The Obama administration’s bioethics commission on Tuesday laid out guidelines for testing anthrax vaccine in children that make such studies extremely difficult and probably impossible.

Studies of the controversial vaccine’s effects on children can be done only if the research poses at most a “minor increase over minimal risk” to their health. That would be the equivalent of becoming sick enough to miss several days of school or getting a chest X-ray.

“Higher risk is unacceptable in the context of pre-event . . . research,” the 13-member commission wrote in a 146-page report.

The advice would appear to preclude immediate studies to test the safety of the three-dose vaccine and the immune system’s response to it, which another federal body, the National Biodefense Science Board, had recommended, pending a review by the bioethics commission.

The panel, comprising mostly physicians and ethicists, based its guidance on two arguments.

One is that children in anthrax studies were not likely to benefit personally because the infection is more of a theoretical than a real threat. The other is that children, because of their age, cannot give informed consent to accept unnecessary risks, as adults are sometimes willing to do.

However, once an anthrax attack occurs, the ethical balance tips, the experts argued. Children might then have something to gain from participating in the research. Planning for those studies should begin now, the experts added.

“This was one of the most difficult bioethical reviews that any national bioethics commission has ever conducted,” said Amy Gutmann, president of the University of Pennsylvania, who chaired the panel.

The one licensed anthrax vaccine in the United States is approved for people ages 18 to 65. It is used principally by the military.

Shortly after the Sept. 11, 2001, terrorist attacks, letters containing powdered spores of the bacterium that causes anthrax were sent to two senators and several news organizations. Nearly two dozen people became ill, and five died. The attack spurred a $5.6 billion program to devise and study “medical countermeasures” against bioterrorism attacks. Anthrax vaccine is one of the countermeasures.

Opinion differs on whether the “minimal risk” standard invoked by the bioethics commission will prevent pediatric anthrax vaccine research in advance of an attack.

“I think it does,” said Michael T. Osterholm, a former Minnesota state epidemiologist who heads the Center for Infectious Disease Research and Policy in Minneapolis. “I think it ultimately puts a hold on any kind of a research protocol in these kids.” He added that he “can understand completely” the argument that studies that occur in advance of any event should not risk children’s health.

“Some will say they put all these regulations in and it’s never going to happen. But I believe that these studies slowly but surely will get done,” said Michael R. Anderson, chief medical officer of UH Rainbow Babies and Children’s Hospital in Cleveland. From 2009 to 2011, he chaired the National Commission on Children and Disasters.

In its report, the Presidential Commission for the Study of Bioethical Issues said it might be possible to do anthrax safety and immune-­response studies exclusively on 18-year-olds. If the vaccine proved to pose no more than minimal risk in them, the studies could be repeated in 17-year-olds, with the process slowly stretching back to infancy.

“Anthrax vaccine in adults generally does not pose more than minimal risk,” Gutmann said. “But that’s too general to extrapolate” to young children without step-wise studies, she said.

John S. Parker, a retired Army major general who heads the National Biodefense Science Board, said it would be possible to find parents willing to enroll their children in anthrax vaccine studies.

“There are people who are first-responders who would very much like to have their families protected,” he said.