"We're seeing a lot more spread. That is concerning to us," said Dr. Jane Seward, of the Centers for Disease Control and Prevention.
Pediatricians are frustrated, saying they are having to spend more time convincing parents the shot is safe.
"This year, we certainly have had parents asking more questions," said Dr. Ari Brown, an Austin, Texas, physician who is a spokeswoman for the American Academy of Pediatrics.
The CDC's review found that a number of cases involved home-schooled children not required to get the vaccines. Others can avoid vaccination by seeking exemptions, such as for religious reasons.
Measles, best known for a red skin rash, is a potentially deadly, highly infectious virus that spreads through contact with a sneezing, coughing, infected person.
It is no longer endemic to the United States, but every year cases enter the country through foreign visitors or Americans returning from abroad. Measles epidemics have exploded in Israel, Switzerland and some other countries. But high U.S. childhood vaccination rates have prevented major outbreaks here.
In a typical year, only one outbreak occurs in the United States, infecting perhaps 10 to 20 people. So far this year through July 30 the country has seen seven outbreaks, including one in Illinois with 30 cases, said Seward, of the CDC's Division of Viral Diseases.
None of the 131 patients died, but 15 were hospitalized.
Childhood measles vaccination rates have stayed above 92 percent, according to 2006 data. However, the recent outbreaks suggest potential pockets of unvaccinated children are forming. Health officials worry that vaccination rates have begun to fall - something that won't show up in the data for a couple of years.
The vaccine is considered highly effective but not perfect; 11 of this year's cases had at least one dose of the vaccine.
Of this year's total, 122 were unvaccinated or had unknown vaccination status. Some were unvaccinated because the children were under age 1 - too young to get their first measles shot.
In 63 of those cases - almost all of them 19 or under - the patient or their parents refused the shots for philosophical or religious reasons, the CDC reported.
In Washington state, an outbreak was traced to a church conference, including 16 school-aged children who were not vaccinated. Eleven of those kids were home schooled and not subject to vaccination rules in public schools. It's unclear why the parents rejected the vaccine.
The Illinois outbreak - triggered by a teenager who had traveled to Italy - included 25 home-schooled children, according to the CDC report.
The nation once routinely saw hundreds of thousands of measles cases each year, and hundreds of deaths. But immunization campaigns were credited with dramatically reducing the numbers. The last time health officials saw this many cases was 1997, when 138 were reported.
The Academy of Pediatrics has made educating parents about the safety of vaccines one of its top priorities this year. That's partly because busy doctors have grown frustrated by the amount of time they're spending answering parents' questions about things they read on the Internet or heard from TV talk shows.
In June, the CDC interviewed 33 physicians in Austin, suburban Seattle and Hollywood, Fla., about childhood vaccinations. Several complained about patient backlogs caused by parents stirred up by information of dubious scientific merit, according to the CDC report.
Questions commonly center on autism and the fear that it can be caused by the measles shots or by a mercury-based preservative that used to be in most vaccines. Health officials say there is no good scientific proof either is a cause. Also, since 2001, the preservative has been removed from shots recommended for young children, and it was never in the measles-mumps-rubella combination vaccine. It can still be found in some flu shots.
Brown said she wrote a 16-page, single-spaced document for parents that explains childhood vaccinations and why doctors do not believe they cause autism. She began handing it out this spring, and thinks it's been a help to parents and a time-saver for her.
"People want that level of information," she said.
At least one outbreak this year of another preventable disease was blamed on lack of immunizations. At least 17 children were sick with whooping cough at a private school in the San Francisco Bay area, and 13 were not vaccinated against the disease, which can be fatal to children.
Associated Press writer Marcus Wohlsen in San Francisco contributed to this story.
On the Net:
The CDC report: http://www.cdc.gov/mmwr