People having SARS-CoV-2, the virus that causes covid-19, may have protection against future infection for a few months, a research published in the JAMA Internal Medicine showed.
The findings could have important public health implications, including decisions about returning to physical workplaces and schools, and prioritising vaccine distribution.
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Researchers from the National Institutes of Health, US, in collaboration with two health care data analytics companies, HealthVerity and Aetion, Inc, and five commercial laboratories obtained antibody test results for more than 3 million people who had a SARS-CoV-2 antibody test between January 1 and August 23, 2020.
This represented more than 50% of the commercial SARS-CoV-2 antibody tests conducted in the United States during that time. Nearly 12% of these tests were antibody positive; most of the remaining tests were negative, and less than 1% were inconclusive.
Antibody tests — also known as serology tests — detect serum antibodies, which are immune system proteins made in response to a specific foreign substance or infectious agent, such as SARS-CoV-2.
About 11% of the seropositive individuals and 9.5% of the seronegative individuals later received a nucleic acid amplification test (NAAT) — sometimes referred to as a PCR test — for SARS-CoV-2.
The research team looked at what fraction of individuals in each group subsequently had a positive NAAT result, which may indicate a new infection. The study team reviewed the results at several intervals — 0-30 days, 31-60 days, 61-90 days, and over 90 days because some people who have recovered from a SARS-CoV-2 infection can still shed viral material for up to three months (although they likely do not remain infectious during that entire period).
During each interval, between 3% and 4% of the seronegative individuals had a positive NAAT test. But among those who had originally been seropositive, the NAAT test positivity rate declined over time.
When the researchers looked at test results 90 or more days after the initial antibody test, only about 0.3% of those who had been seropositive had a positive NAAT result — about one-tenth the rate in those who had been seronegative.
“The data from this study suggested that people who have a positive result from a commercial antibody test appear to have substantial immunity to SARS-CoV-2, which means they may be at lower risk for future infection,” said Lynne Penberthy, associate director of NCI’s Surveillance Research Program, who led the study. “Additional research is needed to understand how long this protection lasts, who may have limited protection, and how patient characteristics, such as comorbid conditions, may impact protection. We are nevertheless encouraged by this early finding.”
Although these results support the idea that having antibodies against SARS-CoV-2 is associated with protection from future infection, the authors note important limitations to this study.
The findings come from a scientific interpretation of real-world data, which are subject to biases that may be better controlled for in a clinical trial. For example, it is not known why people who had tested antibody positive went on to have a PCR test.
In addition, the duration of protection is unknown; studies with longer follow-up time are needed to determine if protection wanes over time.
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