Since the start of the year, a number of measles cases have been reported, many of which have been linked to trips to Disneyland during December. IFLScience first reported on this issue when there were only nine confirmed cases, adding an update last week when the total reached 67. The outbreak shows no signs of slowing down as numbers of confirmed measles cases continues to rise. As of the time of this writing, 87 cases have been reported across seven states and one case in Mexico. A quarter of the 71 cases in California have required hospitalization.

This current outbreak marks a devastating turn of events, as the United States had declared measles as eliminated in 2000. In 2014, however, there were 644 confirmed cases of measles in 27 states; more than had been seen in 20 years. This resurgence of a preventable disease has been attributed to parents opting out of vaccinating their children. Last week, it was discovered that 28 of the 34 cases with known vaccination records had not been immunized.

Not all of those who have been infected have declined vaccination; some may not have been old enough to do so. Since 1989, the MMR (Measles, Mumps, & Rubella) vaccine is delivered in two doses: one at 12-15 months, and another between ages 4-6, typically when children begin Kindergarten. Exceptions can be made for infants as young as 6 months if they are traveling out of the country and will be more at risk. Four of the infected children in California were younger than 12 months and deemed too young to be immunized. An additional 11 are between 1-4 years old, and might not have received the second dose.

However, six of those who became infected had been vaccinated. Two of the six had not received a second dose, but even one dose has been found to be 95% effective. The efficacy improves to 99% with a second dose. Though that’s incredibly high, it is not perfect. Considering how crowded the park is during December and the fact that measles is airborne, it’s not altogether surprising that some vaccinated individuals fell ill.

Local public health officials are currently advocating for parents to keep children away from crowded places where they could be exposed to international travelers, including theme parks and airports. Additionally, they are asking anyone who hasn’t been vaccinated, or whose immunization status is not known, to see their physician to check for immunity or receive the MMR vaccine.