Children are making up an increasing number of patients filling Missouri hospitals during the summer COVID-19 surge, and some doctors worry the return to school will lead to more illnesses.

The fast-spreading delta variant combined with low vaccination rates across Missouri to create a new wave of the COVID-19 outbreak beginning in June and still persists. One difference this time: Children are more prone to get sick.

The number of children in the St. Louis region hospitalized with COVID-19 reached a record 31 on Wednesday before dipping slightly to 27 on Thursday. Ten of the sick children, ages 18 and younger, remain in intensive care units.

"This variant is so contagious it's pretty much attacking any group that's unvaccinated," said Dr. Clay Dunagan of the St. Louis Metropolitan Pandemic Task Force. He said the illnesses also tend to be more severe than in the winter, especially for children with underlying health conditions.

St. Louis isn't alone.

In the Kansas City area, Children's Mercy Hospital reached its capacity Monday. Dr. Barbara Pahud, director of research for infectious diseases, urged parents to have their kids take precautions as they return to school, including vaccinations for those 12 and older.

Hospital leaders in Springfield are also worried about the ramifications of thousands of unmasked students gathering in schools -- only a handful of southwestern Missouri districts require masks. Leaders of both hospitals in Missouri's third-largest city are bracing for the worst.

"My fear is that we will see an increase in cases in the next one to two weeks and we are preparing as a hospital facility at Mercy and CoxHealth for that surge," said Dr. Kayce Morton, a pediatrician at CoxHealth.

Pahud agreed the new school year is cause for worry.

"If we open schools up without social distancing, without vaccination, without masks, we are going to see problems. We are going to see children land in the hospital and that's the last thing we want to do," Pahud said during a Monday briefing.

Unlike much of Missouri, most schools in St. Louis city and county require masks. Still, Dunagan said, he expects to see "a lot of spread in schools."

Missouri continues to lag well behind the national average in vaccinations. Information from the U.S. Centers for Disease Control and Prevention shows while 52.6% of all Americans are fully vaccinated, just 45.3% of Missourians have completed their shots.

The Missouri Foundation for Health and pediatricians from throughout the state recently launched an informational campaign seeking to increase adult vaccinations to help protect children from COVID-19. The concept is simple: Fewer sick adults means fewer opportunities for kids to catch it.

"Vaccination is our best tool to decrease COVID and get it out of here and keep our kids safe," Morton said.

Missouri reported 2,245 newly confirmed cases Thursday and 10 new deaths. Since the onset of the pandemic, Missouri has confirmed 634,296 cases and 10,632 deaths.

Students attending the four campuses of the University of Missouri will not be required to get vaccinations, following a vote by curators Thursday.