So why the rise in parents opting not to vaccinate their children?
"Parents choose not to vaccinate for many reasons," Buttenheim said. "To generalize across this diverse group, they perceive the risks associated with vaccines to be greater than the risks associated with vaccine-preventable diseases. While there is a very strong scientific consensus that this calculation is not correct, we cannot simply ignore or dismiss parental vaccine hesitancy."
One big contributor has been the (now discredited) notion that the measles-mumps-rubella shot might raise autism risk. In 1998, a small but widely publicized study appeared to link childhood MMR vaccination to nine cases of autism. The study appeared in the medical journal The Lancet, which retracted the study in 2010. In January of this year, an investigation by another leading British journal, BMJ, denounced the findings as deliberately fraudulent. But the damage was done.
That, and worries about thimerosal, a mercury-containing preservative once widely used in vaccines, has contributed to parents' fears.
Not all kids with personal belief exemptions are left fully unvaccinated, Buttenheim noted. "We are only able to say whether they have exemptions from one or more vaccines. Children we observe as having personal belief exemptions may have not done, for example [diphtheria, tetanus and pertussis] vaccination, but may be up to date on MMR."
"In general, parents do value vaccines but exemptions do happen," said Rodewald, who is a pediatrician. "Parents have a lot of questions and they want to make sure that vaccines are effective and safe. It's important that health professionals like pediatricians, nurse practitioners and school nurses be able to answer questions for parents so they can make informed decisions; decisions of knowledge and strength."
Because the new study was presented at a medical meeting, the data and conclusions should be viewed as preliminary until published in a peer-reviewed journal.
The U.S. National Institute of Allergy and Infectious Diseases illustrates how herd immunity works.
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