Doctors are planning an unprecedented clinical trial in which the smallpox vaccine will be tested in a small number of children in two states, federal health officials confirmed yesterday.

The idea is to establish vaccine dosages suitable for children in the event that mass vaccination is needed because of a bioterrorist attack.

Tests, which will be conducted at Cincinnati's Children's Hospital Medical Center and Harbor-UCLA Medical Center in Southern California, will mark the first time the vaccine has ever been tested in children, using the rigors of modern science.

Though doctors administered the vaccine successfully to children for two centuries prior to the deadly disease's eradication in 1980, a formal controlled clinical trial had not been performed even in 1960s, a period well within the era of evidenced-based medicine.

Doctors involved in the design of the trial hope testing will begin before the end of the year. Fewer than 50 children are expected to be entered into the trial in both states.

"We're just waiting for the final word, to hear that it's a go. All of the pieces seem to be in place," said Dr. David Bernstein, director of the division of infectious diseases at Cincinnati Children's Hospital Medical Center. Bernstein will be the chief investigator of the study's Ohio arm.

He does not foresee trouble enrolling children into the trial and is in discussion with Cincinnati pediatricians about the test. Because test subjects must be closely monitored and wear special bandages after their inoculation, neither site will accept enrollees from out of state.

"This is a good vaccine and millions of children have been safely immunized with it," said Dr. Michael Lane, a smallpox expert now retired from the Centers for Disease Control and Prevention.

Medical researchers plan to test the Dryvax vaccine, Bernstein said, which was the same one used to eradicate smallpox in an aggressive global campaign in the 1960s and '70s. It is also the same vaccine administered in a nationwide adult clinical trial that began late last year. Findings announced in March showed that Dryvax could be diluted and still maintain its effectiveness.

"It's obvious that we should study this in children if we're going to use it in children," Bernstein said. "Children are not small adults."

But Dryvax is not problem-free. Although doctors believe current medical knowledge may make the vaccine's use safer, it is a live-virus vaccine that has caused encephalitis and brain damage in some recipients. It also carries a 1 in 1 million chance of death, a ratio worked out by Lane and colleagues in a series of groundbreaking studies on the vaccine conducted at the CDC in the 1960s.

Vaccine doses to be used in the pediatric trial are more than 40 years old and are based on viral stocks that first were prepared by the New York City Health Department at the turn of the 20th century. Dryvax, public health officials say, is a mainstay in the National Pharmaceutical Stockpile. The stockpile is the government's cache of medications, vaccines and other supplies that are deployed in the event of an attack. The stockpile is kept in several secret locations.

Dr. Carol Heilman of the National Institute of Allergy and Infectious Diseases, and one of the pediatric trial's designers, said researchers will be seeking some of the same answers that were sought in the adult study.

"We would like to know, for example, if a dilution [of the vaccine] can be effectively administered and still have it maintain its potency," Heilman said.

In the adult trial, lower strengths than would have been administered in the 1950s or '60s proved powerful enough to generate a "take," evidence of an immune response.

Heilman said there are still technical matters that must be resolved before the trial can begin. For that reason, no one can set a starting date, she said.

In addition, the trial's protocol is still under review by the National Institute of Allergy and Infectious Diseases. A core unresolved matter is the age groups of children that will be sought.

Heilman noted that a number of ages have been suggested, but key protocol administrators at the institute and the Food and Drug Administration have yet to settle on a specific age group. The trial is to be jointly overseen by the National Institute of Allergy and Infectious Diseases and the Food and Drug Administration.

She said a separate study is being planned for elderly people and will be designed to address problems associated with declining immunity. The immune system weakens with age, and researchers will attempt to see which dosages works best in older people.

Heilman added that the Institutional Review Board at Harbor-UCLA, a panel that must approve human-subject testing, had some questions about risks and benefits from the proposed pediatric trial, but is in favor of the study. Dr. Joel Ward of the UCLA Center for Vaccine Research will lead the investigation's California arm. He was out of the country yesterday and unavailable for comment.

Dr. Jon Abramson, chief of pediatrics at Wake Forest University and liaison to the CDC's national immunization committee, said the study is vital.

"Can you imagine using any medication or vaccine and never studying how you're going to use it? I could never make that leap of faith. Never having studied it in a pediatric population is inconceivable."