Three sets of parents called Sue Kressly's pediatric practice in Bucks County over the weekend with the same extraordinary question: Their children were not yet one year old, the minimum age to receive the measles, mumps, and rubella vaccine.

Could they get the MMR earlier?

"I think it's a great sign," said Kressly, Pennsylvania chapter president of the American Academy of Pediatrics. "The conversation is changing from whether vaccines can do harm to one about how vaccines can protect you."

For years, pediatricians have been trying to persuade some parents that vaccines protect against diseases that are no longer commonly seen. The parents cited scary reports that they had read online, particularly about a long-debunked study that suggested a link between the MMR vaccine and autism, the all-too-visible neurodevelopmental disorder that can appear in the months after children have been immunized.

"How do you un-ring the bell? How do you un-scare them?" asked Paul A. Offit, an infectious-diseases physician at Children's Hospital of Philadelphia, who has written several books about the issue, gaining the enmity of the anti-vaccine movement.

Offit recalled a 1991 outbreak of measles in Philadelphia, linked to a religious organization that shunned vaccination, that he said resulted in 1,400 cases and nine deaths.

"I lived through that outbreak," he said.

Many young parents today did not. For them, the most fearful news may have come only in the last few months, as an outbreak that began at California's Disneyland spread from unvaccinated visitors to their home states around the country. More than 100 cases have been confirmed from Jan. 1 to 30, on top of 644 last year, the Centers for Disease Control and Prevention reported Monday.

Most cases in recent years have been "imported" by travelers arriving from countries where measles is more common. At greatest risk are children under 1, who are not eligible for the vaccine.

New Jersey has not had any cases this year, after three last year. The Pennsylvania Department of Health, which also reported three cases in 2014, last week reported its first case of 2015, in Cumberland County. It said there was no link to Disneyland.

The state held three immunization clinics in that county last week; 312 people got the MMR vaccine, said Rachel Levine, Pennsylvania's acting physician general.

"Measles is a potentially very serious illness, and it is highly contagious, and it is preventable by the MMR vaccine," Levine said Monday, adding that the vaccine "is extremely safe and highly effective."

Dyanne Westerberg, chairman of family medicine at Cooper Medical School of Rowan University in Camden, said that she periodically has that conversation with parents who come into her university practice in Cherry Hill.

"I tell them that, as a mom, I made sure my children got vaccinated," Westerberg said. "You have to listen to what their beliefs are. And you don't criticize them."

Vaccination rates are well above average locally. For one or more doses of MMR, New Jersey has the third-highest rate in the United States (95.6 percent), according to the 2013 National Immunization Survey, released in August. Pennsylvania's 93.3 percent was also above the national average of 91.9 percent, and Philadelphia's was 95.9 percent.

But statewide rates can be deceptive, because small pockets with lower rates disappear into the average but are capable of triggering outbreaks. California, for example, also has above-average immunization rates, but some communities' rates are far lower because of exemptions.

Nearly every state requires measles immunization for school entry, and also allows parents to opt out for medical or religious reasons. Pennsylvania is one of 20 states that also permit a philosophical exemption.

New Jersey does not. "But the reality is that virtually anyone can get a religious exemption," said Drew Harris, an assistant professor at Thomas Jefferson University's School of Population Health, who has been studying exemption trends in the Garden State.

Several years ago, Harris said, the state Attorney General's Office instructed schools that they could not seek details about whether parents' claims were a "bona fide religious belief," and exemptions tripled.

Celebrity news coverage also seems to get attention. On Monday, Gov. Christie's statement in London that parents should have "some measure of choice" got picked up immediately.

Whether his statement was off the cuff or not, it was quickly followed by staff clarifications of his position, that "with a disease like measles there is no question kids should be vaccinated."