The SIREN research examined the impact of infection on more than 20,000 volunteer health workers from across the UK and a pre-print of the study found only 44 cases of reinfection among 6,614 people.
Two groups of people, one with no evidence of previous infection and the other with evidence of past infection, were followed for up to six months.
The study — which has not yet been peer reviewed — concluded that past infection reduces the chances of catching the virus again by 83% for at least five months.
The cohort were tested regularly for Covid-19.
“About 6,000 of the healthcare workers were people who had evidence of having had SARS-CoV-2 infection… and about 14,000 of the healthcare workers were people who had no evidence of previous SARS-CoV-2 infection,” Tom Wingfield, senior clinical lecturer at the Liverpool School of Tropical Medicine, told the UK’s Science Media Centre.
“The findings suggest that re-infection rates in the positive cohort were 83% lower than the negative cohort during the follow-up period.”
But researchers warned that the protection was not total and that it was unclear how long any immunity lasts. It is also possible that those who have a degree of immunity against the virus may still be able to transmit it to others.
“We now know that most of those who have had the virus, and developed antibodies, are protected from reinfection, but this is not total and we do not yet know how long protection lasts,” Susan Hopkins, senior medical adviser at PHE and co-leader of the study, said according to Reuters.
“Even if you believe you already had the disease and are protected, you can be reassured it is highly unlikely you will develop severe infections. But there is still a risk you could acquire an infection and transmit (it) to others,” Hopkins said.
Hopkins emphasized the point during an interview with the BBC’s Today program on Thursday.
“[Infection] reduces your risk by at least 80%….but it doesn’t eliminate it,” she said.
“We found people with very high amounts of virus in their nose and throat swabs, that would easily be in the range which would cause levels of transmission to other individuals.” Hopkins stressed that people who had previously caught Covid-19 still needed to obey social distancing rules to avoid transmitting the disease.
England is currently under a stringent national lockdown after cases surged over the holiday period. The UK has recorded more than 3.2 million cases of infection.
“What [the study] really highlights is that immediately after infection you probably have a high level of protection but that will decay over time,” Niall Ferguson, a leading epidemiologist at Imperial College London, also told the BBC.
“Transmission now is slower than it would have been in the absence of people getting infected and those people who’ve had the virus before are at less risk of getting infected — and that cumulatively is slowing spread.”
“This study supports the hypothesis that primary infection… provides a high degree of immunity to repeat infection in the short to medium term; with similar levels of prevention of symptomatic infection as current licensed vaccines for working age adults,” the PHE report notes.
“Primary infection also reduces the risk of asymptomatic infection and thus onward transmission; this is particularly important in [that]… healthcare was considered as a potential driver for ongoing community transmission in Wave 1 in the UK.”
The researchers will continue to study antibody responses to infection and the impact of Covid-19 vaccines.