Columbus Public Health first announced an investigation into the outbreak on Nov. 9 after four confirmed measles cases were linked to a child care facility in Franklin County.All of those cases were among unvaccinated children with no travel history, officials said, as Columbus Public Health Commissioner Dr. Mysheika Roberts urged parents to vaccinate their children.By the end of the month, cases were linked to several more sites, including Polaris Mall, a church and a grocery store.It so far appears that all of the children impacted by the outbreak are at least partially unvaccinated, meaning they have only received one dose of the necessary two for the measles-mumps-rubella vaccine, known as MMR, although four children still have an unknown vaccination status.The MMR vaccine is critical in preventing the spread of measles, as 90% of unvaccinated people exposed to the virus will become infected, Columbus health officials warned."