The SIREN study included thousands of UK healthcare professionals who were tested regularly for COVID-19.Credit: Kevin Coombs / Reuters
Most people who contract and recover from COVID-19 are likely to be immune for several months after that, a study of more than 20,000 health professionals in the UK concluded.
The study – published on the prepress server medRxiv on January 14 – found that immune responses from previous infections reduce the risk of getting the virus again by 83% for at least five months.
Over the past year, reports of repeated infections with the SARS-CoV-2 coronavirus have shaken confidence in the immune system’s ability to sustain its defenses against the virus. The study’s interim results, called SIREN, ameliorate some of those fears, Susan Hopkins, a senior medical consultant at Public Health England and principal investigator for the study, told a news conference. The data suggest that natural immunity may be as effective as vaccination, she added, at least during the five-month period the study has covered so far.
The data suggest that repeated infections are rare – they occurred in less than 1% of about 6,600 participants who had already fallen ill with COVID-19. But the researchers also found that people who are reinfected can carry high levels of the virus in their nose and throat, even when they have no symptoms. These viral loads have been linked to a high risk of transmitting the virus to others, Hopkins said.
“Re-infection is quite unusual, so that’s good news,” said immunologist John Wherry, of the University of Philadelphia, Pennsylvania. “But you are not free to walk without a mask.”
Regular screening
SIREN is the largest coronavirus reinfection study that systematically tracks asymptomatic infections, said Hopkins. Participants underwent blood tests for SARS-CoV-2 antibodies and PCR tests for the virus itself every two to four weeks.
Over the five months, the team found 44 possible reinfections. In the group of 14,000 participants who had not been previously infected, 318 people tested positive for the virus.
Some of the cases of reinfection are still being evaluated, Hopkins said. All 44 cases are considered “possible” reinfections and have been classified based on PCR tests combined with screening measures taken to reduce the risk of re-detection of the virus since the initial infection. So far, only two of those 44 cases have passed more stringent tests. The study did not assess whether symptoms were better or worse during the second infection compared to the first, but Hopkins notes that only 30% of people with possible reinfections reported any symptoms, compared with 78% of participants with initial infections.
At the moment, the team does not have enough data to find out who may be at greater risk of reinfection than others. And immunologist George Kassiotis of the Crick Institute in London notes that the study participants were mainly women and quite young. “This group is unlikely to experience the most severe form of COVID-19,” he says, “and may not be representative of the population as a whole.”
The study is ongoing and the researchers plan to collect data over a longer period of time, to get a sense of how long immunity lasts and to investigate the effects of a variant of SARS-CoV-2 called B.1.1.7 that emerged in 2020 and spread rapidly across the country. While there are many reasons to suspect that the existing protection should cover the new variants, it is possible that the immune responses generated against one variant are less effective against another, says Kassiotis. “It is still an open question.”