Though epidemiologists have not yet identified the original source of the Disneyland measles outbreak in December, which has since spread internationally, researchers now say that parents who refuse to vaccinate their kids are probably to blame.
With simple math, researchers were able to show that the vaccination rate among people who were exposed to the measles during the outbreak was no higher than 86 percent, and it might have been as low as 50 percent.
In order to establish herd immunity, between 96-99 percent of the population must be vaccinated, health experts said.
“Even the highest estimated vaccination rates from our model fall well below this threshold,” the MIT and Boston Children’s Hospital team of researchers reported in the JAMA Pediatrics journal on Monday, March 16.
Researchers calculated the range of likely vaccination rates based on key data points, including historical information about infectious diseases. Experts know that in the absence of any vaccination, a single infected person can spread [a measles virus] to between 11 and 18 other people. They are also aware that it takes anywhere from 10 to 14 days for one measles case to lead to another.
According to the study, in the best-case scenario, the vaccination rate among people who encountered the measles as a result of the Disneyland outbreak was between 75 percent and 86 percent, the researchers calculated. If the true effective reproductive number was in the middle of the range, the vaccination rate would have been between 66 percent and 81 percent. If the effective reproductive number was high, the vaccination rate had to have been between 50 percent and 71 percent, according to the study.
The report concluded that the only way to explain how the measles spread from a single person at Disneyland to 142 people in seven states is the current anti-vaccination movement. A substantial number of American parents, for various reasons from religion to personal beliefs, have not had their children fully immunized with the measles, mumps, and rubella vaccine (MMR).
“Clearly, MMR vaccination rates in many of the communities that have been affected by this outbreak fall well below the necessary threshold to sustain herd immunity, thus placing the greater population at risk as well,” researchers concluded.
The wide scope of this national outbreak reflects the rise of the anti-vaccination movement, which continues to grow despite overwhelming medical evidence that vaccines do not cause autism or other developmental problems. In most cases, studies by the Centers for Disease Control and Prevention confirmed, side effects are limited to pain at the injection site, fever, a mild rash or temporary swelling. In rare instances, children may have a severe allergic reaction to the vaccine or develop febrile seizures, temporary arthritis, or immune thrombocytopenic purpura (a blood disorder).
“[The measles outbreak] shines a glaring spotlight on our nation’s growing anti-vaccination movement and the prevalence of vaccination-hesitant parents,” the study’s authors wrote.
In California, three state legislators have introduced a bill that would make it more difficult for parents to opt out of vaccinations by claiming a personal belief exemption. SB 277 would require children to be vaccinated against measles and other infectious diseases before enrolling in California schools.
(With reports from Los Angeles Times)
