Ten children were among the 78 new COVID-19 patients admitted to Baptist Health in Jacksonville, Florida, on Sunday, the hospital said.

The hospital has 584 COVID-19 patients as of Monday. Twenty-one of those patients are children, including six kids who are in intensive care, the hospital said.

-ABC News' Alexandra Faul

Monday marks the first day Canada is reopening to American visitors since the COVID-19 shutdown.

Tourists must provide proof of vaccination and a negative test.

Fifteen million Americans visited Canada in 2019, according to the government agency Destination Canada. The number of travelers dropped to 1.9 million in 2020, with the Canadian economy losing $11 billion in the process, according to Destination Canada.

Dr. Ashish Jha, dean of the Brown University School of Public Health, told "Good Morning America" Monday that he has a five-point strategy to get kids back to school safely:

When asked if the latest surge has peaked, Jha said, "We're hopeful, but we don’t know."

"The numbers are still increasing across much of the country. There are a few states that look like they may be turning" like Louisiana, he said.

-ABC News' Alexandra Faul

Children with COVID-19 used to make up 1% of patients hospitalized at Children's Hospital New Orleans. Now they account for about 20%, Dr. Mark Kline, physician-in-chief at Children's Hospital New Orleans, told "Good Morning America" Monday.

He said about half of the children hospitalized are under 2 years old. Most of the others are between 5 and 10 years old, so too young to be vaccinated.

"This is not your grandfather's COVID," Kline said. "This delta variant is an entirely new and unexpected challenge."

Dr. Peter Hotez, dean for the National School of Tropical Medicine at Baylor, warned on MSNBC that with "schools act[ing] as an accelerant, you should assume we're going to see pediatric intensive care units all across the South completely overwhelmed and even a possibility of small tent cities of sick adolescents and kids."

Hotez said parents need to know that "delta is something different" and "picking off young people like we've never seen."

"If your adolescent kid is unvaccinated, you should assume there's a high likelihood that that child is going to get COVID," he said, adding, "And we haven't even gotten to the 'long COVID' discussion around young people and what that means for their long-term cognitive health."

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