Are contact tracing apps able to get us out of the #COVID19 crisis? A thread. 1/12

The idea behind contact tracing: find the contacts of an infected person - they may have been exposed, and should go into quarantine. I'm reposting the illustration that @ncasenmare made 2/12

Since one case on average causes 2-3 new cases without any measures (the fabled reproduction number R), we get the infamous exponential growth. If we manage to get this to below 1, we can prevent large outbreaks. Thus, transmission prevention is key. 3/12

Models show that rapid contact tracing through apps (with the appropriate quarantining) can bring this number to below 1, if ~60% of the population use it. It needs to be fast (i.e. digital) and it needs to be widespread. See https://science.sciencemag.org/content/early/2020/03/30/science.abb6936.full by @LucaFerrettiEvo et al 4/12

Singapore has developed a contact tracing app. But then why did they just announce a lockdown? Reports indicate that <16% use the app. This is interesting on so many levels (I had mistakenly assumed that the app was mandatory there). https://www.straitstimes.com/singapore/call-for-more-people-to-use-contact-tracing-app 5/12

On the other hand, surveys in Europe suggest that more than 60% would be willing to install such an app. Of course we'll have to see whether that ends up being the case. But this is encouraging. See https://osf.io/7vgq9/  6/12

There is the utterly fascinating possibility that liberal democracies where people have high trust in their governments will see a strong uptake of such an app, and may end up managing the crisis well. Entirely speculative at this point, but fascinating. 7/12

Back to contact tracing: how could this work? Scientist & engineers have proposed, in a grassroots effort, a framework called "Pan-European Privacy-Preserving Proximity Tracing", PEPP-PT. Proximity Tracing, Yes, Giving Up Privacy, No. https://www.pepp-pt.org/  @peppPT 8/12

Many details still to be sorted out. Yesterday, some of the people involved ( @carmelatroncoso et al) proposed a concept of how this could be implemented in a de-centralized fashion: https://github.com/DP-3T/  9/12

Long story short: we think this is doable, hopefully in the near future. PEPP-PT is "just" a protocol, so the operational aspects around it would have to be implemented by health authorities as well. A colossal challenge. But we are in an unparalleled crisis. 10/12

To be clear, contact tracing alone is not a catch-all solution. But I believe that in combination with other sensible measures, it could allow us to go back, step by step, to a normal life. Here is a short 2 minute explainer, again from @ncasenmare 11/12

Lots of open questions, lots of open challenges. It may not work. But from the little I know about epidemiology, and about technology - combined with an incurable dose of optimism - it just may. I think we should try. 12/12