Immune responses from a previous SARS-CoV-2 infection reduce survivors’ risk of reinfection by more than 83 percent for at least five months, according to preliminary data from a study of more than 20,000 UK healthcare workers published by Public Health England. The researchers caution that people previously infected may still be able to transmit the virus.
“Overall I think this is good news,” Imperial College London epidemiologist Susan Hopkins, a senior medical adviser to Public Health England (PHE), tells The Guardian. “It allows people to feel that prior infection will protect them from future infections, but at the same time it is not complete protection, and therefore they still need to be careful when they are out and about.”
Between June and November last year, the researchers monitored, through monthly serological tests and PCR tests twice a month, the infection rates in those who had been infected with the virus before June and those who had not. They found 44 potential reinfections, including 13 symptomatic cases, among the 6,614 who’d had the virus before, and 318 cases among the 14,173 who had no evidence of past infection. The authors concluded from these results that prior exposure to SARS-CoV-2 provides 94 percent protection against symptomatic reinfection, and 75 percent protection against asymptomatic reinfection.
The researchers also found that people who become reinfected can carry a high viral load in their noses and throats, even in asymptomatic cases, which correlates with a higher risk of spreading the virus to others, says Hopkins.
“Reinfection is pretty unusual, so that’s good news,” University of Pennsylvania immunologist John Wherry tells Nature. “But you’re not free to run around without a mask.”
“The immunity gives you a similar effect to the Pfizer vaccine and a much better effect than the AstraZeneca vaccine and that is reassuring for people. But we still see people who could transmit and so we want to strike a note of caution,” Hopkins tells The Guardian. In clinical trials, two doses of the Pfizer vaccine were 95 percent effective at preventing infection, compared with 62 percent from two doses of the Oxford/AstraZeneca vaccine. The Pfizer vaccine has been approved for emergency use in the UK, Canada, Mexico, the US, Switzerland, and the EU, while the Oxford/AstraZeneca vaccine has been approved for emergency use in the UK, Argentina, India, and Mexico.
None of the individuals with potential reinfections had PCR-based evidence of a first infection, but all harbored antibodies against SARS-CoV-2 at the outset of the study, which has not yet been peer-reviewed or published in any medical journal. The authors tried to measure antibodies that were specific to SARS-CoV-2, but prior studies show that antibodies against other coronaviruses may cross-react to show a false positive result for SARS-CoV-2 antibodies, which is why the researchers termed these cases “possible reinfections.”
The PHE team does not yet have enough data to understand who might be at highest risk of reinfection. Francis Crick Institute immunologist George Kassiotis tells Nature that participants in the study were mainly women, and mostly under the age of 60. “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.”
Correction (January 15): We omitted Mexico from the list of countries that have approved the Pfizer vaccine. The Scientist regrets the error.