The study found that out of the 1,416 measles cases reported in the U.S. between 2000 and 2015, more than half (56.8%) were in unvaccinated individuals. And among those (for whom detailed medical records and the reasons for non-vaccination were available) 70% had a non-medical exemption from parents. More importantly, most of these unvaccinated cases make up the very dangerous "first wave" of the epidemic, Omer says, which is what triggers a mass spread.
Measles is a highly contagious disease spread through coughing and sneezing, which can cause a high fever, cough, red and watery eyes, mouth sores, and a rash. It usually goes away within three weeks, but in vulnerable patients it can cause pneumonia, brain swelling, and even death. The vaccine is 97% effective at preventing measles, so in rare cases vaccinated people can still be infected. "This is why vaccine refusal is so risky, because no vaccine is 100% effective even if everyone does the right thing and gets it," Omer says.