Measles Returns: 280,000 US Kindergarteners Unvaccinated as Coverage Falls Below Herd Immunity Threshold

At least 280,000 US kindergarteners missed the MMR vaccine, Dr. Paul Offit told The Guardian. National coverage fell to 92.4%, below the 95% needed for herd immunity — the level at which enough people are protected to stop spread. Measles is so contagious that one case can infect 12 to 18 susceptible people. It's the worst US measles year since 1991, with outbreaks in Utah and South Carolina.