Just under 2 percent of people who donated blood to the American Red Cross this summer had coronavirus antibodies, showing that very few people in the United States have been exposed to the virus that’s ravaged the country. The organization started offering antibody tests to donors in early June, and it published the results of the program in the Journal of the American Medical Association this week.

The Red Cross tested just under 1 million blood donations for antibodies over the course of the summer. When the antibody testing program started, about 1.18 percent of people who donated blood were positive. By the end of August, 2.58 percent of the people donating blood had antibodies.

Tracking the number of people with antibodies gives researchers and public health officials a window into how far the virus has spread. Many people with COVID-19 don’t have symptoms or aren’t able to get tested when they’re sick, so the number of cases identified through regular testing doesn’t capture the full extent of an outbreak.

These results are similar to those from other surveys in the US, which found that only a few percent of people in most areas have antibodies to the virus. Notable exceptions include New York City, which suffered a large outbreak and has high antibody rates. The Red Cross doesn’t collect much blood in New York City, which it says in its report might skew the study’s results.

When the Red Cross started offering antibody tests, the main research goal was to find out how many people in the US had been exposed to the virus. But the organization also hoped that the study would be an incentive to bring more people in to donate blood since donations had dropped off in the spring and early summer during the pandemic. The initial response showed it worked: after it offered antibody tests for two weeks, donor appointments jumped by 150 percent, a spokesperson told The Verge in June.

The number of donors who were giving blood for the first time jumped from 11 percent to 17 percent after the Red Cross introduced antibody testing as well. First-time donors were more likely to have antibodies than repeat donors: about 3 percent of the first-time donors had them. That could mean people who thought they’d been sick with COVID-19 were turning to blood donation as a way to find out if they had antibodies — which, like the limited collection in New York City, could have influenced the findings.

Blood banks are useful for research because they have a ready-made collection of blood samples available. (Donors usually sign off on their blood being used in studies as part of the consent process.) But blood donors aren’t fully representative of the general population: not everyone can donate, and donors have to be healthy. The Red Cross study is one picture of the antibody rates across the US, but like any survey, it has limitations.

The Red Cross plans to follow up with donors who had antibodies to see how their antibody levels change over time. Some studies have found that levels gradually decline, but people with lower counts may still be protected from future infection. The Red Cross is also participating in another blood donor antibody study, run by the Centers for Disease Control and Prevention, which will include other blood banks.