Dr. Richard Besser, president and CEO of the Robert Wood Johnson Foundation and former acting director of the CDC, said parents should not get to choose if their kids wear masks in school.

"Allowing it to be an issue of personal choice is fine if it only affected your child, but it doesn't. It affects everyone around your child as well," Besser told "Good Morning America" Tuesday.

"There's a lot we don't know about this virus," Besser said. "I urge parents, I urge schools, I urge governors, not to underestimate what we're dealing with."

Mississippi's top health official warned Monday that the state's top-level hospitals have no ICU beds left, and things are going to get worse.

Mississippi State Health Officer Dr. Thomas Dobbs cited the latest stats on the growing number of COVID-19 cases, stating there were 6,912 new cases and 28 deaths recorded.

"Keep in mind - this will translate into around 500 new hospitalization in coming days, and we have ZERO ICU beds at Level 1-3 hospitals, and we have

Texas Gov. Greg Abbott announced several measures Monday to curb the state's growing COVID-19 cases and hospitalizations.

Health care personnel from other states will be coming to Texas to assist the Texas Department of State Health Services with their recent wave of cases, Abbott said.

The governor sent a letter to the Texas Hospital Association urging them to suspend elective surgeries.

He also announced the health department will open more antibody fusion centers and vaccine sites for residents.

Arkansas saw its highest number of people hospitalized with the coronavirus since the pandemic began, according to state health data.

The number of hospitalizations rose by 103, its biggest one-day increase, to 1,376, which is five hospitalizations higher than the previous record set in January, the state health data showed.

Gov. Asa Hutchinson tweeted that only eight ICU beds remained in the entire state. He encouraged more people to get a vaccine.

As of Monday, 49.3% of eligible residents in Arkansas have received one vaccine shot, according to the U.S. Centers for Disease Control and Prevention.

The proportion of hospitalized COVID-19 cases among vaccinated people has risen sharply since spring, coinciding with the sudden rise of the delta variant, according to the CDC's internal briefing slides reviewed by ABC News.

The finding doesn’t mean the vaccines aren’t working. People are still considerably less likely to wind up in the hospital or die if they are vaccinated, health officials have said.

The reality of more breakthrough cases also could be tied to the larger number of Americans getting vaccinated. While breakthrough cases remain relatively uncommon, the more people in the U.S. who are vaccinated increases the chances for breakthrough cases to be counted.

According to the CDC, vaccinated individuals who end up hospitalized tend to be elderly, medically vulnerable or live in long-term care facilities. They also are more likely to be asymptomatic and are hospitalized for reasons other than COVID-19.

On July 17, CDC Director Rochelle Walensky estimated that 97% of hospitalizations were occurring among people who weren’t immunized.

-ABC News' Anne Flaherty, Cheyenne Haslett, Eric Strauss