BBC News
By David Shukman
Wearing a high grade mask known as an FFP3 can provide up to 100% protection.
By contrast, there is a far greater chance of staff wearing standard issue surgical masks catching the virus.
Professional bodies have long campaigned for staff to be given better personal protective equipment.
The data was gathered during a programme of regular testing for Covid at the trust.
The results are published in a pre-print paper that has not been peer-reviewed.
For most of last year, the hospital followed national guidance which specifies that healthcare workers should wear surgical masks, except in a few limited situations.