By and large Long Covid remains a mystery. But piece by piece we’re starting to form a picture. The ­latest piece of the jigsaw comes from a study by Imperial College ­London where researchers reveal small deficits in the performance of cognitive and memory tasks in people who had recovered from Covid-19, compared with those who’d never had it.

The study also shows that cognitive deficits are worse for people who were hospitalised, who had ongoing long-term symptoms, and who were infected with earlier virus variants.

These insights come from the ­Imperial-led study, called REACT Long COVID, on more than 140,000 participants, who undertook at least one cognitive task, many having had Covid-19 at various levels of severity and persistence.

Participants in the study were asked to perform an innovative online cognitive assessment that can detect subtle changes in different aspects of brain function, such as memory, reasoning, executive function, ­ attention and impulsivity.

The study revealed small deficits that are still detectable a year or more after infection, even in people who had a short illness. They are larger for people who had symptoms lasting 12 weeks or more, those who’d been in hospital, or those who were infected with one of the early variants of the virus.

Surprisingly, people who had longer lasting symptoms that had resolved by the time they did the cognitive assessment still showed small deficits similar in size to those of people who had a shorter duration illness.

The deficits were in many areas of cognition, most notably in recent memory and, in some tasks, testing executive and reasoning abilities, such as those that require spatial planning or verbal reasoning. First author of the study Professor Adam Hampshire, from Imperial College London, expressed public concern about the potential long-term effects of Covid-19 on cognitive function, alongside the concern of healthcare professionals and ­policymakers. Until now, however, it’s been difficult to gather good data.

He pointed out their online platform made it possible to measure multiple aspects of cognition and memory on a large scale, and so researchers were not only able to detect small deficits in cognition but also the effect of illness duration, virus variant and hospitalisation.

Professor Paul Elliott, senior author and director of the REACT programme, concluded that though the cognitive impact of Covid-19 appears to have reduced since the early stages of the pandemic given the large numbers of people who were infected, it’ll be important to carry on monitoring the long-term clinical and cognitive fallout from the Covid-19 pandemic. We’ll never stop.