The agency does not yet know how many shots each of those 76 had, “but under extreme disease pressure we know there can be vaccine failures,” Dr. Messonnier said.

She suggested that adults likely to encounter the virus, including health workers, travelers and anyone in affected neighborhoods, get a blood test that can show how immune they are to measles, mumps and rubella.

[Here’s our full coverage about the measles outbreak.]

Anyone born before 1957 is assumed to have had the measles as a child and therefore immune.

Americans born between 1957 and 1989 are in 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.

Vaccination 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.

Some states permit religious exemptions, even though no major religion opposes vaccination, and a few states also permit “philosophical” or “personal choice” exemptions.