year.

While the researchers had no information on the participants’ thinking skills before they got Covid, cognitive tests two to three years later showed that on average patients’ IQs were 10 points lower than expected for their age, education and other factors. One in nine showed signs of “severe cognitive deficits”, which equate to an IQ 30 points lower than expected.

Questionnaires completed by participants showed that many felt moderate to severe levels of depression (47%), fatigue (40%) and anxiety (27%) two to three years after Covid. Rather than improving over time, the symptoms were on average worse two to three years after infection than at six to 12 months.

“What we already knew before doing the study was that Covid-19 was associated with a greater risk of cognitive problems, depression and anxiety compared with other respiratory infections,” said Dr Max Taquet, an author on the study from the University of Oxford. “We found that in our cohort there was a substantial neuropsychiatric burden at two to three years.”

More than one in four people in the study changed occupation after catching the virus, often because they could no longer face the cognitive demands of their job.

Those who fared worse over the years were often among the sickest six months after infection, but the severity of the original illness did not appear to affect their long-term outcome.

Although the proportion of patients reporting depression, anxiety and fatigue rose after six months, cognitive problems appeared to improve. Six months after infection, 44% had objective cognitive deficits compared with 33% at two to three years.

The work, published in Lancet Psychiatry, led the authors to stress the importance of diagnosing and managing long Covid symptoms early to reduce the risk of people developing a more complex disorder.

According to the Office for National Statistics, the number of people with long Covid continues, to grow with 2 million people in England and Scotland now experiencing symptoms. The brain fog many report is equivalent to a six-point drop in IQ, one recent study found.

Danny Altmann, a professor of immunology at Imperial College London, who was not involved in the study, said the findings were “profound and alarming” and warned people not to be complacent in the era of vaccines and milder Covid variants.

The risk of long Covid has dropped from about 10% in the first wave of infections to about 2.5% today, but that still amounts to a “huge number of cases”, Altmann said. “There could not be a more stark warning that Covid-19 is ongoing and can still do awful things to you, so it’s worth staying boosted and avoiding reinfection.”

The scientists urge caution over the results, however. Only 19% of the 2,500 people invited on to the study took part, and if those who agreed fared much worse or much better than the wider group, it would skew the results. It is also unclear whether people who developed long Covid later in the pandemic, after vaccination, and without being hospitalised, would experience as severe health problems.