| In early 2020, it was assumed that most people who contracted Covid would fully recover in two to three weeks. Months later, we learned that a significant proportion of survivors are plagued by lingering symptoms that we now recognize as long Covid. In 2021, analyses of electronic health record data showed how Covid patients, especially those treated in hospitals, were at higher risk of a constellation of problems — from diabetes and depression to heart attacks and strokes — months later. The research, though, was based on observational data that linked or associated a SARS-CoV-2 infection with subsequent health effects. To test if the coronavirus actually causes a new health problem requires experimentally infecting people and comparing them with uninfected "controls." After a rigorous review of the ethics, UK researchers did precisely that in early 2021. Their so-called human challenge study was the first — and likely only — research to involve inoculating healthy, young, unvaccinated adult volunteers with the original coronavirus strain. The research yielded unique and important insights into why some people manage to avoid infection and why others can spread it widely. But the latest finding might be the most eye-opening. Each participant, aged 18 to 30 years old, completed 11 tasks on an iPad during two consecutive days before they were all inoculated nasally with the SARS-CoV-2 virus. That exposure led to a mild infection in roughly half of them. While none of the participants noticed any lasting cognitive effects after their inoculation, the results of further testing showed otherwise. Five rounds of cognitive testing after the participants left quarantine found the infected group had measurable reductions in memory and executive function compared with those who weren't infected. The differences were small, but persistent, with the changes still evident a year after the experiment. The effect size equated roughly to a difference of 6 IQ points, said Adam Hampshire, one of the study's senior authors. The finding broadly aligned with his earlier research — involving more than 81,000 participants — during the UK's first pandemic wave in 2020, he added. "I would say that these are small decrements that are comparable in scale to the fluctuations that we see day-by-day in a person's cognitive abilities," Hampshire told me. "It is likely a person would not perceive a difference of this scale over the longer term." Time will tell whether the deficit eventually disappears or whether the volunteers will be left with a slight, but permanent reduction in cognition. The results can't be extrapolated across the broader population since our immune defenses have been strengthened by vaccinations and previous infections and newer virus variants may carry a lower risk of neurological complications. Still, they corroborate a growing body of evidence showing that the SARS-CoV-2 virus — and its indirect social and economic effects — have taken a toll on people's minds and bodies. —Jason Gale | 
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