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TUESDAY, Oct. 3, 2017 (HealthDay News) -- Less than 20 years ago, health experts thought it was only a matter of time before measles was completely eradicated in the United States. But over the past 15 years, the disease has gained a new foothold in the United States, likely due to parents choosing not to vaccinate their children, a new study suggests.
From 2001 to 2015 measles cases in the United States remained very low (less than 1 case per million people), but there's still been a signficant rise, the study found.
Overall, measles incidence doubled -- from 0.28 per million in 2001 to 0.56 per million in 2015. Infants and young children were hit hardest, and most cases were among the unvaccinated, the research showed.
The findings suggest that, "being unvaccinated rather than failure of vaccine is the main driver of measles spread," said lead researcher Nakia Clemmons, an epidemiologist at the U.S. Centers for Disease Control and Prevention.
"Thanks to high vaccination rates, measles cases among Americans has remained rare since the disease was eliminated," Clemmons said.
Measles outbreaks, however, can and do still occur in the United States, she said.
"Measles is still commonly transmitted in many parts of the world. Unvaccinated U.S. residents returning from overseas travel and foreign visitors to the U.S. may develop measles and expose people in United States. When measles gets into communities of unvaccinated people, outbreaks are more likely to occur," Clemmons said.
The best way to protect children against measles and other vaccine-preventable diseases is to ensure they are up to date on vaccines, she said.
"Getting vaccines at the recommended ages protects children at the earliest age possible from serious vaccine-preventable diseases," Clemmons said.
Dr. Paul Offit is director of the Vaccine Education Center at Children's Hospital of Philadelphia.
"The reason measles has come back is not because the virus has mutated. It's not because the vaccine isn't effective. It's because a critical number of parents have chosen not to vaccinate their children," said Offit, who wasn't involved with the study.
Measles is now endemic in the United States at a very low level, he said. "It's now spreading from one American to another."
Offit, however, doesn't expect that measles will reach the epidemic proportions it once had. "We used to have 3 to 4 million cases a year, 48,000 hospitalizations and 500 deaths a year, he said. "Measles was a scourge in this country."
Since measles is the most contagious of all the vaccine-preventable diseases, it makes sense it's the disease to come back first. It could be the harbinger of the return of other diseases, Offit said. "It's the canary in the coal mine," he said.
Using data from the CDC, the researchers looked at the number of measles cases in the United States that occurred after the disease was nearly eradicated.
Between 2001 and 2015, nearly 1,800 cases of measles were reported. People who suffered from the disease were an average of 15 years old and nearly half were girls, the researchers found.
Nearly 70 percent of those who came down with measles were not vaccinated. It was unknown if another 18 percent who contracted measles had been vaccinated or not, Clemmons said.
About 27 percent of measles cases were imported from other countries; the rest were acquired in the United States.
Measles was most likely to affect infants ages 6 to 11 months and toddlers 12 to 15 months. The rate of measles started to drop beginning at 16 months, the researchers said.
The number of measles cases each year varied between 24 and 658, the study showed.
Offit said it's not unusual for parents to be wary of vaccines.
Kids are subjected to 26 different vaccinations and as many as five shots at one time to prevent diseases most people don't see and using biological fluids most people don't understand, he said.
"It's not surprising there's been pushback. Vaccines are a victim of their own success," Offit said.
The report was published as a letter in the Oct. 3 issue of the Journal of the American Medical Association.
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