In one of the first large studies exploring immunity from natural infection with COVID-19, researchers in Denmark found some surprising trends—including evidence that even those who previously contracted the disease should still get vaccinated.
In the study, published March 17 in medical journal The Lancet, researchers took advantage of repeated coronavirus test results from about 4 million people in order to track how often those previously infected with SARS-CoV-2—the virus that causes COVID-19—were reinfected. Since the test results were tied to people’s health records, the scientists could anonymize the data and compare those who tested positive during Denmark’s first COVID-19 wave last spring with those who tested positive during a second wave in the autumn. That revealed key findings about reinfection rates in the country.
In the spring, 2% of the 530,000-plus people who were tested for SARS-CoV-2 were positive. Among them, 0.65% tested positive again later that year, compared to 3.3% of those who had tested negative during the first wave. That suggests infection with SARS-CoV-2 provides about 80% protection against reinfection. What’s more, there was no evidence that this immunity waned over the study’s six month follow-up period.
But when the researchers broke down the data by age, they learned that this protection wasn’t uniform—protection was only 47% for people over 65 years old.
“The result in the elderly surprised me,” says lead researcher Steen Ethelberg of the Statens Serum Institut, Denmark’s equivalent of the U.S. Centers for Disease Control and Prevention. “That was, ‘wow, this is actually important; we have to communicate this because you can obviously not count on being protected if you are a senior citizen and have the disease.’”
To adjust for the possibility that the tests were picking up the same infection twice, Ethelberg’s team analyzed the results with differing times between the testing. But it came up with similar results, showing previous infection provided about 80-82% protection from reinfection for most adults.
The team’s data was collected through last year, before major new SARS-CoV-2 variants emerged, so the findings don’t apply to immunity after infection from these strains. However, Ethelberg is planning to repeat the study with more recent data to see if similar patterns hold up for those infected with the variants.
For now, the findings strongly suggest that even if people over age 65 years have experienced COVID-19, they still need to be vaccinated, since their protection from their natural infection may not be sufficient on its own. “They need to take care, and shouldn’t believe that they are immune and still protect themselves,” says Ethelberg. While less urgent, that also goes for anyone of any age who has recovered from COVID-19. “It’s well known that coronavirus infection do not induce 100% immunity,” he says.
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