Most U.S. counties have seen a decline in measles, mumps and rubella (MMR) vaccination rates over the last five years, according to a new report published Monday.

“If vaccination rates continue to decline, measles is likely to return to endemic levels in the U.S.,” wrote the research team led by senior researcher Lauren Gardner, a professor at Johns Hopkins Whiting School of Engineering who is an expert in using data and modeling to understand the spread of disease.