(CNN) -- Just as thousands more students return to classrooms this week, another study shows how easily children can get coronavirus.
More than 97,000 children in the US tested positive for coronavirus in the last two weeks of July, according to a new report published by the American Academy of Pediatrics and the Children's Hospital Association.
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.
At least 86 children have died as a result of the disease since May, according to the new report.
Last week, a 7-year-old boy with no pre-existing conditions died from COVID-19 in Georgia. In Florida, two teenagers died earlier this month, marking the state's sixth and seventh coronavirus deaths among children 17 and under.
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
Florida is still grappling with one of the worst infection rates in the country. As of Monday, more than 17% of tests taken turned out to be positive -- the fifth highest rate of any state in the US, according to Johns Hopkins data.
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.
Just a week after the school reopened, North Paulding is moving to online-only classes for at least Monday and Tuesday.
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.
The teen who shot the photo of the packed hallway, Hannah Watters, said she's received threats for posting the image online. But the sophomore said she's not deterred and just wants everyone to know how risky the situation is.
"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.
And another Georgia school district, the Barrow County School System, scrapped plans to reopen in-person next week after more than 90 staff members were forced to quarantine due to a confirmed, suspected, or possible exposure to COVID-19.
'This is all wholly preventable'
More than 5 million Americans have been infected with coronavirus, and more than 162,000 have died, according to Johns Hopkins data.
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."
The rate of daily COVID-19 deaths dipped nationwide in early July, but has steadily increased since then, according to Johns Hopkins data.
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
Contrary to President Trump's predictions, it's not possible to have a safe and effective coronavirus vaccine ready by Election Day, vaccine experts said.
Vaccinologists told CNN that timeline is unrealistic after reviewing data from Moderna, the first company to start Phase 3 clinical trials of a COVID-19 vaccine in the US.
"I don't see how that would be possible," said Dr. Paul Offit, a vaccinologist at Children's Hospital of Philadelphia.
Moderna must first finish enrolling participants -- possibly by the end of September -- give them their first shots, wait 28 days before giving a second round of shots, and then wait two weeks to see if the vaccine is effective.
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.
Dr. Peter Hotez, a vaccinologist at Baylor College of Medicine, reviewed the data and also said "there's just no way" a vaccine will be publicly available by November 3. Health experts say it's more likely the public could get a vaccine in 2021.
'We're just not being as cautious'
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."
At this rate, the Institute for Health Metrics projects the US death toll from coronavirus will reach 300,000 by December 1. But if 95% of Americans started consistently wearing face masks when leaving their homes, "more than 66,000 lives would be saved."
CNN's Naomi Thomas, Rosa Flores, Sara Weisfeldt, Elizabeth Cohen, Tina Burnside, Chandler Thornton, Melissa Alonso and Hollie Silverman contributed to this report.