There are other ways to have Covad immunity than getting the virus and recovering from it. Many people have T Cell immunity already: https://medicalxpress.com/news/2020-06-immunity-covid-higher-shown.html And then there may be a genetic immunity. Europeans received that immunity by going through 2 Bubonic Plagues. Around 10% of Europeans carry a mutation that prevents HIV from entering cells and damaging their immune system. Herd immunity may be closer than realised.