Coronavirus infection leads to immunity that’s comparable to a COVID-19 vaccine — at first

One of the enduring questions of the COVID-19 pandemic is how much immunity people are left with after recovering from a coronavirus infection. New research suggests the level of protection is comparable to getting a vaccine — at least for a few months.

Among a group of hundreds of thousands of Americans who tested positive for a SARS-CoV-2 infection, the risk of developing a subsequent infection more than three months later was about 90% lower than for people who had not been previously infected and therefore had no immunity to the virus, according to researchers from the National Cancer Institute.

For the sake of comparison, when the vaccines made by Pfizer-BioNTech and Moderna were tested in Phase 3 clinical trials, they reduced the risk of developing COVID-19 by at least 94%.

The findings, published Wednesday in JAMA Internal Medicine, could help inform plans for returning workers to their offices, sending students and teachers back to school campuses and allowing more of the economy to reopen.

“I think we knew this, that immunity [after natural infection] lasts a long time,” said Dr. Monica Gandhi, an infectious-disease specialist at UC San Francisco who was not involved in the new research. “But it’s still very exciting.”

There are three important things scientists need to know to understand the biological value of coronavirus antibodies, said Dr. Mitchell H. Katz, who leads NYC Health and Hospitals. They are: Do antibodies protect against infection? Can they be reliably detected with current tests? And, if they do offer some protection, how long does it last?

The new study “provides reassuring answers to the first and second questions,” Katz wrote in an editor’s note that accompanied the study.

To investigate coronavirus immunity, the cancer researchers examined the results of more than 3 million blood tests administered to Americans between the start of the pandemic and Aug. 23. A total of 378,606 of those tests were positive for SARS-CoV-2 antibodies — a sign that the person who provided the sample had an active coronavirus infection.

Among the millions of people who were tested, some — about 11% of those who tested positive and 9.5% of those who tested negative — later took a different test to look for evidence of the coronavirus’ genetic material in patient samples, which are typically gathered via the nose, throat or from saliva.

The researchers used these results to see whether people who’d had a coronavirus infection were any less likely than their uninfected counterparts to have SARS-CoV-2 particles in their system. For their analysis, they sorted the results into four groups based on the gap between the antibody test and the genetic test.

After running the numbers, the researchers found that between 3% and 4% of those who originally tested negative for coronavirus antibodies later tested positive with the genetic test.