"We're seeing a lot more spread. That is concerning to us," said Dr. Jane Seward, of the U.S. 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 have the vaccines.
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 some Americans pick it up while traveling abroad and bring it home. 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, deputy director of the CDC's Division of Viral Diseases.
None of the 131 patients died, but 15 were hospitalized. Childhood vaccination rates for measles continue to exceed 92 percent, but outbreak pockets seem to be forming, health officials said.
Of this year's total, 122 were unvaccinated or had unknown vaccination status. Some were unvaccinated because the children were under age 1, making them too young to get their first measles shot.
In 63 of those cases -- almost all of them 19 or younger -- the patient or their parents refused vaccination, the CDC reported.
In Washington state, an outbreak was traced to a religious conference, including 16 school-aged children who were not vaccinated because of parents' beliefs. Eleven of those kids were home schooled and not subject to vaccination rules in public schools.
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. Last year, there were only 42 U.S. cases.
The American Academy of Pediatrics has made educating parents about the safety of vaccines one of its top priorities this year, in part because busy doctors have grown frustrated at the amount of time they've been 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 it can be caused by the mercury-based preservative that used to be in most vaccines. Since 2001, the preservative has been removed from shots recommended for young children.
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.
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