https://www.scientificamerican.com/article/what-covid-19-antibody-tests-can-and-cannot-tell-us/

Dozens of antibody tests for the novel coronavirus have become available in recent weeks. And early results from studies of such serological assays in the U.S. and around the world have swept headlines. Despite optimism about these tests possibly becoming the key to a return to normal life, experts say the reality is complicated and depends on how results are used.

Antibody tests could help scientists understand the extent of COVID-19’s spread in populations. Because of limitations in testing accuracy and a plethora of unknowns about immunity itself, however, they are less informative about an individual’s past exposure or protection against future infection.