With the COVID-19 pandemic causing blood shortages, some blood banks now offer routine antibody tests to attract donors.
A new research letter in JAMA uses those test results to estimate current herd immunity levels.
Herd immunity is an indirect defense against infection that occurs when enough people are immune to a disease to strongly inhibit its spread.
That safeguard depends on factors like a virus's transmission rate and the antibody levels required to confer protection.
When researchers measured those levels in almost 1 million American Red Cross blood donations, they found less than 2% contained SARS-CoV-2 antibodies.
Although not a randomized sample, the research suggests only a small portion of Americans have been exposed to the virus and, short of widespread vaccinations, herd immunity lies a long way off.
