This paper argues for the regular testing of members of at-risk groups more likely to be exposed to SARS-CoV-2 as a strategy for reducing the spread of Covid-19 and enabling the resumption of economic activity. We call this ‘stratified periodic testing’. It is ‘stratified’ as it is based on at-risk groups, and ‘periodic’ as everyone in the group is tested at regular intervals. We argue that this is a better use of scarce testing resources than ‘universal random testing’, as recently proposed by Paul Romer. We find that universal testing would require checking over 21 percent of the population every day to reduce the effective reproduction number of the epidemic, R’, down to 0.75 (as opposed to 7 percent as argued by Romer). We obtain this rate of testing using a corrected method for calculating the impact of an infectious person on others, where testing and isolation takes place, and where there is self- isolation of symptomatic cases. We also find that any delay between testing and the result being known significantly increases the effective reproduction number and that one day’s delay is equivalent to having a test that is 30 percent less accurate.


Cleevely, M., Susskind, D., Vines, D., Vines, L. & Wills, S. (2020). 'A Workable Strategy for Covid-19 Testing:Stratified Periodic Testing rather than Universal Random Testing'. COVID Economics Vetted and Real-Time Papers, Centre for Economic Policy Research Press, Issue 8, 22 April 2020.
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