Only 0.7 percent of children aged 19 to 35 months received absolutely no vaccinations in 2013, according to the National Immunization Survey. But that number masks some of the lower levels of uptake for some vaccines; about 92 percent of children that age, for example, received a dose of the MMR (measles, mumps, and rubella) vaccine.
Some parents forgo vaccines for economic reasons. Maybe they can’t afford to make the trip to the doctor or they don’t have insurance that would pay for vaccines. That helps explain why, generally, vaccination rates are slightly lower for children living in poverty compared to those who aren’t.
And a really small segment of the population skips vaccines for medical reasons — if, for example, a child has a compromised immune system that couldn’t handle vaccines.
Others opt out of childhood vaccine requirements for school children for religious or philosophical reasons. This map, made by Mother Jones with CDC data, estimates non-medical exemption rates across the United States for the 2012–2013 school year:
This map doesn’t capture the variation within states. Some communities might have much lower vaccination rates than the rest of the state. It’s these types of clusters that most worry public health officials, since disease outbreaks can take off there and eventually begin trickling out to other parts of the country. (This is what happened during the 2014 Ohio Amish measles outbreak and the 2014–2015 Disneyland measles outbreak.)
These outbreaks in small communities exemplify why it’s so important for everyone who is able to get vaccinated to do so. If enough people are inoculated, the population reaches a coverage threshold known as herd immunity that prevents illnesses from spreading and turning into an epidemic.