Bethesda, MD—People who were previously infected with SARS-CoV-2, the virus that causes coronavirus-19 (COVID-19), maintain meaningful levels of protection against reinfection, at least for a few months, according to a new study.

The JAMA Internal Medicine report suggests that finding could explain why reinfection appears to be relatively rare. Perhaps more importantly, it could help public health officials determine when it is safe to return to physical workplaces and schools, as well as help prioritize vaccine distribution, according to researchers from the National Cancer Institute (NCI).

The study team conducted an observational descriptive cohort study of clinical laboratory and linked claims data; their goal was to evaluate evidence of SARS-CoV-2 infection based on diagnostic nucleic acid amplification test (NAAT) among patients with positive versus negative test results for antibodies.

Researchers created cohorts from a deidentified data set including commercial laboratory tests, medical and pharmacy claims, electronic health records, and hospital chargemaster data. Patients then were categorized as antibody-positive or antibody-negative according to their first SARS-CoV-2 antibody test in the database.

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.

For purposes of the study, primary end points were post-index diagnostic NAAT results, with infection defined as a positive diagnostic test post-index, measured in 30-day intervals (0-30, 31-60, 61-90, >90 days). Researchers also tracked demographic, geographic, and clinical characteristics at the time of the index antibody test, including recorded signs and symptoms or prior evidence of COVID-19 diagnoses or positive NAAT results and recorded comorbidities.

Ultimately, the cohort included 3.25 million unique patients—56% female with a median age of 48 years—with an index antibody test between January 1, 2020, and August 23, 2020. That represented more than half of all commercial SARS-CoV-2 antibody tests conducted in the United States during that time period.

The study found that patients with a negative antibody test result tended to be older than those with a positive result—mean age 48 years versus 44 years. Of index-positive patients, 18.4% converted to seronegative over the follow-up period.

Results indicate that, during the follow-up periods, the ratio (95% CI) of positive NAAT results among individuals who had a positive antibody test at index versus those with a negative antibody test at index was 2.85 (95% CI, 2.73-2.97) at 0 to 30 days, 0.67 (95% CI, 0.6-0.74) at 31 to 60 days, 0.29 (95% CI, 0.24-0.35) at 61 to 90 days, and 0.10 (95% CI, 0.05-0.19) at more than 90 days.

“In this cohort study, patients with positive antibody test results were initially more likely to have positive NAAT results, consistent with prolonged RNA shedding, but became markedly less likely to have positive NAAT results over time, suggesting that seropositivity is associated with protection from infection,” the authors concluded. “The duration of protection is unknown, and protection may wane over time.”

“The data from this study suggest 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,” explained Lynne Penberthy, MD, MPH, 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.”

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