The spread of the Covid-19 pandemic is all the more rapid because it is difficult to identify those carrying the virus. During the incubation period – and for even longer in the absence of pronounced symptoms – these individuals are unaware that they are spreading the disease. To limit the number of victims of the pandemic, the strategy adopted by most affected countries is therefore social distancing or even lockdown – a strategy that can only be applied for a limited period, given its economic, social and human costs. Today, the most viable way out of this impasse appears to require widespread testing of the population. Such testing would make it possible to isolate those carrying the virus and allow others to emerge from lockdown. However, production capacity for PCR (Polymerase Chain Reaction) tests remains limited. Although capacity is increasing, it is not yet sufficient to enable testing that is systematic and frequent enough to allow the lifting of these stringent public health measures. The utility of each test can, however, be maximised by applying it to a pool of samples from several individuals. This technique, which has already been tried and tested in another context, has been the subject of initial successful trials on the coronavirus. We demonstrate how the testing method must be calibrated to maximise the utility of each available test.
Easing lockdown restrictions and tackling the Covid-19 pandemic: grouping tests together for greater efficiency
This note sets out how the testing method should be calibrated to maximise the utility of each available test, taking into account the constraints imposed by the Covid-19 pandemic.
Reference IPP Policy Brief No. 54
Presentation
Key Results
- To halt the spread of the epidemic, lockdown is currently at the heart of the strategy adopted by the hardest-hit countries, but it is too costly to be sustainable. Mass testing could provide a way out of this impasse.
- The shortage of tests currently limits this screening to a few tens of thousands of people per week in France, and test production is not increasing fast enough to allow for an end to lockdown.
- The pooled testing technique maximises the utility of each test. A pooled test, carried out on a mixture of samples taken from n people, allows the lockdown to be lifted for all members of the group if the result is negative.
- The method requires precise calibration of group sizes to optimise its effectiveness. It is all the more useful when prevalence is low, i.e. when the proportion of the population carrying the virus at the time of testing is low.
- If the prevalence is on average 2 per cent (its true value is unknown and estimating it is one of the aims of this note), each test can, on average, enable 18 people to come out of lockdown and thus return to work.
- The usefulness of testing justifies massive investment in increasing production, particularly as mass testing significantly boosts its effectiveness.
- Our approach demonstrates the value of the tried-and-tested method of group testing, which should be trialled on a larger scale.
