More than 1,400 healthcare workers at an Oxford hospital Trust took part in the research.
Two doses of either the Pfizer/BioNTech or Oxford/AstraZeneca vaccine offer similar protection against coronavirus as natural immunity after infection, new research suggests.
None of the 1,456 healthcare workers at Oxford University Hospitals (OUH) NHS Foundation Trust who had received two vaccines had a symptomatic infection when followed up more than 14 days after their second vaccination.
The study saw the same high level of protection in unvaccinated healthcare workers who had contracted Covid-19 naturally.
They had 98% fewer symptomatic infections than unvaccinated individuals who had not been infected before, researchers say.
Most of the healthcare workers in the study had only received one vaccine to date.
According to the research, protection against symptomatic infection was at 67% after a first dose of vaccine, which was received by 11,023 hospital staff.
In total, 13,109 OUH healthcare workers have participated in the study of symptomatic and asymptomatic staff.
Of these, 8,285 have received the Pfizer/BioNTech vaccine (1,407 of them two doses), and 2,738 the Oxford/AstraZeneca jab (49 of them receiving both doses).
Dr Katie Jeffery, OUH director infection prevention and control, said: “We are grateful to the thousands of staff who work at OUH’s hospitals who have taken part in the testing programme.
“It has provided a rich source of data that continues to shed light on the nature of this new virus and how immunity to it is conferred.
“In this case it is significant that two doses of the vaccines offer similar levels of protection to natural immunity, and that we saw no symptomatic infections among those staff who had had two vaccine doses.
“Data from studies such as this are important as they provide information which may feed into national policy. It also highlights that healthcare workers and other groups at increased risk of infection should take up second vaccines as soon as these are available.”
The study, which has not been peer-reviewed, is published on the MedRxiv pre-print server.
It has been carried out jointly by researchers and clinicians from OUH and a number of University of Oxford departments, including the Nuffield Department of Medicine and the Big Data Institute, with support from the NIHR Oxford Biomedical Research Centre.
Researchers found no evidence that either vaccine or natural infection provided less protection against the Kent variant of the virus.
The research also suggests both vaccination and previous infection may reduce transmission, after finding that the rate of positive tests was lower after both.
And even when people did get infected after a previous infection there was evidence the amount of virus present was reduced, making transmission less likely.
Professor David Eyre of the University of Oxford’s Nuffield Department of Population Health, said: “These findings suggest that evidence of vaccination or previous infection with detectable antibodies could potentially be used to develop a more personalised approach to easing of lockdown restrictions.”