https://wwwnc.cdc.gov/eid/article/27/9/21-1042_article https://wwwnc.cdc.gov/eid/article/27/9/21-1042_article > one scenario is that their immune system mounte

https://wwwnc.cdc.gov/eid/article/27/9/21-1042_article https://wwwnc.cdc.gov/eid/article/27/9/21-1042_article > one scenario is that their immune system mounted a strong nasal/mucosal response in the upper respiratory system, based on exposure to prior coronaviruses. Another scenario is they got a small dose and their innate immune system worked. A bigger dose would just restart the process from scratch. I'm not claiming the vaccine is a miracle cure, but it's layer #1 of a defense in depth strategy. In any case, studies have shown infection + vaccine is even better.