The proportion of young kids who don’t receive vaccinations is on the rise, according to two new Centers for Disease Control and Prevention reports.

Though the share of children under age 2 who haven’t received any vaccines is admittedly low, it has roughly quadrupled over the past decade and a half. About 47,700 children born in 2015 (or 1.3%) had not been vaccinated by 2017, compared to just 0.9% of kids aged 19 to 35 months in 2011 and 0.3% in 2001.

The CDC’s Advisory Committee on Immunization Practices advises routine vaccination by age 2 against “14 potentially serious illnesses.” Vaccine coverage (the estimated proportion of people who’ve been given specific vaccines) was still “high and stable overall,” the first CDC report said, pushing 90% for varying doses of the poliovirus; measles, mumps and rubella; hepatitis B and varicella vaccines.

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Vaccine coverage was lower among kids who were uninsured or Medicaid-insured than among those with private insurance, according to the report; kids living in more rural areas also tended to have lower vaccine coverage.

“This is something we’re definitely concerned about,” pediatrician Amanda Cohn, the senior adviser for vaccines at the CDC, told the Washington Post. “We know there are parents who choose not to vaccinate their kids . . . (and) there may be parents who want to and aren’t able to.”

Meanwhile, kindergarteners’ median vaccine coverage was around 95% for the 2017-18 school year, the second report found. But the proportion of kids exempted from immunization requirements — nearly all states grant religious exemptions, and 18 states grant philosophical exemptions — is also rising, with a median 2.2% of kindergarteners exempted from at least one vaccine.

“Although the overall percentage of children with an exemption was low,” the report authors noted, “this was the third consecutive school year that a slight increase was observed.”

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The reasons parents hesitate, delay or refuse to vaccinate their kids fall into four broad categories, according to one 2016 study: “religious reasons, personal beliefs or philosophical reasons, safety concerns, and a desire for more information from healthcare providers.” “Parental concerns about vaccines in each category lead to a wide spectrum of decisions varying from parents completely refusing all vaccinations to only delaying vaccinations so that they are more spread out,” the authors wrote.

Celebrities like Jenny McCarthy, Jim Carrey and Rob Schneider have also received criticism for their public comments about vaccines.

Unvaccinated children are more vulnerable to contracting diseases, and can put other people’s health at risk. And vaccinating them can even save money, according to a study of children born from 1994 to 2013: “Vaccination will prevent an estimated 322 million illnesses, 21 million hospitalizations, and 732,000 deaths over the course of their lifetimes, at a net savings of $295 billion in direct costs and $1.38 trillion in total societal costs,” the report said.