President Donald Trump claimed Thursday that "young people are almost immune" to Covid-19—a statement that came the same day the World Health Organization chief stressed that children can be infected with the coronavirus.

WHO Director-General Tedros Adhanom Ghebreyesus, speaking during a media briefing in Geneva on Thursday, said, "Although older people are at a higher risk of severe disease, younger people are at risk too."

"We have said it before and we’ll say it again," said Tedros, "young people are not invincible."

"Young people can be infected; young people can die; and young people can transmit the virus to others," he added.

Tedros's comments stood in stark contrast to Trump's assertion the same day downplaying the threat of Covid-19 to children as the president reiterated his call for schools to reopen for the fall term with students physically present.

The president was asked by the press on Thursday, "How can you assure people that schools will be safely reopened?"

"Can you assure anybody of anything?" responded Trump.

"Young people are almost immune to this disease," Trump added. "They're stronger. They're stronger. They have a stronger immune system. It's an incredible thing. Nobody has ever seen this before. Various types of flu will hurt young people more than older people."

Just "a tiny percent" of children get the virus, said Trump, "so we have to have our schools open. We have to protect our teachers. We have to protect our elderly. But we do have to have our schools open."

The "tiny percent" Trump referred to in his comments may be a reference to CDC data from Feb. 12 to April 2 showing that 1.7% of U.S. coronavirus cases, about 2,500, were in children.

While children are likely to exhibit milder symptoms than adults, infected kids may develop multisystem inflammatory syndrome in children. Further, the disease's long-term effects on the young and old are still unclear.

Reporting by the New York Times on Friday raises further concern about the risks to children and employees should schools physically reopen. 

Citing epidemiological research out of the University of Texas at Austin, the Times reported:

Based on current infection rates, more than 80 percent of Americans live in a county where at least one infected person would be expected to show up to a school of 500 students and staff in the first week, if school started today.

In the highest-risk areas—including Miami, Fort Lauderdale, Nashville and Las Vegas—at least five students or staff would be expected to show up infected with the virus at a school of 500 people.

Faced with potentially unsafe conditions for students, teachers, and staff, the threat of "safety strikes" looms.

"Let's be clear. Just as we have done with our healthcare workers, we will fight on all fronts for the safety of our students and their educators," Randi Weingarten, president of the American Federation of Teachers, told the union at its conference this week.

"But if authorities don’t protect the safety and health of those we represent and those we serve, as our executive council voted last week, nothing is off the table," she said, "not advocacy or protests, negotiations, grievances or lawsuits, or, if necessary and authorized by a local union, as a last resort, safety strikes."