People over 65 are more likely to catch COVID-19 twice, study suggests, showing the limits of natural immunity

• Research from Denmark suggests that people over 65 might be less resistant to reinfection by COVID-19.
• The findings show that people need to get vaccinated even if they’ve already been sick, two experts said.
• The risk of reinfection remains very low, less than 1%, per the study.
• See more stories on Insider’s business page.

People over the age of 65 seem more likely to be reinfected with COVID-19, according to research published on Wednesday.

The study emphasized that the risk remains low, but is possible. Its authors said that it shows that people ought to seek vaccination even if they have already had the virus.

The authors found that the general population had around 80% protection from the virus after being infected once. This dropped to 47% for those over 65.

The study, published in The Lancet, looked at data collected in Denmark between February and December 2020. Around 4 million people took around 10 million PCR tests to check for COVID-19 infection.

The authors looked for the records of people who took PCR tests in both the first wave of COVID-19 in Denmark, between March and May, and during the second wave, between September and December.

They compared those who had two positive tests to those who only tested positive once.

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In line with previous findings, the authors found that the rate of reinfection was very low, lower than 1% among the general population. Antibodies gave around 80% protection which lasted at least 6 months, they concluded.

This compared to much lower protection among those aged 65 and older, around 47%, according to this study.

There are a few limitations to the study. It is also possible for instance, that the PCR tests picked up “lingering” traces of virus rather than a genuine reinfection.

Importantly, this study does not inform about the risk of reinfection by variants, some of which are thought to evade immunity.

The P.1. variant, which was first identified in Brazil, for instance, is thought to be able to reinfect people who have had the disease before.

This data is a confirmation, “if it were needed”, that natural protection from infection might not be good enough, Dr. Rosemary Boyton and Dr. Daniel Altman, immunologists at Imperial College London, said in a comment about the piece.

However, because their immunity has been primed by infection, some research suggests that those who have been infected before might only need one shot of two dose-regimen vaccines, such as Pfizer and Moderna, Insider’s Aria Bendix and Hilary Brueck reported.