Quick and effective testing and tracing could reduce the reproduction number (R number) by up to 26%, a study suggests.

But Imperial College London researchers warn that testing alone is unlikely to bring the R number below one at current levels of immunity.

Therefore other interventions such as continuing physical distancing will be needed, they say.

The R number represents how many people someone infected with Covid-19 is likely to go on to infect.

In the study, researchers from Imperial’s Covid-19 Response Team looked at the potential impact of different testing and isolation strategies on transmission of the coronavirus.

An effective test and trace system depends strongly on the coverage of testing and the timeliness of contact tracing, they say.

According to the study, if 80% of cases and contacts are identified and there is immediate testing following symptom onset and quarantine of contacts within 24 hours, then the R number could potentially be reduced by up to 26%.

However, the researchers say the test and trace system in the UK is currently falling short of that.

Professor Nicholas Grassly, from Imperial’s School of Public Health, said: “Effective testing is key to controlling the coronavirus pandemic.

“We need to use testing to prevent transmission in two ways – first, to identify infected individuals and their contacts to reduce transmission through isolation and quarantine, and second, to detect outbreaks so that local lockdowns can be applied when needed.

“Our results show that test and trace can help reduce the R number but needs to be carried out effectively and quickly to do so.

“Test and trace alone won’t be enough to control transmission in most communities and other measures alongside will be needed to bring the R number below one.”

Last week, NHS Test and Trace figures showed that, in the 10 weeks since the launch, 81.5% of contacts had been reached and asked to self-isolate.

Some 18.5% of contacts were not reached.

Researchers say optimal testing strategies should include regular screening of high-risk groups such as health and social care workers during periods of sustained transmission.

Weekly screening of these groups, irrespective of symptoms, by testing them for Covid-19 is estimated to reduce their contribution to SARS-CoV-2 transmission by 23%.

This is on top of reductions achieved by self-isolation following symptoms, provided results are made available within 24 hours.

The researchers developed a mathematical model of SARS-CoV-2 transmission based on infectiousness and PCR test sensitivity over time since infection.

According to this, if everyone with symptoms compatible with Covid-19 self-isolated and self-isolation was 100% effective in reducing onwards transmission, self-isolation of symptomatic individuals would result in a reduction in R of 47%.

Weekly screening of healthcare workers and other high-risk groups, irrespective of symptoms, by use of PCR testing is estimated to reduce their contribution to SARS-CoV-2 transmission by 23%, on top of reductions achieved by self-isolation following symptoms, assuming results are available within 24 hours.

Dr Margarita Pons-Salort, from Imperial’s School of Public Health, said: “We looked at how testing can be used to control transmission.

“Although regular screening of asymptomatic individuals in high-risk groups, as well as contact tracing (test-trace-isolate) of the wider population can help reduce transmission, control of Covid-19 cannot rely on these strategies alone.

“Among others, the effectiveness of these strategies depends a lot on the timeliness to provide test results and to find and quarantine contacts.

“This means that to have a real impact on transmission, testing strategies need to be implemented very well.”

– The research, published in Lancet Infectious Diseases, was supported by the UK Medical Research Council.