Measles vaccination rates for young children may be far lower than publicly reported, a troubling development that could mean the United States is closer than expected to losing its “elimination status” for the extremely contagious disease.

“We are experiencing an extremely concerning decline in measles vaccination in the very group most vulnerable to the disease,” said Benjamin Rader, a computational epidemiologist at Boston Children’s Hospital, an assistant professor at Harvard Medical School and the author of a recent study that looked at children’s vaccination rates.