A steady uptick in the number of measles cases in the U.S. in recent years has primarily been driven by people who haven’t been vaccinated.

Those are the findings of a new report published in JAMA Pediatrics which found that measles, which was eradicated nationally in 2000, has made a return largely because of parents who have refused to vaccinate their children.

In the latest findings, researchers from the Centers for Disease Control and Prevention analyzed 1,789 measles cases among U.S. residents reported to the CDC from January 2001 through December 2015. They found that nearly 70 percent, or 1,243 individuals, were unvaccinated. Babies and toddlers had the least protection. Of 163 infants ages 6 to 11 months who became sick, only two had been vaccinated. Among 106 toddlers ages 12 to 15 months, 95 were unvaccinated.

The study found that the rate of measles has accelerated with time, with 667 cases recorded in 2014, a record since it was declared eradicated. The authors’ works shows another troubling trend: the proportion of cases caused by unvaccinated travelers from overseas has declined over time, with just 15 percent of cases imported to the country in 2015 compared to 47 percent in 2001.