Another more immediate use of antibody testing could be finding recovered patients to serve as donors of antibody-rich blood serum, so-called “convalescent plasma” or “convalescent serum,” which some scientists have argued could be used to treat the disease or to confer short-term immunity to the virus for high-risk workers. Data from clinical trials of the treatment will be available soon, but early reports from its use in severely ill patients are promising, and the idea has a century-old track record of safety and efficacy for treating infectious diseases. However, finding recovered patients who are eligible and available to donate can be a logistical challenge, and each donor can provide only enough serum for treating 2 or 3 patients, so its usefulness is limited by the number of prospective donors we can identify.

As of this writing, we know of only about 195,000 recovered patients in the United States — about one for every six currently ill patients — so finding large numbers of recovered patients through antibody testing could allow for this treatment to become more widely available for those who need it the most. The simple tests used in these surveys may not be enough to determine if people have sufficient levels of antibody in their blood to serve as donors, but it could at least help identify prospective donors who could receive follow-up tests to determine their eligibility. As long as there are millions of unidentified people who have recovered from Covid-19, we should not allow a shortage of donors to hold back the availability of convalescent plasma therapy.