New vaccines are often evaluated in trials in which some participants receive a placebo. But not all studies can be designed this way.

The Department of Health and Human Services last week announced a new standard for testing the safety of vaccines, a “radical departure from past practices.”

All new vaccines will be evaluated against a placebo, an inert look-alike that serves as a point of comparison, the department said. Health Secretary Robert F. Kennedy Jr., as well as many anti-vaccine groups, has long argued that placebo-controlled trials were the only way to fully understand vaccine side effects.

To scientists who have spent their careers evaluating vaccines, the plan did not seem so radical. New vaccines are often tested against a placebo in clinical trials. One researcher has created a crowdsourced spreadsheet of more than a hundred examples.

But it also concerned vaccine experts that Mr. Kennedy seemed not to recognize the circumstances when placebo groups are neither ethical nor practical. The idea is widely accepted by scientists and enshrined in ethics frameworks for medical research.

“He’s asking for something that’s not ethical,” said Arthur Caplan, a leading bioethicist at the New York University Grossman School of Medicine.

Why Are Placebos Used in Vaccine Trials?

Randomized placebo-controlled trials are often described as the “gold standard” of research: they allow scientists to tease out whether the effects they observe result from the drug itself or some other factor, such as the expectationof treatment.