evidence: "We’re not just seeing cases, we’re seeing transmission, which means vaccine levels aren’t what we’d like them to be," said Saskia Popescu, an assistant professor of epidemiology and public health at the University of Maryland School of Medicine. Measles is a highly contagious viral infection, spread through contact with mucus that has been coughed or sneezed, or through breathing the same air as an infected person hours after they have left a room. In 2000, U.S. health officials had declared the end of the disease’s endemic spread, eliminated by decades of vaccination campaigns. In the years since, the measles vaccine has saved an estimated 57 million lives worldwide, according to the CDC. READ MORE: As COVID cases rise, doctors worry about the consequences of misinformation But now, data show a dangerous decline in vaccination among schoolchildren, both in the U.S. and in Europe. "That is a huge wakeup call," said Dr. Syra Madad, an epidemiologist, senior director at New York City Health + Hospitals and fellow at the Harvard Kennedy School’s Belfer Center for Science and International Affairs. She said when it comes to vaccination rates, each percentage point "can translate into thousands of children."