It'll likely be a long time before children are vaccinated against COVID-19, even though vaccinating kids could eventually play an integral role in reducing the virus' spread.

The big picture: None of the leading contenders in the U.S. are being tested for their effectiveness in children. Even once one of them gains authorization from the Food and Drug Administration, there will only be a limited number of available doses.

It's normal for vaccines to be tested on adults before being tested on children.

Why it matters: Children have a relatively low risk of severe coronavirus infections. But they can still spread the disease to more vulnerable adults. Vaccinating kids could play an integral role in reducing community spread, and of reopening schools.

• For now, the consensus is that those vulnerable adults — which could include teachers and school staff — will likely be able to get a vaccine early in the process. Eventually, though, vaccinating children would be helpful.
• "If you protect children, then you will reduce community spread and protect adults, so it's more of a herd immunity rather than a specific immunity question," explains John Moore, a professor of immunobiology at Weill Cornell Medical College. "If you need 70% protection in herd immunity, at some point you will need to include children in that."

What they're saying: Randi Weingarten, president of the American Federation of Teachers, recently told Axios that her union would support requiring in-school teachers to take a COVID-19 vaccine, once one has been approved and is readily available.

A mandatory vaccine for kids would be a long way away — we'd need to have one before we could even consider mandating it — but would likely be controversial, as so many vaccines have become.

• "We don’t know if it's safe, or how kids will respond to it, so I can’t say next year that they definitely should get it," said Rhea Boyd, a pediatrician andpublic health advocate. "I don’t think we would require it in a year from now, and maybe not in five years from now.”

Regulators and vaccine developers will face a series of difficult decisions, once it comes time to start testing a vaccine in children — likely the first big controversy in this phase of the vaccine race.

• Move too fast, and parents will question whether the product is safe. Move too slow and the virus will be with us longer.
• "I think they’ll try to come up with a compromise that gives them some safety information in the minimal amount of time possible," says The Mayo Clinic's Rick Kennedy, who studies the development of immune responses after vaccination.

The bottom line: We know much less about a child vaccine than we do about an adult one, but do know that the ethical issues could prove even thornier.