Blood donations show that the United States is still nowhere near herd immunity

To better understand how widely the coronavirus has spread in the United States, some researchers are turning to an unusual source of data: blood donations.

In an effort to encourage more donations, many blood collection centers have been offering to test donated blood for antibodies to the coronavirus, which indicates a past infection with SARS-CoV-2, the virus that causes COVID-19. Of the nearly 1 million Americans who donated blood to the Red Cross from June 15 to August 23 and were tested, only 1.82 percent had the antibodies. That finding suggests that the vast majority of Americans have yet to be infected with the virus, researchers report September 14 in JAMA.

Blood donations aren’t a random sample of the population, but the data can give researchers an idea how much of a population has been exposed to the virus, a concept known as seroprevalence, and how susceptible different populations remain to continuing outbreaks.