Coronavirus: Asymptomatic cases detected in London NHS staff
The case for routinely testing NHS staff is "overwhelming", leading cancer scientist at the Crick Institute Dr Charles Swanton has said.
His team identified NHS staff testing positive at the peak of the pandemic who were "completely asymptomatic".
But the study was not able to show whether these staff without symptoms passed their infections on to others.
The Department of Health said it was being "guided by the evidence" on routine testing.
Dr Swanton said while the prevalence of infection in healthcare staff was now low, routine screening would be necessary "ahead of a second wave".
Healthcare workers in the study, lead by Dr Catherine Houlihan, were swabbed between 26 March and 8 April, at the peak of the pandemic.
They were then followed up for a month.
Of 200 University College London Hospital staff tested, 36 were positive initially, of whom 16 (38%) did not report any noticeable symptoms at the time of the test or at any point afterwards.
For the 20 people who did develop symptoms, on average these developed four days after the positive test, the study.
The same people were also tested for antibodies in their blood, suggesting a past Covid infection.
By the end of the study period, 45% (87 staff members) had developed antibodies.
This level of exposure to the virus among healthcare staff was much higher than they had expected and than previous studies had suggested, Dr Swanton said. "The case for asymptomatic healthcare worker testing seems to be overwhelming."
A government spokesperson said: "We have engaged with SAGE on the appropriate frequency of repeat testing, with NHS Trusts continuing to routinely and strategically test asymptomatic frontline staff."
People working in patient-facing and resident-facing roles in health and social care were six times more likely than the general population to test positive for coronavirus, according to figures published by the Office for National Statistics on Tuesday.
The evidence suggests people without symptoms are capable of passing on the virus, but establishing how big a role they play is still challenging.
On 11 June the government's Scientific Advisory Group on Emergencies stated infectiousness did "correlate with the duration of disease/severity" - in other words, it does seem that people with symptoms transmit the disease more.
That doesn't mean symptomless people don't transmit the disease at all.
But equally, the fact that a significant proportion of positive tests are in people without symptoms, doesn't mean that a significant number of infections are spread by those people.
It's believed that just 10-15% of people are responsible for about 80% of infections.
A number of NHS leaders have called for routine testing of staff at least once - if not twice - a week, to assure them they are not carrying the virus without knowing it.
Asymptomatic testing is currently available to care home staff and residents.
A pilot launched on Thursday will look into the testing of other "high-contact" occupations, like taxi drivers and sales assistants without symptoms.