The top US health department plans to require placebo testing for all vaccines in an effort to offer "straightforward" public health information, but experts say such testing could limit availability and raise ethical concerns.

In a statement first given to the Washington Post, the Department of Health and Human Services (HHS) said this week, "All new vaccines will undergo safety testing in placebo-controlled trials prior to licensure — a radical departure from past practices".

The agency did not provide details on which "new vaccines" would be included.

But officials have suggested that updated Covid-19 shots may be included, which vaccine experts say could slow down vaccine access.

Peter Lurie, a former official with the Food and Drug Administration (FDA), said "it's hard to tell exactly what is being proposed."

"But, broadly, if they mean that every modification to an existing vaccine would require a new placebo-controlled trial, they are treading in ethically dubious territory and likely to deny Americans life-saving vaccines at some point."

HHS has not offered details on the timing of the placebo plan or specify the vaccines involved.

An HHS spokesperson told the BBC in a statement that health secretary Robert F Kennedy Jr's goal of "radical transparency" means being "honest and straightforward about what we know — and what we don't know — about medical products, including vaccines".

The statement said none of the childhood vaccines recommended in the US - except the Covid shot - had undergone "inert placebo" testing, meaning "we know very little about the actual risk profiles of these products".

But public health experts say the statement is misleading, as childhood vaccinations, including ones for Hepatitis A and B, rotavirus, polio and the mumps, were all initially tested against a placebo. In fact, all new immunizations already go through the trials - a type of random testing where one test group receives the immunization, and the other gets a placebo, like a saline shot.

But newer versions of the shots may not go through the same process, because it is considered unethical to withhold a shot known to be safe from a particular group, and because the shot is only being tweaked in a minor way, vaccine experts said.