The researchers found a 40% increase in child coronavirus cases in the states and cities that were studied during those two weeks.

The age range for children differed by state, with some defining children as only those up to age 14 and one state -- Alabama -- pushing the limit to 24.

While more classrooms are scheduled to reopen this week, some students who just started the school year must go back to virtual learning because of new infections among students and teachers.

Florida schools will reopen despite CDC guidance

But schools in at least 12 Florida counties will reopen this week for in-person learning, according to the Florida Education Association and school district websites.

Last month, the director of the Centers for Disease Control and Prevention said officials in counties where the test positivity rate is higher than 5% might consider keeping classrooms closed to students.

Yet out of those 12 counties where schools will reopen this week, at least nine had test positivity rates higher than 5% as of Saturday, according to the Florida Department of Health.

Students who returned to school must stay home again

North Paulding High School in Georgia made headlines after a student posted a photo of a packed hallway, with virtually no one appearing to wear a mask.

At least "six students and three staff members who were in school for at least some time last week ... have since reported to us that they have tested positive," Principal Gabe Carmona wrote in a letter to parents.

"We could have just delayed opening like many other schools," she said. "They kind of sent us to school and used us as guinea pigs to see what would happen later on."

The Cherokee County School District in Georgia said 19 students and four teachers or staff members have tested positive for since the start of the school year one week ago.

At least 16 schools across the county are impacted, the district said. Students and teachers affected must quarantine for two weeks.

'This is all wholly preventable'

Yet many Americans still aren't heeding the basic advice that would significantly reduce the spread -- and would help get more students back into classrooms safely, doctors say.

Why the CDC wants you to wear a mask

"We're still generating 50,000 to 70,000 new cases a day, 1,000 Americans are dying every day," said Dr. Ashish Jha, director of the Harvard Global Health Institute. "This is all wholly preventable."

Nationwide, 299 deaths were reported on July 5. On August 5, that number had jumped to 1,372. And for five days in a row last week, more than 1,000 Americans died each day.

'No way' for a vaccine by November

"I don't see how that would be possible," said Dr. Paul Offit, a vaccinologist at Children's Hospital of Philadelphia.

After that, researchers have to wait to see who gets sick with Covid-19, and who doesn't. Some volunteers will be getting a placebo instead of an actual vaccine.

Health officials have tied a resurgence in new cases to social gatherings among younger people, who can pass the virus to vulnerable populations.

Dr. Anthony Fauci had a message for those who might not take safety measures seriously: "Don't be the weak link in the chain."

"That's the message we've got to get to young people," he said during a webinar with the Alliance for Health Policy. "Obviously they're not doing anything deliberately and maliciously, but what they're doing is inadvertently -- they're propagating the outbreak."

And many Americans are now moving around as much as they did before the pandemic, said Dr. Christopher Murray, director of the Institute for Health Metrics and Evaluation at the University of Washington.

"If you look at the mobility data collected from cell phones in many parts of the country, we're almost back to pre-Covid levels of mobility," Murray said. "So we're just not being as cautious as other people are in other countries."

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97,000 children tested positive for Covid-19 in 2 weeks as more schools plan to reopen - CNN