They initially ran antibody tests and discovered that some people were being re-infected with the virus. Now saying that is not the case. This is a big deal and good news for the likelihood of herd immunity.
https://www.businessinsider.com/cor...activated-cases-not-reinfected-experts-2020-4
https://www.businessinsider.com/cor...activated-cases-not-reinfected-experts-2020-4