While immunization rates among young children in the U.S. remain high, the percentage who haven't received recommended vaccines has been growing, and so have vaccine exemption rates, the CDC says.

In a recent report, the CDC found that the percentage of 2-year-olds who'd received no vaccinations grew to 1.3% among those born in 2015, up from 0.9% for those born in 2011. Meanwhile, the median rate of kindergarten children with a vaccine exemption has also risen—for the third year in a row—to reach 2.2%, the CDC said in another study.

"Continued evaluation of prevalence and reasons for nonvaccination is needed, as are improvements in access to and delivery of age-appropriate vaccinations to all children," wrote the researchers in the first report, which focuses on children aged 19-35 months.

Researchers did find some clues from the data they gleaned. Young children of that age group living outside of metropolitan areas showed vaccination rates 2.6 to 6.9 percentage points lower for various vaccines compared with those of their urban peers. The percentage of children who'd received no vaccinations at all was also higher in rural areas.

Uninsured children and those covered by Medicaid were less likely to be vaccinated, even though they’re eligible for free vaccines under the national Vaccines for Children (VFC) program. The researchers cited "unfamiliarity with the VFC program and how to access it, transportation, child care, and convenience of clinic hours" as possible reasons for the disparity.

A measles outbreak that originated from Disneyland in California back in 2014 turned the spotlight on low childhood vaccination rates against that disease. Even still, in 11 states, less than 90% of children aged 19-35 months had received at least one dose of measles, mumps and rubella vaccine (MMR) in 2017, the CDC found.

Looking at kindergarten-age kids in 49 states and the District of Columbia in the 2017-2018 school year, Colorado, Idaho, Kansas and D.C. reported median two-dose MMR completion rates below 90%. D.C. landed at the bottom with just 81.3%. The country’s capital also recorded the lowest rates for DTaP and varicella, whereas national "median kindergarten vaccination coverage was close to 95%" for all three vaccines, the CDC said.

However, what concerns the researchers more is the percentage of children with a vaccination exemption. For the third consecutive school year, that rate has increased slightly, this time to 2.2%. Only 0.2% had exemptions for medical reasons. Although CDC scientists couldn’t pinpoint the reason for that increase, they suspected "the ease of the procedure for obtaining exemptions or parental vaccine hesitancy" might have played a part.

In a report published in June in the journal PLOS Medicine, even some of the country’s largest metropolitan areas were identified as nonmedical exemption "hotspots." These include Seattle, Washington; Phoenix, Arizona; Houston and Austin, Texas; and Pittsburgh, Pennsylvania, among others.

"Given the effectiveness of closing the exemption policy, evidence suggests that other states should consider discontinuing the [nonmedical exemption] option and protect schoolchildren from vaccine-preventable diseases," the study’s authors wrote.

Scientists and health officials have cautioned that growing antivaccine activity or parental hesitancy would render children—and potentially, a wider population—vulnerable to certain preventable diseases. A 2017 study based on Twitter data from 2009 to 2015 revealed that antivaccine messages were already breeding in affluent areas like New York and California, and in cities with large numbers of new mothers.

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