Other scenarios have already been given, but one risk to be considered is younger siblings of classmates who are not yet vaccinated. Not far from where I live,

Other scenarios have already been given, but one risk to be considered is younger siblings of classmates who are not yet vaccinated. Not far from where I live, a 3-month-old baby died of whooping cough. The DTaP vaccination against pertussis is given at age of 3 months, so the baby had no immunity yet; some scolded the baby's parents for being anti-vaxxers but that was cruel - they were simply following the standard vaccination schedule. The legitimately not-yet-immunized baby was killed by being exposed to someone who most likely had refused vaccination. This is difficult to prove, of course, and case details are not public.