This modelling study from Sweden suggests 26% of Stockholm county will have been infected by May 1st ( https://www.folkhalsomyndigheten.se/contentassets/2da059f90b90458d8454a04955d1697f/skattning-peakdag-antal-infekterade-covid-19-utbrottet-stockholms-lan-februari-april-2020.pdf).">https://www.folkhalsomyndigheten.se/contentas... However, our estimates of under-reporting suggest only 5-10% have been infected so far ( https://cmmid.github.io/topics/covid19/global_cfr_estimates.html).">https://cmmid.github.io/topics/co... So what& #39;s going on? 1/

The model uses a study that found 2.5% of a random sample tested positive in Stockholm County early April. As only 150–200 cases were being reported each day during that period, it suggests there was a lot more infection out there. But how much exactly? 2/

The key issue here is that the number currently infected (i.e. prevalence) isn& #39;t the same as the number of new cases (incidence). To account for this, the model fits to both observed cases and the number infected (i.e. the 2.5% prevalence estimate). And here& #39;s the issue... 3/

To work out under-reporting we need to compare that 2.5% to how many active infections were being reported at same point. The model assumes people are infectious for 5 days, i.e. it effectively compares 5d worth of reported cases (i.e. ~800) with the 2.5% prevalence estimate. 4/

This suggests about 70-80 infections for each reported case, which leads to 26% estimate. But are people really infected for only 5d? People may be *infectious* for relatively short period, but they can test +ve for longer: possibly 2 weeks on average https://www.medrxiv.org/content/10.1101/2020.04.05.20053355v2">https://www.medrxiv.org/content/1... 5/

This suggests we shouldn& #39;t be comparing 2.5% with cases over 5 days - we should be looking at the previous 2 weeks. Looks like ~2400 cases were reported during this period, suggesting around 25 infections per reported case. In other words, 1/3 of the value they estimate... 6/

This would imply 5–10% will have been infected by 1 May, consistent with our estimate (and Imperial: https://mrc-ide.github.io/covid19estimates/#/details/Sweden).">https://mrc-ide.github.io/covid19es... The above is of course just a rough estimate, but it shows that it& #39;s always worth checking results against other data sources. 7/7