Among more than 3.2 million people in the United States who took a Covid-19 antibody test, those who tested positive were far less likely to be diagnosed with the virus later. Of the antibody-positive group, 0.3% were later found infected, beyond 90 days. Among those who tested negative, 3% were.
The study appeared Wednesday in JAMA Internal Medicine. Its authors came from the National Cancer Institute and the companies LabCorp, Quest Diagnostics, Aetion Inc. and HealthVerity. They looked at test results from January through August of last year: 11.6% of people came back positive for antibodies and 88.3% negative.
Dr. Douglas Lowy, principal deputy director of the National Cancer Institute and one of the authors, called the gap a 10-fold reduction, then added a warning. “I would have caveats around that,” he said. “In other words, it could be an overestimate of the reduction. It could be an underestimate of the reduction.”
“To me, the big message is — there’s a reduction,” Lowy said. “The main takeaway is that being antibody positive after natural infection is associated with partial protection against a new infection.”
What the study cannot say
It is an observational study. It shows an association between a positive antibody result and a lower chance of infection about 90 days later, but it cannot prove cause, and more research is needed on that point. Nobody yet knows how long any protection from antibodies lasts.
Variants are another open question. “Now that there are these variants of concern, what are the implications there? The short answer is we don’t know,” Lowy said.
His advice for people who have recovered, or who tested positive for antibodies, was the same: get vaccinated when it is your turn.
An editorial in the same journal issue, by Dr. Mitchell Katz of the NYC Health + Hospitals health care system, struck a similar note. Most patients who recover from Covid-19 have antibodies, and reinfection appears rare so far. But it remains unclear for “how long antibody protection will last because of natural infection,” Katz wrote.
“For this reason, vaccination against SARS-CoV-2 is recommended regardless of antibody status,” he wrote. “How long the antibody protection provided by vaccines will last is also unknown. To know how long protection will last with antibodies because of natural infection or vaccination is something only time will tell.”
