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(CNN) — It’s the first day of school for more than 1.8 million students, and some will be part of a science experiment not on the curriculum.

Most of the biggest districts reopening Tuesday are starting the school year online only. But some will have students in classrooms — subjecting students and teachers to an experiment that has had mixed results so far.

Several schools have already had to temporarily shut down again after COVID-19 outbreaks this school year. Others, including some universities, have managed to keep their cases low after testing every student returning to school.

About 513,000 children have been infected with coronavirus as of September 3, according to the American Academy of Pediatrics and the Children’s Hospital Association. In the last two weeks of the study, the number of child cases jumped by 16%, or 70,630 cases.

“These numbers are a chilling reminder of why we need to take this virus seriously,” said American Academy of Pediatrics President Dr. Sally Goza in the groups’ weekly report on pediatric coronavirus cases.

“While much remains unknown about COVID-19, we do know that the spread among children reflects what is happening in the broader communities,” she added.

While COVID-19 deaths are rare among children and young adults, many young people are suffering long-term effects from the disease. And even those without symptoms can easily spread coronavirus to others.

Waiting for outcomes from Labor Day weekend

One big factor that could impact how rampantly Covid-19 spreads this fall is the outcomes from Labor Day weekend celebrations.

We won’t know the effects of Labor Day behavior for weeks. It takes up to 14 days for newly infected people to develop symptoms, and any new hospitalizations or deaths typically happen days or weeks after that.

To avoid mistakes made during the Memorial Day and July Fourth holidays, many Americans opted to stay home or socialize safely this Labor Day weekend. Others weren’t as cautious.

In Brooklyn, authorities shut down a bar over the weekend after deputies found nearly 300 people packed inside, CNN affiliate WABC reported. In Nashville, thousands flocked to the bars operating with restrictions, prompting the mayor to close down portions of a road to allow for more social distancing, CNN affiliate WKRN said.

In Rehoboth Beach, Delaware, crowds of visitors made social distancing nearly impossible, visitor Kristin Lefebver told CNN affiliate WBOC.

“In general I thought crowds were down this summer, until we hit Labor Day weekend,” Lefebver said. “As far as walking along the avenue, walking along the beach, being in the ocean, we’re quite on top of each other this weekend.”

Vaccine makers’ pledge could quash Trump’s claim

Despite President Donald Trump’s repeated claims that vaccines could be rolled out by Election Day, more health experts are saying that’s unrealistic.

“The reality is I do not see a way that we will have enough data for our Data and Safety Monitoring Board to say we have a vaccine, we can approve it, and it’s ready to roll out before Election Day,” said Dr. Carlos del Rio, a professor at Emory University School of Medicine and one of the investigators for the Moderna vaccine.

“This seems to be more of a political stunt than a public health gain. And we need to focus on the science … and let science drive the process.”

The only way a vaccine could be available by Election Day “is if there are so many infections in the clinical trial sites that you get an efficacy answer sooner than you would have projected,” Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Tuesday during an event sponsored by Research America.

“It’s unlikely that we’ll have a definitive answer at that time,” he added. “More likely by the end of the year.”

Moderna is one of nine vaccine makers that has issued a joint pledge Tuesday vowing to not cut corners in terms of safety just to get a vaccine out.

The pledge comes after US Food and Drug Administration Commissioner Dr. Stephen Hahn told the Financial Times that the FDA could consider emergency use authorization or approval for a COVID-19 vaccine before critical Phase 3 trials are complete.

The nine companies promised they would “Only submit for approval or emergency use authorization after demonstrating safety and efficacy through a Phase 3 clinical study that is designed and conducted to meet requirements of expert regulatory authorities such as FDA.”

Many health experts have said a safe and effective COVID-19 vaccine might not be publicly available until 2021.

What to do before a vaccine

Doctors worry that as the weather cools down, more Americans might start socializing indoors — where coronavirus can spread more easily.

One easy way to minimize the spread is with face masks. If 95% of Americans wore face masks consistently, about 70,000 lives could be saved between August and December, according the University of Washington’s Institute for Health Metrics and Evaluation.

The problem is not everyone is heeding that message, IHME Director Dr. Christopher Murray said.

“We’re seeing a rollercoaster in the United States,” Murray said.

“It appears that people are wearing masks and socially distancing more frequently as infections increase, then after a while as infections drop, people let their guard down and stop taking these measures to protect themselves and others — which, of course, leads to more infections. And the potentially deadly cycle starts over again.”

The effectiveness of face masks is obvious in countries that have lower rates of COVID-19 deaths than the US does, said Dr. Jonathan Reiner, a professor of medicine at George Washington University.

CDC official confirms America’s COVID-19 deaths are really COVID-19 deaths

“When you look at countries where the mortality is a fraction of what it is in the United States, the common theme from the very beginning of the pandemic was universal masking,” he said.