The mechanism of protection may be heterologous immunity. H.I is a consequence of an encounter with a specific antigen, which can alter the subsequent immune response to a different antigen. It can occur with infections or vaccination. It is fascinating how the Covid-19 virus spreads rampantly through adult communities but almost stops in its tracks at schools - it's like there is herd immunity already in the schoolyard. This was demonstrated in a study that came out a week ago in Australia.