In the first seven weeks of 2015, measles struck 141 people in 17 states and Washington, D.C. Most people in the United States are protected against the often severe fever and rash by having had one or more doses of the measles, mumps and rubella vaccine. But a small fraction of people either can’t get the shot — they are too young or have weak immune systems — or choose not to.
The immunity of the crowd usually protects those people: Since most of the population has had measles or been vaccinated, an outbreak is unlikely to spread. But measles is so infectious that the fraction immune needs to be around 90 percent to provide “herd immunity.” Seventeen states did not meet that goal in 2013, the most recent year for which data have been compiled.
California, the location of most of this year’s cases, did meet the federal government’s vaccination goal for children, suggesting that pockets of lower rates of vaccination exist.
Approximate fraction of 19- to 35-month-old children needing measles vaccination to achieve herd immunity
Fraction of 19- to 35-month-old children in the United States in 2013 who had received at least one dose of the MMR vaccine
Fraction of 19- to 35-month-old children in Colorado, Ohio and West Virginia in 2013 who had received at least one dose of MMR
Source: L.D. Elam-Evans et al/Morbidity and Mortality Weekly Report 2014