Doctors reported the first cases of COVID-19 reinfection in August, confirming that surviving the coronavirus once wasn't a guarantee that you can't get it again.
But given the rarity of reinfection, we knew that the COVID-19 immunity must be robust — and we're now starting to learn exactly how robust.
Subscribe to Freethink for more stories like this.
On January 14, Public Health England (PHE) released the interim results of the largest study of COVID-19 reinfection to date, called SIREN.
According to the SIREN data, natural COVID immunity is about as effective as the best coronavirus vaccines at preventing symptomatic infections — 95% or more after five months — but that doesn't mean coronavirus survivors should let down their guard.
Studying COVID-19 Immunity
PHE recruited more than 20,000 U.K. healthcare workers for the SIREN study, including 6,600 who had already had COVID-19.
Every two to four weeks, they tested workers for both active COVID-19 infections and coronavirus antibodies — immune system proteins that would indicate that a person had already had a COVID-19 infection.
During the study, PHE discovered two "probable" cases of COVID-19 reinfection and 42 "possible" cases. Researchers said that the possible cases would need more testing to prove that the positive test result wasn't the lingering remnants of the original infection.
Assuming all 44 cases were reinfections, the COVID-19 immunity prompted by natural infection is about 94% effective at preventing a symptomatic case of the disease, putting it on par with the vaccines developed by Moderna and Pfizer.
Overall, natural immunity was 83% effective at preventing any second bout of COVID-19, and 75% effective against asymptomatic infection. Currently available vaccines haven't been evaluated for asymptomatic infections, so how natural COVID-19 immunity stacks up there is unknown.
"Everyone should consider themselves a potential source of infection."
The SIREN study wasn't without its limitations — because it was only five months long, we don't know how long natural immunity might last or whether it fades with time. However, researchers believe it is unlikely that such strong immunity will rapidly disappear.
Additionally, the study's participants were disproportionately women and people under the age of 60.
"This group is unlikely to experience the most severe form of COVID-19 and may not be representative of the population as a whole," George Kassiotis, an immunologist at the Francis Crick Institute, told Nature.
Attention, Coronavirus Survivors
The SIREN study isn't over yet, though. The researchers plan to follow-up with participants for 12 months, studying the duration of natural COVID-19 immunity and the effectiveness of vaccines.
Coronavirus survivors should be able to breathe easier knowing their chances of having a symptomatic reinfection are slim — but they should also keep doing that breathing behind a mask, for now, to avoid asymptomatic transmission.
"These data reinforce the message that, for the time being, everyone should consider themselves to be a potential source of infection for others and should behave accordingly," Eleanor Riley, an immunology professor at the University of Edinburgh, said in a statement.
We'd love to hear from you! If you have a comment about this article or if you have a tip for a future Freethink story, please email us at [email protected].