Outbreaks of tens of thousands of cases have occurred recently in poor or war-torn countries like Madagascar, Ukraine and Yemen. But case numbers are also climbing in wealthy countries with modern health care systems, like Israel, Britain, France and Italy. Deaths from measles have occurred in those countries.
Before measles vaccination became widespread in the United States in 1963, up to four million Americans got measles each year, the C.D.C. said. Of the roughly 500,000 cases that were reported to medical authorities each year back then, about 48,000 were hospitalized, 4,000 developed encephalitis, and 400 to 500 died.
Nationally, since the mid-1990s, more than 91 percent of American children have been vaccinated against measles.
Anyone born before 1957 is assumed to have had the disease as a child and to be immune to it.
Americans born between 1957 and 1989 are in something of a middle ground. Some got the early “killed virus” vaccine, which later proved to be too short-lived and was replaced by a “weakened virus” vaccine. Until 1989, it was routine to give one shot; now children get two.
One shot of the new vaccine provides 93 percent immunity in the overall population, while two shots drive that up to 97 percent, which is considered more than enough to keep the virus from spreading.
But each individual’s immune system is different, so some Americans worried about the current outbreak have visited their doctors for a simple blood test that can show how immune they are to measles, mumps and rubella.
Immunization levels vary from state to state, largely dependent on how easy state legislatures make it to get exemptions. All states permit exemptions for children who are allergic to the vaccine, have a compromised immune system or have another medical reason to avoid it.