The next phase of the smallpox vaccination program should be put on hold while health authorities investigate possible severe side effects, a CDC advisory panel recommended Thursday.

The original plan, proposed by President Bush in December, called for health care workers to be vaccinated first, followed by up to 10 million first-responders - police officers, firefighters and emergency medical workers - and then the general public.

The Advisory Committee on Immunization Practices, which meets several times a year to recommend changes in vaccination policy to the Atlanta-based CDC, voted unanimously to continue only current efforts to vaccinate public health response teams and hospital workers and not expand the program. Nearly 38,000 health care workers already have been vaccinated, but that figure is well short of the 500,000 called for in the original plan.

The panel cited the recently discovered occurrence of post-vaccination myocarditis, an inflammation of the heart muscle, and pericarditis, inflammation of the lining around the heart, as ``a new and unanticipated safety concern'' that mandates further investigation before expansion of the program should continue.

``The committee has believed from the very beginning that we need to put safety above and beyond any other issue,'' said Dr. John F. Modlin, a professor of pediatrics and medicine at Dartmouth Medical School who chairs the committee.

``This will allow us to buy some time, and to perhaps better understand both sides of the equation,'' he said, noting that further research is needed to determine the health risks involved.

In a news conference Thursday, CDC Director Julie Gerberding said she and Health and Human Services Secretary Tommy Thompson would consider the committee's recommendations ``respectfully,'' while also weighing national security concerns.

``We have traditionally held their advice in very high regard, and it does have a very strong, persuasive impact on the decisions that we make,'' Gerberding said. ``But it is not the only source of advice that we receive, and as we've been saying all along with smallpox, this is a situation where we are balancing public health issues . . . with homeland security issues.''

Since the smallpox vaccination program began with military personnel in December, nine people have had heart attacks after immunizations and three have died, according to the CDC, but it is not clear whether the deaths were related to the vaccinations.

All of the heart attack victims were middle-aged, and many of them had other health problems, such as clogged arteries, diabetes or smoking, that probably put them at risk.

Of the 454,856 military personnel who have received the smallpox vaccination, at least 50 experienced possible myocarditis or pericarditis symptoms, Col. John D. Grabenstein of the Military Vaccine Agency told the committee. At least 22 of the 37,608 civilian recipients of the vaccine experienced symptoms, according to Dr. Juliette Morgan of the CDC.

Morgan was one of several doctors and officials from the CDC to discuss the smallpox vaccinations with the committee before the panel voted on the findings.

The precise wording of the committee's recommendations has not been finalized, but CDC officials said the resolution probably would be passed on to Gerberding today.

The committee also pointed out that surveillance, training of response teams and careful planning at hospital, community and regional levels were important parts of preparedness.

``The question of how broad the smallpox program needs to be is something we've all paid careful attention to,'' Gerberding said.

``This is not about a number, it is not about should we have 40,000 people or 400,000 people or 4 million people,'' she said. ``It's about how do we get prepared.''

``This is different than normal immunization policymaking, because you have to consider more than public health issues,'' said Dr. Walt Orenstein, director of the CDC's National Immunization Program.

Though many of the presenters at the meeting discussed the health risks associated with smallpox vaccinations, CDC officials said overall health problems occurring as a result of the vaccine were far lower than expected.

CDC officials also noted Wednesday that not enough health care workers have been vaccinated.

``In some quarters, there is a perception that the threat of smallpox has diminished,'' Dr. Raymond A. Strikas of the National Immunization Program told the panel.

Last month, a committee from the Institute of Medicine made similar recommendations to the CDC, advising a ``pause'' in the smallpox vaccination program to ensure safety and evaluate preparedness in case of an attack.