They probably are immune due to repeated bits of infection, a normal phenomenon denied all of us who were locked down and deprived of any normalcy. There is such a thing as too much cleanliness.

That's why children are mostly immune; they touch everything, wash hands only when prompted, and nature builds their immunity just as it did before anybody ever heard of a vaccine. I have several times seen mothers pick up a pacifier in a grocery store or on a sidewalk and pop it right back into the baby's mouth.