People who have already been infected with the coronavirus are as protected against reinfection as those who have received the best Covid-19 vaccines, according to a survey of 20,000 British health workers, the largest in the world so far.
Public Health England regularly tests two matching groups of volunteers between June and November – 6,000 health workers who were previously infected with the coronavirus and 14,000 who were not.
A comparison of the infections in the two groups, described in preliminary results published on Thursday, found that the previous infection provided at least 83% protection against reinfection. It provides better than 94 percent protection against symptomatic Covid-1
Susan Hopkins, a senior PHE medical adviser, said she was “strongly encouraged” by the finding that the infection provided powerful – though incomplete – protection against reinfection for at least five months.
“The natural infection looks as good as the vaccine, which is very good news for the population,” she said.
Although the study could not provide data on possible protection after five months, Professor Hopkins was optimistic that it would last “much longer than the few months that people have been speculating about” during the early stages of last year’s pandemic.
“This will give a level of immunity in the community that will reduce transmission,” she said.
During the study, 44 people from the previously infected group of 6,000 tested positive at least three months apart, suggesting they were “potentially reinfected.” However, as genomic analysis was not available to confirm that the different viruses were responsible for the two infections, they were not considered proven reinfections. The same virus may have been incubated for a long time within the same individual, although researchers believe this is less likely in most cases.
Eleanor Riley, a professor of virology at the University of Edinburgh, said the study also showed that people who recovered from Covid-19 were less likely to pass the virus on to others unknowingly because the natural infection appeared provides about 75% protection against asymptomatic reinfection. “This is good news in terms of the long-term trends of the pandemic,” she said.
Still, Professor Hopkins urged people “not to misunderstand these early discoveries.”
“If you think you are already sick and protected, you can be sure that you are very unlikely to develop a severe infection, but there is still a risk of contracting the infection and passing it on to others,” she said.
The cut-off point for the preliminary analysis at the end of November came too early for researchers to examine how well the vaccines – the first of which was approved for use in the UK in December – protected health workers in the study group.
The researchers also failed to assess the impact of the new and more contagious variant B.1.1.7 on the rate of reinfection. PHE aims to expand the research project over the next three months to include 100,000 health professionals and address both issues.