(CNN) There has been a small increase in vaccine exemption rates among kindergarteners in the United States, according to a new report from the US Centers for Disease Control and Prevention.
The finding, published in the CDC's Morbidity and Mortality Weekly Report on Thursday, shows that rates of young children not receiving vaccinations against certain diseases, including measles, continued to climb in recent years.
For the 2018-2019 school year, the national percentage of kindergartners with an exemption from one or more required vaccines was 2.5%, which was a slight increase from 2.3% during the previous school year and 2.1% in the 2016-2017 school year, the report found.
The percentage of kindergartners with an exemption from one or more required vaccines ranged from 0.1% in Mississippi to 7.7% in Idaho and Oregon, the report found.
Nationally, among those vaccine-exempt kindergartners, only 0.3% had a medical exemption while 2.2% had a nonmedical exemption, according to the report.
"Measles outbreaks affecting school-age children across multiple states during the 2018-19 school year underscore the importance of both school vaccination requirements for preventing disease spread and school coverage assessments to identify pockets of undervaccination," the researchers wrote in the report.
Those recent measles outbreaks occurred even though the measles virus was declared eliminated from the United States in the year 2000, meaning there was no continuous transmission of the disease for greater than a year. Measles can be prevented with the measles, mumps and rubella vaccine, or MMR, according to the CDC.
"Although the overall percentage of children with an exemption increased slightly for the second consecutive school year, children with exemptions still represent a small proportion of kindergartners nationally and in most states," the researchers wrote. "More importantly, in 25 states, the number of nonexempt undervaccinated kindergartners exceeded the number of those with exemptions."
The report noted that nonexempt students include those provisionally enrolled, in a grace period or otherwise without documentation of vaccination. A grace period refers to a set number of days during which a student can be enrolled and attend school without proof of complete vaccination or exemption, but provisional enrollment allows a student without complete vaccination or exemption to attend school while completing a catch-up vaccination schedule. In many states, nonexempt undervaccinated students are attending school in a grace period or are provisionally enrolled, according to the report.
The new report summarized vaccination coverage data on about 3.6 million kindergarteners during the 2018-2019 school year. The data was collected by state and local immunization programs in 49 states: all states except Alaska, for which data was not reported to the CDC.
The report also included vaccine exemption data on some 3.6 million kindergarteners in all 50 states.
Federally funded immunization programs collaborate with departments of education, school nurses and other school personnel to assess vaccination coverage and exemption status of children enrolled in public and private kindergartens and then to report that data to CDC.
The new report found that nationally, during the 2018-2019 school year, vaccination coverage among kindergartners for the two-dose MMR ranged from 87.4% in Colorado to at least 99.2% in Mississippi.
The recommended coverage for MMR is at least 95%, according to the report.
For the diphtheria and tetanus toxoids and acellular pertussis vaccine or DTaP, coverage ranged from 88.8% in Idaho to at least 99.2% in Mississippi, according to the report.
For the varicella or chickenpox vaccine, the report found that coverage ranged from 86.5% in Colorado to at least 99.2% in Mississippi.
Nationally for the 2018-2019 school year, coverage among kindergarteners for those vaccines was: 94.7% for two doses of the MMR; 94.9% for the required doses of DTaP; and 94.8% for the varicella vaccine, according to the report.
The report had some limitations, including that the data are based on documentation from schools so some children might not be included in the data, such as those who are homeschooled.
More research is needed to directly compare regions or states -- since in the new report, comparability is limited due to variation in states' requirements, data collection methods and definitions of grace periods or provisional enrollment when it comes to vaccine requirements.
"Today's data reinforces that childhood vaccine coverage remains high," Ranee Seither in CDC's Immunization Services Division and first author of the report said in an email on Thursday.
"Most parents continue to protect their children from 14 vaccine preventable diseases. But while nationwide vaccination coverage is high, and even high in many states, we still see pockets of under-vaccinated communities," Seither said.
While the CDC regularly reports on statewide exemptions of vaccinations, the agency could better predict possible measles outbreaks -- or outbreaks of other infections -- that result from declining vaccine coverage by also examining vaccine exemptions on a more local level, such as by county or school, said Dr. Peter Hotez, dean for the National School of Tropical Medicine at the Baylor College of Medicine in Houston and co-director of the Texas Children's Hospital Center for Vaccine Development, who was not involved in the new report.
Hotez and his colleagues conducted their own county-level examination of non-medical vaccine exemptions across the United States, published last year in the journal Plos Medicine.
That study found that the number of nonmedical "philosophical-belief" vaccine exemptions had risen between 2009 and 2017 in more than half of the 18 states that allowed such policy at the time: Arkansas, Arizona, Idaho, Maine, Minnesota, North Dakota, Ohio, Oklahoma, Oregon, Texas and Utah.
Overall that previous study showed "at least 100 counties, including 14 urban counties, that are highly vulnerable to measles," Hotez said.
"Sure enough, measles erupted in seven of those 14 counties, so we wound up making the first good measles prediction map," he said. "I would recommend instead that the CDC publish at least county or even school based exemption rates in addition to state-level exemptions, and they should do this on an annual basis."
Other research, published in the medical journal JAMA in 2016, 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 who led that separate research, told CNN last year.
However, "if it's difficult to obtain an exemption, then you have lower rates of refusal and lower rates of disease," he said.