https://www.wired.com/story/new-covid-19-antibody-study-results-are-in-are-they-right/

Last month, residents of Santa Clara County, California, stuck at home and newly reacquainted with the Facebook scroll, may have noticed an unusual proposition pop up in their feed: a targeted ad from researchers at Stanford’s medical school offering blood antibody tests for Covid-19. The curious who clicked through were offered a plan to arrive at a parking lot somewhere in the county, where a volunteer would give them a quick finger prick through their car window.

So-called serological tests work differently from Covid-19 diagnostic tests, which require a nose or throat swab and look for viral RNA. Instead, they check a person’s blood for evidence of an immune response to the virus, which can be found even in people with no symptoms or those who have already recovered from the disease. Researchers have eagerly awaited those kinds of tests to gain insight into the true infection and fatality rates across age groups, and to help answer questions about things like asymptomatic spread and how long antibodies last—a key part of understanding how Covid-19 immunity might work. In early April, more than 3,000 people took the Stanford researchers up on their offer.