NEW YORK (BLOOMBERG) - Mass testing for the coronavirus might do no more to stop the pandemic than quarantines and contact tracing alone, according to a new study.

While testing is critical in assessing the spread of the virus and the risks of lifting lockdowns, the containment benefits might be limited to certain high-risk groups such as healthcare workers, according to the report from Imperial College London researchers.

The study wades into a thorny political debate, with the World Health Organisation and other groups urging earlier more widespread testing. Governments in Britain, the United States and other places have faced criticism that a shortage of diagnostics allowed the virus to spread. Countries with aggressive testing regimens, like Germany and South Korea, have shown effectiveness in controlling it.

Britain abandoned widespread testing in the community last month, instead focusing on trying to delay the spread of the virus through social distancing. A study by other Imperial College researchers, showing the potential for hundreds of thousands of deaths if business went on as usual, was seen as instrumental in persuading the government to lock down.

"There has been substantial pressure on the UK government and others to 'test, test, test' in response to the Covid-19 pandemic," Mr Nicholas Grassly, an Imperial professor and one of the authors of the new study, said in an e-mailed statement.

"We find that testing is most useful when targeted at high-risk groups, such as healthcare and care-home staff."

Weekly screening of healthcare workers and others at higher risk is estimated to reduce their contribution to the spread of the illness by up to 33 per cent, the study found. But widespread testing is unlikely to limit overall transmission more than contact tracing and quarantine based on symptoms alone, it found.

Governments are scrambling to ramp up testing, which is seen as key to safely reopening economies.

So-called immunity passports based on tests for antibodies or infection could help get people back to work but face significant technical, legal and ethical challenges, according to the report. Officials and scientists in a number of countries are considering giving certificates to people who've recovered from Covid-19 that would allow them to escape restrictions, while the uninfected might have to remain isolated until a vaccine or treatment is found.