Almost 20 years after measles was eliminated in the U.S., 2019 could see the highest rates of the dangerous disease in three decades, an expert has warned.

A combination of the efforts of the small but extremely vocal anti-vaccine movement and barriers to obtaining shots in some communities has sparked outbreaks of the potentially deadly—but entirely preventable—disease across the country.

In the decade before the deployment of the first measles vaccine in 1963, an estimated 3 million to 4 million people in the U.S. became infected with the disease. It killed between 400 to 500 people each year. An outbreak in school-age children in 1989 prompted immunologists and pediatricians to push for a countrywide immunization program. The campaign was a success: By 2000, the disease had been eliminated, defined as reduced to zero incidences for more than 12 months, falling short of eradication in which this effect is seen worldwide.

But efforts to prevent measles have unraveled, and 2019 could be the “worst year for measles” since 1989, William Moss, a specialist in epidemiology and immunology at Johns Hopkins Bloomberg School of Public Health, told Newsweek. “[This forecast] points to the fact that we are losing ground to this disease that once killed millions of children each year,” said Moss.

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