As of March 2023, when the Office for National Statistics stopped collecting data on this condition, 1.879 million individuals had self-assessed as having long covid - symptoms lasting more than 12 weeks following acute covid-19 infection. Of these, the proportion of individuals with symptoms lasting two years or more is around 42%, suggesting a decline in new cases of long covid but a persistence of those with ongoing symptoms.1 Some systematic reviews and meta-analyses have reported that up to a third of such individuals have persistent symptoms of cognitive impairment,23 but estimates vary widely and are complicated by methodological heterogeneity - eg, study size, assessment approach, follow-up duration, and different sampling frames (from self-reported surveys4 to large retrospective matched cohort studies of health records5), as discussed in a recent meta-analysis.6