One of the persistent issues of the COVID-19 pandemic is the amount of immunity that people have after recovering from a coronavirus infection. New research suggests that the level of protection is comparable to that of getting a vaccine – at least for a few months.
Among a group of hundreds of thousands of Americans who tested positive for SARS-CoV-2 infection, the risk of developing a subsequent infection more than three months later was about 90% less than for people who had not previously been infected and therefore without immunity to the virus, according to researchers at the National Cancer Institute.
For 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 on Wednesday in JAMA Internal Medicine, may help inform plans for workers to return to their offices, sending students and teachers back to school campuses and allowing more savings to be reopened.
“I think we knew that, that immunity [after natural infection] it lasts a long time, ”said Dr. Monica Gandhi, an infectious disease specialist at the University of California at San Francisco who was not involved in the new research. “But it is still very exciting.”
There are three important things that scientists need to know to understand the biological value of antibodies against coronavirus, said Dr. Mitchell H. Katz, who heads New York’s hospitals and health care. They are: Do antibodies protect against infections? Can they be reliably detected with current tests? And, if they offer any protection, how long does it last?
The new study “provides reassuring answers to the first and second questions,” wrote Katz in a note from the editor who accompanied the study.
To investigate coronavirus immunity, cancer researchers examined the results of more than 3 million blood tests administered to Americans between the start of the pandemic and 23 August. A total of 378,606 of these 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 – subsequently did a different test to look for evidence of the genetic material of coronavirus in patient samples, which are typically collected through the nose, throat or saliva.
The researchers used these results to see if people who had a coronavirus infection were less likely than their uninfected counterparts to have SARS-CoV-2 particles in their system. For their analysis, they classified 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 antibodies to the coronavirus, subsequently tested positive with the genetic test. This occurred in all four time intervals: 0 to 30 days, 31 to 60 days, 61 to 90 days and more than 90 days. The consistency was probably a reflection of the relatively stable rate at which people in their communities were currently being infected, the researchers said.
Compare that to people who originally tested positive for antibodies to coronavirus. The results of the genetic tests were positive at very high rates in the first 30 days (11.3%), which the researchers said was probably a sign that the remaining viral particles were still being eliminated from their systems.
However, the rate of positive genetic testing plummeted to 2.7% in the second month after infection and dropped to 1.1% in the third month. And after those 90 days, only 0.3% of people with a previous coronavirus infection had another infection detected with a genetic test.
This rate of coronavirus infection was 10 times lower than for people who had probably not been infected previously.
This level of protection appears to be comparable to the benefits offered by the Pfizer and Moderna vaccines in their clinical trials, the study authors pointed out.
“Of course, protection induced by a safe vaccine is clearly preferable,” they were quick to add, “since the population risk of a serious result from an authorized or approved vaccine must be orders of magnitude less than the natural infection.”
While the findings may be of great interest to scientists, they are unlikely to make much of a practical difference at this stage of the pandemic, said Gandhi. That’s because vaccines appear to be at least as protective as a past infection and are already being launched.
These findings “could have been used two months ago,” she added.
Although this study did not measure how long the benefits of a previous infection last beyond 90 days, other lines of evidence suggest that it takes time to decrease. For example, Gandhi pointed to an article in Science that found that the immune cells triggered by SARS-CoV-2 remained in the body for at least eight months.
The findings may help explain why new infections are falling in Los Angeles County in the wake of a devastating increase in vacations. Dr. Roger Lewis, director of hospital demand modeling COVID-19 for the LA County Department of Health Services, estimated that about 1 in 3 people in the county now have coronavirus immunity.
Still, the study’s authors said more research would be needed to get a clearer picture of natural immunity to coronavirus.
“Factors that influence the risk of reinfection – such as variable viral strains, the patient’s immune status or other characteristics of the patient’s level – should be evaluated in subsequent studies that include follow-up beyond 90 days,” they wrote.