How common is COVID reinfection? People over 65 at much greater risk, study finds
A new analysis of millions of COVID-19 test results in Denmark found that natural protection against coronavirus reinfection lasts at least six months, with no evidence that it wanes within that time.
However, immunity is drastically lower for adults over 65, who were found to have only 47% protection against repeat infection compared to 80% for younger people, according to a study published Wednesday in the journal The Lancet.
Using only 2020 data, the researchers note their analysis focused on the original coronavirus strain, not the more contagious variants that have since emerged around the world.
The findings suggest that people who have already had COVID-19 should still get vaccinated, and they highlight the importance of protecting older adults with continued physical distancing and vaccine prioritization.
“Our study confirms what a number of others appeared to suggest: reinfection with COVID-19 is rare in younger, healthy people, but the elderly are at greater risk of catching it again,” study co-author Dr. Steen Ethelberg, a senior infectious disease researcher focusing on viruses that hop from animals to humans at the Statens Serum Institut in Denmark, said in a statement. “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.”
The researchers first studied data that compared reinfection rates during Denmark’s first and second wave of coronavirus surges. Among people who had COVID-19 during the first wave between March and May 2020, about 70 people were reinfected, or about 0.65%, during the second wave from September to December 2020.
The age differences were stark, however.
Only 0.60% of people under 65 were reinfected, while 0.88% of people aged 65 or older tested positive a second time.
Similar results were found when the researchers analyzed coronavirus test data throughout the entire pandemic last year, which included 2.5 million people. In all, people younger than 65 benefited from 80% protection against reinfection while those 65 and older had only 47% protection.
Imperial College London infectious disease professors Rosemary Boyton and Daniel Altmann, who were not involved in the study, said the findings were “relatively alarming.”
“Only 80% protection from reinfection in general, decreasing to 47% in people aged 65 years and older are more concerning figures than offered by previous studies,” they said, according to the statement. “These data are all confirmation, if it were needed, that for SARS-CoV-2 the hope of protective immunity through natural infections might not be within our reach and a global vaccination programme with high efficacy vaccines is the enduring solution.”
On the bright side, the researchers found that protection against reinfection remains stable for at least six months, but because the coronavirus only started infecting humans about a year ago, much remains unknown about how long immunity from natural infection lasts.
“The closely related coronaviruses SARS and MERS have both been shown to confer immune protection against reinfection lasting up to three years, but ongoing analysis of COVID-19 is needed to understand its long-term effects on patients’ chances of becoming infected again,” study co-author Dr. Daniela Michlmayr said in the statement.
Because detailed information about COVID-19 patients’ cases are only recorded if they are admitted to a hospital, the researchers said it wasn’t possible to assess whether the severity of someone’s symptoms affected their level of protection against reinfection.
This story was originally published March 19, 2021 at 3:03 PM with the headline "How common is COVID reinfection? People over 65 at much greater risk, study finds."
