People who have already contracted coronavirus are as protected against reinfection as those who have received the best Covid-19 vaccines, according to a survey of 20,000 UK healthcare workers, the largest study in the world so far.
Public Health England regularly tested two matched groups of volunteers between June and November — 6,000 health workers who had previously been infected with coronavirus and 14,000 who had not.
A comparison of infections in the two groups, described in preliminary results released on Thursday, found that prior infection provided at least 83 per cent protection against reinfection. It gave better than 94 per cent protection against symptomatic Covid-19, matching the figures for the most effective Covid-19 vaccines.
Susan Hopkins, PHE senior medical adviser, said she was “strongly encouraged” by the finding that infection gave powerful — though not complete — protection against reinfection for at least five months.
“Natural infection looks as good as a vaccine, which is very good news for the population,” she said.
Although the study could not provide data on possible protection beyond five months, Prof Hopkins was optimistic that it would last for “much longer than the few months people were speculating about” during the early stages of the pandemic last year.
“It will give a level of immunity in the community that will reduce transmission,” she said.
During the study 44 people in the previously infected group of 6,000 tested positive at least three months apart, suggesting that they had been “potentially reinfected”. But because genomic analysis was not available to confirm that different viruses were responsible for the two infections, they were not considered proven reinfections. The same virus might have been incubated over a long period within the same individual, though the researchers thought this was less likely in most cases.
Eleanor Riley, professor of virology at the University of Edinburgh, said the study data also suggested that people who had recovered from Covid-19 were less likely to transmit the virus to others unknowingly, because natural infection appeared to provide about 75 per cent protection against asymptomatic reinfection. “This is good news in terms of the long-term trends of the pandemic,” she said.
Still, Prof Hopkins urged people “not to misunderstand these early findings”.
“If you believe you already had the disease and are protected, you can be reassured it is highly unlikely you will develop severe infection but there is still a risk that you could acquire an infection and transmit to others,” she said.
The cut-off point for the preliminary analysis in late November came too soon for the researchers to investigate how well vaccines — the first of which were approved for use in the UK in December — protected health workers in the study group.
The researchers were also unable to assess the impact of the new and more contagious B.1.1.7 variant on rates of reinfection. PHE aims to expand the research project over the next three months to include 100,000 health workers and consider both of those questions.