"Another idea is that people have bigger deductibles and that may have created some decreases in the use of these services as parents decide they don't want to spend money," Thomas said.
The report relied on voluntary reporting from 1,000 health plans covering 118 million Americans, in addition to data from Medicaid, which provides free or co-payment-only health coverage for some low-income people who could not otherwise afford it. (In certain cases, children may be eligible for coverage even if parents are not.)
The authors found a drop in several routine childhood vaccinations. Measles, mumps and rubella (MMR) vaccines decreased from 93.5 percent in 2008 to 90.6 percent in 2009; diphtheria, tetanus and whooping cough rates fell from 87.2 percent to 85.4 percent in that one-year period; and the proportion of kids getting vaccinated for chickenpox fell from 92 percent in 2008 to 90.6 percent in 2009.
Dr. Gabrielle Gold-von Simson, assistant professor of pediatrics at NYU School of Medicine, believes the success with the Medicaid rates "is due to the vaccines-for-children programs and other programs that are dedicated to supplying vaccines for children at low or no cost."
"I think that's a public health success in a way," she added.
But experts are worried that the downward trend in more middle-class families, if it continues, could jeopardize the public's health.
"People have to understand there's only one disease that we have eliminated from the earth so far and that's smallpox," Frenck said. That means that other diseases, including polio, are still lurking and could infect anyone who is not vaccinated.
Witness the recent pertussis or whooping cough in California, which has sickened more than 6,200 people -- the most cases reported in 60 years -- and killed 10 infants, according to the state health departmen
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