Originally Published: 09 NOV 23 14:00 ET

Updated: 09 NOV 23 14:14 ET

By Amanda Musa, CNN

(CNN) — The percentage of kindergartners who received their state-required vaccines for measles remained below the federal target last school year, and the rate of vaccine exemptions for children reached the highest level ever reported in the United States, according to new data published Thursday by the US Centers for Disease Control and Prevention.

Coverage for four key vaccines — the measles, mumps, and rubella (MMR) vaccine; diphtheria, tetanus, and acellular pertussis (DTaP) vaccine; poliovirus vaccine; and varicella vaccine for chickenpox — declined in a majority of states for the 2022-23 school year, the report said. Lower levels of vaccine coverage raise the risk of outbreaks, leaving the country vulnerable to diseases that can cause severe illness and death.

Vaccine coverage rates fell below 95% from the 2019-20 to 2021-22 school years, declining to about 93%. Last school year, coverage remained at about 93% for reported vaccinations, including 93.1% coverage for MMR and poliovirus, and 92.7% for DTaP, the report said.

The percentage of kindergartners — children around 5 and 6 years old — who are exempt from state-required vaccinations increased to 3% for the 2022–23 school year from 2.6% in 2021–22. Exemptions increased in 40 states and Washington, DC, according to the CDC.

Ten states reported exemptions for at least one vaccine that exceeded 5% of kindergartners. Exemptions beyond 5% limit the level of vaccination coverage possible, the researchers wrote, which raises the risk of an outbreak of vaccine-preventable diseases.

National MMR coverage of about 93% during the 2022–23 school year translates to approximately 250,000 kindergartners who are at risk for measles infection, the new report said. About 4% of kindergartners nationwide were not fully vaccinated with MMR or formally exempt from getting it, but were allowed to attend school during a grace period for provisional enrollment.

“Because clusters of under-vaccinated children can lead to outbreaks, it is important for immunization programs, schools, and providers to make sure children are fully vaccinated before school entry, or before provisional enrollment periods expire,” the CDC researchers wrote.

There are some limitations to the data, including limited state comparisons due to the variation in state vaccination requirements, including the type of vaccine and the number of doses, and data collection methods. The CDC does not require school vaccinations.

All states have laws requiring specific vaccines for students and all school immunization laws grant exemptions to children for specific medical reasons. But 44 states and Washington, DC, also grant religious exemptions, and 15 states allow philosophical or moral exemptions for children, according to the National Conference of State Legislatures. In the new study, non-medical exemptions accounted for more than 90% of reported exemptions and approximately 100% of the increase in the national exemption rate.

The researchers also noted that the 2022-23 kindergarten class reached age-eligibility to complete most state-required vaccinations during the Covid-19 pandemic, when many vaccinations were interrupted or delayed.

Currently, school requirements do not include the Covid-19 vaccine, which is explicitly banned from being included in school mandates in at least 20 states.

However, that vaccine did become part of the CDC’s recommended immunization schedule for both children and adults this year.

Aside from school requirements, the CDC recommends routine vaccination against 14 diseases for children before they turn 2 years old.

CNN’s Deidre McPhillips contributed to this report.