“It’s crucial to understand why measles matters,” said Dr. Kate O’Brien, director of the WHO’s Department of Immunization, Vaccines and Biologicals. “Its high transmissibility means that even small drops in vaccine coverage can trigger outbreaks, like a fire alarm going off when smoke is detected first.” That is, measles is often the first disease to pop up when vaccination rates overall drop. "When we see measles cases, it signals that gaps are almost certainly likely for other vaccine-preventable diseases like diphtheria or whooping cough or polio, even though they may not be setting off the fire alarm just yet," O'Brien said at a media briefing Monday, ahead of the release of the WHO's Progress Toward Measles Elimination report, published Friday in its Weekly Epidemiological Record.