Understanding the impact of increased testing

We will see a jump in confirmed cases of COVID-19 as the government rolls out testing, which for many people will feel strange alongside the tight social control measures. This is how confirmed case numbers relate to testing and preventive measures:

Professor Devi Sridhar, Professor of Global Public Health, Usher Institute, College of Medicine and Veterinary Medicine, Edinburgh University

“Other countries’ experience has shown that the more you test, the more cases you will find. We should see it as a positive development if we see increased numbers of cases in the coming weeks as a result of more testing, as it’s better to get a hold on who has the virus in the community. If we increase testing, we’re changing the parameters for who and what we’re testing for: from mainly severe cases in hospitals to more mild and asymptomatic cases in society. We’ll be widening our net.

We should expect to see a delay of 2-3 weeks before interventions such as social distancing will have an effect on the numbers of cases. This is because the cycle of a person spreading the virus, incubation time, through to another person having mild symptoms takes time – around 2 to 3 weeks.

Overall, we should expect to see interventions decreasing hospital admissions, in about 3-4 weeks time – this is the purpose of them, to reduce the strain on the health care system and to buy time meanwhile for rapid research.”

Professor Sir David Spiegelhalter, Chair, Winton Centre for Risk and Evidence Communication, University of Cambridge

“There’s a lot of people now who have the virus, but are not part of the statistics because they have not been tested. So the true number of people with the virus is maybe 5 or 10 times as many as being reported as ‘confirmed cases’.

When more tests have been done, we can expect a jump in the numbers of ‘confirmed cases’, simply because more people who have the virus are being found. With more tests being done, we should have a better idea of how many people are being infected, and be able to judge whether all the tough measures are working.”

Professor Christl Donnelly, Professor of Statistical Epidemiology, Faculty of Medicine, Imperial College London and Professor of Applied Statistics, Department of Statistics, University of Oxford

“As current testing is limited, greater testing will increase the detection rates as well as increasing confirmed case numbers. Interventions aimed at reducing transmission will, if successful, reduce case incidence after a delay, compared to what would happen otherwise. An intervention might reduce the rate of increase in incidence even if it doesn’t lead to decline in incidence.”

Published 26 March 2020