(CNN) - The percentage of young kids in the U.S. who received no vaccine doses has continued to rise, as have vaccine exemption rates, according to two reports published Thursday by the U.S. Centers for Disease Control and Prevention.
While coverage for a number of vaccines "remained high and stable overall," the segment of unvaccinated kids under age 2 rose from 0.9 percent for those born in 2011 to 1.3 percent for those born in 2015, according to one report. In 2001, only 0.3 percent of kids between 19 and 35 months had received no vaccine doses.
"Although the number of children who have received no vaccinations by age 24 months has been gradually increasing, most children are still routinely vaccinated," wrote authors from the CDC's National Center for Immunization and Respiratory Diseases. "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."
Still, coverage in this age group surpassed 90 percent for a number of vaccines, including poliovirus, Hepatitis B, varicella, and measles, mumps and rubella.
Those who were least likely to be fully vaccinated: uninsured children and those on Medicaid, compared to privately insured kids, despite being eligible for a federally funded program that administers free vaccines. Kids outside of metropolitan areas were also less likely to be covered when compared to those in major cities.
"These disparities were larger for vaccines that require a booster dose in the second year of life," the report said, citing the diphtheria, tetanus and pertussis vaccine as an example.
While the report does not identify the reasons behind the trend, the authors say that it could have something to do with lack of knowledge about how to access free vaccines; transportation; how close clinics are and when they operate; child care; and a shortage of pediatricians and other health care providers in rural areas.
Vaccination rates varied across states, with rotavirus (a common cause of stomach flu in children) coverage highest in Rhode Island (85.1 percent) and lowest in California (64.7 percent). Coverage for measles, mumps and rubella was highest in Massachusetts (98.3 percent) but lowest in Missouri (85.8 percent).
The second report focused partly on vaccine exemption rates among kindergartners. This past school year was the third consecutive one that saw a "slight increase" in the overall proportion of kids with an exemption, the report says.
"The median percentage of kindergartners with an exemption from at least one vaccine was 2.2 percent," the report said, and nonmedical vaccine exemption rates ranged from 0.1 percent in California to 7.5 percent in Oregon.
Reasons for the exemptions weren't included but may involve "parental vaccine hesitancy" and how easy it is to get an exemption in any given state.
Of the states included in the kindergarten analysis (41 or 49 states depending on the vaccine, plus Washington DC), the nation's capital reported the lowest median rates of vaccine coverage for varicella; diphtheria, tetanus and pertussis; and measles, mumps and rubella. Mississippi had the highest rates.
"During the 2017-18 school year, median kindergarten vaccination coverage was close to 95% for MMR, DTaP, and varicella vaccine," the authors wrote.
Previous research showed that certain "hot spot" metropolitan regions also show a high proportion of nonmedical exemptions and so may be especially susceptible to outbreaks, including Seattle, Portland and Phoenix. High numbers of unimmunized children living in large cities with busy international airports may contribute to the risk of a swift spread of disease, according to researchers.
Other research has shown that states with philosophical exemptions had both higher rates of refusal and higher rates of disease.
"Clusters of refusal overlap clusters of outbreaks," Saad B. Omer, a professor of global health and epidemiology and pediatrics at Emory University, previously told CNN.
However, "if it's difficult to obtain an exemption, then you have lower rates of refusal and lower rates of disease," he said.
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