May 14 (Reuters) - U.S. Health and Human Services

Secretary Robert F. Kennedy Jr. plans to require all new

vaccines be tested against a placebo in human trials, and has

said that almost no shots used in the United States have

undergone such rigorous testing, which is not accurate.

The American Academy of Pediatrics has said childhood vaccines

are carefully studied in randomized controlled trials -

including with placebos - to ensure they're safe and effective.

Here is what you need to know about how vaccines are tested and

approved in the United States:

What has Kennedy said about vaccine testing?

In an appearance with television personality Dr. Phil McGraw

in April, Kennedy said that "the only vaccine that was ever

safely tested in a clinical trial against a placebo was the

COVID vaccine... None of the others were ever tested against

placebo."

He repeated that claim during heated Congressional testimony on

Wednesday.

What is a randomized controlled trial?

In order to receive approval from the U.S. Food and Drug

Administration, most new medical treatments must be tested in

large trials to confirm their effectiveness and safety.

In such trials, participants are randomly assigned to either

a group receiving the new medicine or a control group. That

ensures that any differences in outcomes can be attributed to

the new treatment.

In many randomized trials, a new therapy is compared to an

older, well established therapy. If no standard therapy already

exists, the new treatment is compared to a placebo - an inert

sham treatment without any therapeutic or physiological effect.

Are new vaccines always compared to placebos?

Not always. When a brand new vaccine is developed to protect

against a disease that has no preventative therapy on the

market, the FDA requires placebo-controlled trials to prove the

vaccine is safe and effective.

Drugmakers also develop vaccines to improve upon an existing

shot, either by updating an already-approved vaccine or by

identifying a mechanism for defending against the virus that

offers better protection and/or fewer side effects.

In those cases, the newer vaccine is compared to an existing

vaccine. It is considered unethical to randomly assign

volunteers to receive an inert placebo, leaving them - and the

unvaccinated people they may come in contact with - vulnerable

to a disease when a protective vaccine is available.

After successful testing in randomized controlled trials,

all vaccines are then monitored in "real world" studies, which

usually include more diverse patient populations and reflect

actual use in routine healthcare settings.

Have all childhood vaccines been tested against a placebo?

No. Childhood vaccines currently recommended for use in the

United States that have been tested against placebos include

Sanofi's Daptacel, used to protect against diphtheria,

tetanus, and pertussis (whooping cough), Sanofi's immunization

against respiratory syncytial virus (RSV) Beyfortus, and GSK's

Rotarix and Merck's ( MRK ) RotaTeq, the two rotavirus

vaccines licensed for use in the United States.

Some of the currently available vaccines were tested by

comparing them to already-approved vaccines.

For example, the very first combined vaccine against

measles, mumps and rubella (MMR), approved in 1971, was compared

in randomized controlled trials to a measles-only vaccine and a

placebo. But most subsequent MMR vaccines were not compared to

placebos because that would have left study participants

vulnerable to preventable diseases.

What do experts say about Kennedy's proposal?

Kennedy's proposal to require that all new vaccines undergo

safety testing in placebo-controlled trials "is ethically

problematic and will slow testing down for no good reason," Dr.

Seema Shah, director of research ethics at Ann & Robert H. Lurie

Children's Hospital of Chicago, said in a statement.