Measles, Mumps and Rubella are completely different from coronaviruses. As is the MMR vaccine will do nothing against COVID-19. However against SARS-1 they did modify it as a recombinant with SARS-1 and it produced SARS-1 antibodies.
The reason why American Samoa has no COVID-19 cases is because they completely locked down the territory and are not letting anyone enter.