More possible measles cases are being reviewed in Tarrant County, Texas, where at least 21 people have been sickened this month at the Eagle Mountain International Church, whose ministers have been critical of vaccination. Local officials say several more cases of infections with fever and rash have been reported, but not confirmed.

“It’s concerning. It’s something we jump on,” said Russell Jones, a Texas state epidemiologist who’s been tracking the situation. “It could get into the schools.”

Public health officials say that the northeast Texas outbreak is just the latest in a small but growing number of places -- think San Diego, Calif.Boone and Hamilton counties, Ind., and, most recently, Brooklyn, N.Y. -- where vaccine resistance has sickened children and put the wider community at risk for potentially deadly infectious diseases.

“The rate of change has sort of accelerated,” said Dr. Saad Omer, an infectious disease epidemiologist at Emory University in Atlanta who studies clusters of vaccine exemptions in public schools.

“Tarrant County was not on my radar,” he added.

He’s more familiar with places such as Marin County, Calif., where rates of new kindergarteners excused from mandatory shots nearly doubled from 4.2 percent in 2005 to 7.8 percent at the start of last year, county figures show. There, at the small New Village School in Sausalito, 74 percent of entering kindergarteners said no to vaccinations.