How many COVID-19 cases have countries been detecting relative to testing effort? Here's plot of total number of reported tests per confirmed case over time - for Germany, Italy, Japan, Korea and UK. Try and guess which is which... (data from @ECDC_EU & @OurWorldInData) 1/

Here's the plot with labels. Surprised at the results? It shows that while COVID-19 testing is important, what's really important is how many as-yet undetected infections those tests are picking up... 2/

The first locally transmitted case in the UK was reported end of Feb, when UK had so far performed over 400 tests per confirmed case. Clearly infections had gone undetected (as in many countries) and it was these missed infections that would lead to the current epidemic... 3/

A recent relevant example here is Ebola. We estimated a few years ago that if cases were being missed, and could create superspreading events, even 'ring vaccination' of at-risk contacts might not prevent a large outbreak: https://wwwnc.cdc.gov/eid/article/22/1/15-1410_article 4/

So what are the future options for COVID-19? We could test more widely - but even if we tested everyone often, many cases would have already spread infection by the time they had positive test. So unless we're also tracing at-risk contacts, the outbreak would likely continue...5/

Maybe we could work on tracing more contacts? UK residents have average 10 contacts per day (with a lot of variability), and cases can be infectious for several days, so effort required would depend on the outbreak size. More on manual contact tracing: https://www.lshtm.ac.uk/newsevents/events/coronavirus-covid-19-outbreak-insights-leading-expert-prof-yik-ying-teo 6/

Perhaps contact tracing could be streamlined with an app? There are good ideas emerging in this area (e.g. https://twitter.com/trvrb/status/1245240645003833345?s=20), but we'd still need majority of population to use the app (& quarantine immediately when asked) to slow transmission. 7/

In practice, a combination of these things could help reduce transmission, and hopefully make aspects of normal routine possible (as in Singapore until recently). But there won't be an quick, simple solution - maintaining control will likely require several ongoing measures. 8/

More on the factors that influence the success of isolation, testing and contact tracing: https://www.thelancet.com/journals/langlo/article/PIIS2214-109X(20)30074-7/fulltext 9/9