Vaccine Misinformation is Dangerous
You might be reading this because you’ve got questions about vaccines after reading something elsewhere. There’s a lot of misinformation out there. Let’s start with the impact that not vaccinating one person can have on others.
Vaccines are most effective when everyone is vaccinated. But not everyone can be. Infants, for example, are too young, while people with damaged immune systems such as cancer patients also can’t be vaccinated. Fortunately, if a high enough percentage of everyone else has been vaccinated, spreading diseases such as the measles to people who haven’t been vaccinated becomes unlikely. This is called herd, or community, immunity.
Herd immunity is achieved when about 95% of the population are vaccinated against a disease like the measles. That’s how countries like Canada managed to become nearly measles-free, with only a very small number of new cases. Almost all of those cases were caught by people travelling abroad. When people stop vaccinating themselves and their children, however, herd immunity is lost and diseases spread like wildfire to people too young or too sick to be vaccinated.
Measles is not a disease to take lightly. Nine out of 10 people who have not been vaccinated can be expected to catch it. In 2017, it killed an estimated 110,000 people around the world, and in developing countries about one in 100 children who catch measles will die from it.

Source: https://www.cdc.gov/globalhealth/measles/globalmeaslesoutbreaks.htm