Reinfection with COVID-19 is relatively rare, though more common in people older than 65, a large new Danish study finds.
Protection against reinfection appears to last more than six months, but the overall degree of protection does highlight the importance of the vaccination campaign.
Researchers led by Prof Christian Holm Hansen, of the Statens Serum Institut in Copenhagen, conducted a population-level observational study; results were published in The Lancet. During the pandemic’s first surge before June 2020, 533,381 people were tested for SARS-CoV-2 infection, and 11,727 of them (2.20%) were PCR positive.
Among eligible PCR-positive individuals from that first surge, 72 individuals (0.65%) tested positive again during the second surge between September and the end of the year. Of those who tested negative during the first surge, there were 16,819 people who tested positive (3.27%) during that second wave.
Protection against repeat infection was found to be 80.5%. However, this dropped dramatically with increasing age. The protection against repeat infection in those aged 65 and older was only 47.1%.
There was no evidence that protection against reinfection varied by sex, and it did not appear to wane over time, according to the authors.
“Since older people are also more likely to experience severe disease symptoms, and sadly die, our findings make clear how important it is to implement policies to protect the elderly during the pandemic,” said senior author Dr Steen Ethelberg.
In a linked commentary, Drs Rosemary Boyton and Daniel Altmann, of Imperial College London, noted that the degree of protection against reinfection is “more concerning… than offered by previous studies.”
The largest previous datasets on the topic came from Qatar, but it was based on a programme of testing in the context of symptomatic disease, while Denmark’s was a free testing program open to all. Thus, the PCR-positive cases in the Danish dataset are likely to include far more asymptomatic cases that likely offer more marginal levels of protective immunity.
Still, the new data do provide clarity on the path forward. “The quality, quantity, and durability of protective immunity elicited by natural infection with SARS-CoV-2 are poor relative to the much higher levels of virus-neutralising antibodies and T cells induced by the vaccines currently being administered globally,” Drs Boyton and Altmann wrote.
There is also no information in the Danish study regarding the effect of the novel variants now known to be circulating around the world.
“For SARS-CoV-2 the hope of protective immunity through natural infections might no be within our reach, and a global vaccination programme with high efficacy vaccines is the enduring solution.”