People who have been infected with Covid-19 have 83% less chance of being infected again, according to a study of health care workers in the United Kingdom.
However those conclusions come with a caveat: the study has only been running for five months, so is unable to predict what effect might be felt over a longer time period.
Since the start of the pandemic, stories have emerged in the media of people becoming reinfected with the virus, while a previous infection should have conferred immunity. That has led to doubts in some quarters as to the efficacy of a vaccine and how long immunity might last after vaccination.
The study by Public Health England looked at 20,000 health care workers, 6,600 of whom had previously been infected, and found that only 1% from that group became infected a second time. Some of them, however, carried a large viral load in the nose and throat, making it possible they could infect others, despite having no symptoms themselves.
“Reinfection is pretty unusual, so that’s good news,” said immunologist John Wherry of the University of Pennsylvania in Philadelphia, who also took part in the study.
“But you’re not free to run around without a mask.”
According to Susan Hopkins, a senior medical adviser at Public Health England who leads the study, the interim results suggest that natural immunity of 83% is almost as effective as the 90% or so achieved by vaccines – at least over a period of five months.
The study, named SARS-CoV-2 Immunity and Reinfection Evaluation (SIREN), is ongoing, and the largest study of Covid reinfection, with systematic screening for asymptomatic reinfection.
Every two to four weeks, participants have blood tests for SARS-CoV-2 antibodies, as well as a PCR test to detect the virus itself.
Over the 5 months, the team found 44 reinfections labelled ‘possible’. In the group of 14,000 participants who had not been previously infected, 318 people tested positive for the virus.
The possible reinfections are subjected to further testing, with only two of the 44 found so far being re-graded to ‘probable’.
In the meantime, the study will turn its attention to the so-called British variant officially labelled B.1.1.7, to determine whether the protection offered by SARS-CoV-2 also works against new variants – a question of major importance to vaccine development.