Three Measles cases were confirmed in Montgomery County, Texas, all in unvaccinated people from one family exposed to a traveler-linked case from out of state.
Two MMR doses are about 97% effective, the health department said, underscoring that none of the infected individuals had been vaccinated.
Symptoms typically appear 7 to 14 days after exposure and include fever above 101°F, cough, runny nose, red watery eyes and a rash.
Health officials urged anyone exposed or symptomatic to isolate and call a healthcare provider before seeking testing to avoid spreading the airborne virus.
Measles was declared eliminated in the United States in 2000, but imported infections still seed local outbreaks because the virus remains common in other parts of the world.