Your innate system is probably very strong and/or you got a low dose. If you are young your innate system can handle C19. That is why kids dont get sick. Also mucosal immune system is separate from systemic. This is why muscular vaccines that help systemic system keep you from getting sick but do not stop virus growth and spread in mucosal areas (nose, throat). So you may have a very strong innate mucosal immune system against coronaviruses.