Thread by @bealelab

Can regular testing help with reducing the spread of Covid? Yes it definitely can, in the right context. It's not a panacea, but with education and other sectors of the economy opening up before we are fully vaccinated it's vital we try to make this work. bbc.co.uk/news/uk-566320…

First, the basics. You will never detect infection in the first day or two after exposure. Then the viral load goes up massively, then – after that – symptoms start. The idea behind these programmes is to detect cases before symptoms start, and to detect asymptomatic cases.

3/n This illustration from @michaelmina_lab shows how this can take place. You could use a high sensitivity PCR test, or a lower sensitivity lateral flow test.

4/n real world data looks like this, from the @SAFERuclh analysed by @AdamJKucharski and colleagues. PCR testing definitely works really well in this context. medrxiv.org/content/10.110…

5/n a great real-world example of asymptomatic testing by PCR is at @Cambridge_Uni where students pool swabs to make this practical at scale. It's hugely effective at preventing outbreaks. cam.ac.uk/coronavirus/st…

6/n so what about lateral flow tests? They are less sensitive than PCR, so it's crucial to perform them more frequently, otherwise you can easily miss the peak viral load. They have a big advantage in being quicker though.

7/n In defined populations where the motivation is clear and people act responsibly there is every reason to believe these tests work. Results from the larger scale tests in secondary schools are likely available to Government by now. If these are encouraging, it would make >

8/n sense to expand this to other sectors of the economy that are opening up. Can we make this work at an even wider scale?

9/n Messaging is crucial. This is not a screening programme. You do not participate for your own individual benefit. It is a *collective* effort to reduce asymptomatic and pre-symptomatic transmission. It must be part of a package of measures designed to suppress transmission.

10 (and final) I would like to see health professionals and scientists do the best they can to help make this work. It's not a panacea, but it certainly can help and we should do our best to support uptake of tests and associated responsible behaviour.