Having had COVID-19 once now looks like decent armor against catching it again, at least for a while. A Public Health England study found 83% protection from reinfection, and that held for at least 5 months after a person first became sick. The catch: people with that protection may still carry the virus and pass it on.
The SIREN study (SARS-CoV-2 Immunity and Reinfection EvaluatioN) has regularly tested healthcare workers across the UK since June for new infections and for antibodies, which suggest a past infection. It has covered 20,787 staff from 102 NHS trusts, clinical and non-clinical. On recruitment, 6,614 of them tested positive for antibodies.
Between 18 June and 24 November, scientists spotted 44 potential reinfections in that antibody-positive group: 2 classed as “probable” and 42 as “possible,” depending on the confirming evidence. If all 44 were confirmed, protection works out to 83%. If only the 2 probable cases held up, it would be 99%. Work continues to narrow the range.
Both probable cases reported symptoms during the first wave without being tested then, and both said their symptoms were milder the second time. None of the 44 had a PCR test in the first wave, but all tested positive for antibodies at recruitment.
Protected, but maybe still infectious
Early evidence from the next stage of the study suggests some antibody-positive people carry high levels of virus in the nose and throat. PHE therefore urged everyone to keep following the rules, even those who have already had COVID-19. The agency noted that cases were at their highest level to date and the R number was above 1 across the country.
Professor Susan Hopkins, PHE’s Senior Medical Advisor and the SIREN study lead, said: “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. Crucially, we believe people may still be able to pass the virus on.”
Many people who caught the disease in the first wave may be vulnerable again now, since protection lasts 5 months on average and researchers are still checking whether it goes beyond that. The analysis predates the wide spread of the new variant VOC202012/01, so whether past infection guards against it is under laboratory study. The study will follow participants for 12 months. This first report says nothing about vaccine responses; those come later.
Experts split on how to read the numbers
The results were released as a preprint, not yet peer reviewed, and outside scientists read them differently.
Prof Eleanor Riley of the University of Edinburgh saw good news: “a primary infection with SARS-CoV-2 provides at least 94% protection against symptomatic reinfection for at least 5 months.” She put protection against asymptomatic reinfection at about 75%, but warned that you cannot assume you are not infectious because you have had the virus.
Prof Paul Hunter of the University of East Anglia took the opposite view of the same data. “What this shows is that reinfection is common even in this short time period,” he said, and he expects the risk to grow with longer follow-up. Only 22% of people without a previous infection were asymptomatic, compared with 66% of those with one.
Prof Danny Altmann of Imperial College London calculated protection of 76-87% over 5 months. He contrasted the 44 UK reinfections with a Qatar study that found 243 reinfection cases among 133,000 people, or 0.18%. Dr Julian Tang of the University of Leicester called the result reassuring for healthcare workers, with over 80% immunity to reinfection for at least 5 months, and said that having the vaccine after recovery is not a problem.
Sources
- Past COVID-19 infection provides some immunity but people may still carry and transmit virus
- roundups & rapid reactions
