Children may be included in clinical trials to determine if an experimental Covid-19 vaccine works, FDA Commissioner Stephen Hahn told a Senate panel Tuesday.

A potential vaccine should only be tested on children once a candidate has shown promise in healthy adult volunteers, bioethicists said.

Talk of a potential decision to test the unapproved vaccine in children comes as some Republicans, such as Sen. Rand Paul (R-Ky.) called for reopening schools in the fall during Tuesday’s Senate health hearing. Just a few days earlier, New York Gov. Andrew Cuomo warned that three children died and more are becoming seriously ill with unconventional and dangerous symptoms.

There are 125 vaccine candidates in development, including one developed by scientists at the National Institute of Allergy and Infectious Diseases in the National institutes of Health that went into human testing in March.

But this study, known as a phase I trial, only involves healthy, adult volunteers because it’s the first time scientists have introduced the experimental vaccine into the human body. Testing in children would only happen in future larger studies that determine if the vaccine meets the Food and Drug Administration’s approval standards.

“That’s under discussion between FDA and NIH at this time because we do realize it’s important for us to understand what this is in children,” Hahn said during the Senate Health, Education, Labor, and Pensions Committee hearing on getting back to work and school.

The phase I study is fully enrolled, and NIAID Director Anthony S. Fauci said he expects to move into the phase II/III trials by late spring and early summer. “And if we are successful, we hope to know that in the fall and early winter,” Fauci told the Senate panel.

Steven Joffe, a bioethicist and a pediatric oncologist at the University of Pennsylvania medical school, said generally the initial testing to determine safety and whether the potential vaccine invokes an immune response, known as immunogenicity, should be done in healthy volunteers.

“But, for pediatric diseases (and we should consider COVID a pediatric disease both because kids get sick and because they seem to be sources of spread), vaccine testing in kids should follow very closely on the heels of vaccine testing in adults,” Joffe said in an email. “In other words, once you have that initial safety and immunogenicity data, testing in kids should follow at most a half step behind testing in adults.”

Many of the vaccine candidates in the pipeline probably won’t work, Benjamin Wilfond, director of pediatric bioethics center at Seattle Children’s Hospital, said in an interview.

“We don’t want to be exposing kids to the additional burdens and risks of being in a vaccine trial until we have more confidence that it’s going to be working relatively well and is relatively safe,” Wilfond said. “Whereas we might be more willing to do that in adults, to allow them to take on more risk.”

Generally, there’s an intentional focus on when to include children based on the quality of evidence about safety and efficiency. “This will be a moving target based upon the studies,” he said.

Wilfond said he was happy to hear Hahn say the FDA is considering whether to involve children. Research often hasn’t involved children, leading to drugs that are approved for adults or for youngsters over a certain age.

“Historically, this has a been a big problem,” he said, adding Hahn’s remarks are consistent with the direction in which the FDA has been going when including children in research trials.