(The Center Square) – A record high number of kindergartners started last school year with an exemption from one of the vaccines U.S. health authorities require.

The overall percentage of children with an exemption increased from 2.6% during the 2021-22 school year to 3% during the 2022-23 school year, the highest exemption rate ever reported in the U.S., according to a Centers for Disease Control and Prevention report published Thursday.

During the 2022-23 school year, nationwide vaccination coverage among kindergarten children remained about 93% for measles, mumps, and rubella vaccine (MMR); diphtheria, tetanus, and acellular pertussis vaccine (DTaP); poliovirus vaccine (polio) and varicella vaccine, which protects against chickenpox. That’s similar to the 2021-22 school year, lower than the 94% coverage in the 2020-21 school year, and lower still than the 95% coverage during the 2019-20 school year, when children were vaccinated before the COVID-19 pandemic, according to the report.

Exemptions from one or more vaccines increased in 40 states and Washington D.C. and increased by at least 1 percentage point in seven states.

Nonmedical exemptions accounted for more than 90% of reported exemptions, and 100% of the increase in the national exemption rate.

“It is not clear whether this reflects a true increase in opposition to vaccination, or if parents are opting for nonmedical exemptions because of barriers to vaccination or out of convenience,” according to the report. “Whether because of an increase in hesitancy or barriers to vaccination, the COVID-19 pandemic affected childhood routine vaccination.”

Health officials warn that declines in vaccination rates could lead to outbreaks.

“Because clusters of undervaccinated 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,” according to the report.