“We had a sense of security with a 98 percent [statewide] immunization rate, but what we are now understanding is that in pockets of schools, that may be a false sense of security,” said Dr. Jack Ross, chief of infectious diseases at Hartford Hospital. “We are becoming ripe for the introduction of measles into populations with large numbers of unvaccinated children, and we are losing the protection and benefit of ‘herd immunity.' ”