Disease may continue to spread unless more people get shots to protect against infection, experts warn
WebMD News from HealthDay
By Steven Reinberg
MONDAY, March 16, 2015 (HealthDay News) -- Pockets of unvaccinated children appear to have fueled the recent measles outbreak traced to Disney theme parks in California, researchers report.
"The Disneyland outbreak is quite possibly a direct consequence of the growing anti-vaccination movement in the United States," said study author Maimuna Majumder, a research fellow at Boston Children's Hospital.
Although the person who started the outbreak has not been identified, the researchers analyzed outbreak data and found the rapid spread of the disease indicates that most of those infected were not vaccinated or were incompletely vaccinated.
As of March 6, 142 of the 173 measles cases in the United States had been tied to the Disney outbreak that began last December, according to the U.S. Centers for Disease Control and Prevention.
Although the national average for measles, mumps and rubella (MMR) vaccination in the United States is quite high -- about 92 percent -- pockets of under-vaccinated children are breeding grounds for outbreaks, Majumder explained.
The researchers found that "substandard" vaccination is likely to blame for the Disney measles outbreak. "Substandard vaccination means that less than 96 to 99 percent of the population exposed to measles during the course of the outbreak were vaccinated," she said.
Because measles is so contagious, that high level of vaccination coverage is necessary to prevent outbreaks, Majumder noted. "In our study, we found that MMR vaccination rates among the exposed population [in the outbreak] may be as low as 50 percent and likely no higher than 86 percent," she added.
"If the anti-vaccination movement in the U.S. continues to grow, the likelihood of outbreaks will increase -- as will their scale and scope," Majumder said.
The report was published as a research letter in the March 16 online edition of the journal JAMA Pediatrics.
Dr. Paul Offit, chief of the division of infectious diseases at Children's Hospital of Philadelphia, said, "As herd immunity erodes, it's the most contagious diseases that come back first."
Herd immunity occurs when most of the population has been vaccinated for an infectious disease, thereby providing some protection for people who are not vaccinated.
source : Low Vaccination Rates Likely Behind Disney Measles Outbreak: Study