The number of children who have not received vaccines for preventable diseases has quadrupled since 2001, to an estimated 100,000 individuals, according to a report released by the Centers for Disease Control and Prevention (CDC).

Using data from the National Immunization Survey-Child, the CDC report published Thursday found that in 2017, “the percentage of children with no vaccinations by age 2 years increased from 0.9% for children born in 2011 to 1.3% for those born in 2015.” In 2001, that figure was only 0.3%. The survey found that children were least likely to have received vaccinations for hepatitis A and rotavirus.

While 1.3% may seem like a small figure, any rise in the number of unvaccinated children is a cause for concern, according to Dr. Amanda Cohn, senior advisor for vaccines at the CDC. “We want to see coverage as high as possible,” Cohn says.

Cohn says that the reasons for this increase are multifaceted. “Some parents may choose not to vaccinate their child, but there are other parents who may want to get their child vaccinated but may not have access to a health care provider, or understand how to access vaccines,” she says.

The report found that uninsured children were more likely to be totally unvaccinated compared to those with private health insurance or Medicaid. In the 2017 survey, 7.1% of uninsured children received no vaccinations, compared to just 0.8% of those with coverage. Among the unvaccinated children surveyed, 17.2% were uninsured.

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In a separate report also issued Thursday, the CDC gathered data on vaccine exemption rates across the country for kindergarteners. While median rates of vaccination coverage for kindergarteners is around 95%, this was the third consecutive year that the CDC observed an increase in exemption rates. Those numbers vary widely from state to state: The rate of kindergartners with an exemption from one or more required vaccines ranged from 0.1% in Mississippi — which is one of three states that does not permit vaccine exemptions for religious or philosophical reasons — to 7.6% in Oregon, which allows exemptions for both. While the CDC did not speculate on the reasons why this increase occurred, the report states that “parental vaccine hesitancy” may be responsible.

Anti-vaccination sentiments were popularized by a 1998 study claiming a correlation between MMR vaccine and an increase in autism rates. While the study was retracted and the doctor behind it was subsequently discredited, a 2018 study published in PLOS Medicine found that non-medical refusal rates of vaccines have continued to increase in some metropolitan areas across the country. Evidence in that study also suggested that urban areas with higher rates of exemption are at a greater risk for diseases preventable by vaccine.

Write to Wilder Davies at wilder.davies@time.com.