There are some people who may not be vaccinated, for medical reasons. These people are protected from deadly and debilitating diseases by herd immunity. This means that enough people are vaccinated/immune, so the people who cannot be have a low chance of someone else spreading it to them.
Measles is a great example of this. Before vaccinations, measles killed and estimated 2.6 million people a year. Now, measles kills only 122,000 people a year. Unfortunately, measles is still one of the leading causes of death among young children, even though a safe and cost-effective vaccine is available. The reason for this is because young children are too young to be vaccinated , and are vulnerable. They depend on all who are able to be vaccinated.
Vaccinations work. Mandate them.