A panel of medical experts overseeing the Bush administration's smallpox immunization campaign advised yesterday against expanding the effort to millions of emergency response workers, saying a series of unexpected heart complications raises concerns about the safety of the vaccine.

The recommendation by the Advisory Committee on Immunization Practices comes as many state and federal health officials say privately that the program is at a standstill, falling far short of President Bush's desire to vaccinate millions. At its meeting in Atlanta yesterday, the advisory panel approved a one-page resolution saying it would be "unwise to expand beyond its current, pre-event smallpox vaccination recommendations" and begin inoculating first responders "because of the new and unanticipated safety concerns."

Joseph Henderson, the chief bioterrorism official at the Centers for Disease Control and Prevention, said the agency is conducting a six-month assessment of its smallpox preparedness efforts and expects to suggest adjustments to the White House with more focus on education and training, emergency response drills and faster reporting of suspicious outbreaks.

Investing almost solely in vaccinations "is not a practical approach right now," Henderson said in an interview. Nevertheless, he said, CDC is "committed to the president's decision" and intends to press for more volunteers to be immunized.

In a news briefing, CDC Director Julie L. Gerberding said the administration will review the panel's recommendation but is likely to encourage more people to be immunized.

"The more people we have vaccinated, the better off we'll be, and the fact that we have almost 40,000 people vaccinated is I think a tremendous step forward compared to where we were just six months ago," she said. "So we've made enormous progress, but we have more to do."

When Bush announced in December his plan to resume smallpox vaccinations, his health advisers outlined an ambitious timetable for inoculating nearly 500,000 health care workers in early 2003 and as many as 10 million medical and emergency response personnel by summer. So far, fewer than 40,000 people have been immunized.

Skepticism about the true threat of a biological attack, partisan wrangling over a compensation package for people harmed by the vaccine and public health crises such as the outbreak of severe acute respiratory syndrome dampened participation in the program, Henderson said.

States' participation has varied -- 34 people in Rhode Island have been immunized, compared with more than 3,700 in Florida, according to the June 13 CDC report. The District has immunized about 105, Virginia 867 and Maryland 735, according to the CDC.

From the outset, health experts knew -- and worried -- about complications from the smallpox vaccine, made from a live virus called vaccinia. But what startled the medical community were the few dozen cardiac cases, in particular some heart attacks and several cases of swelling in or around the heart.

The first hints of heart complications came in the military, which immunized more than 450,000 personnel before the war in Iraq. At least 27 military employees and 21 civilian workers suffered heart inflammation after being vaccinated, prompting the administration to tighten its screening procedures.

The panel and other medical experts have urged more extensive studies on the possible link between the vaccine and heart trouble before continuing the program.

Although smallpox was believed to be eradicated in the late 1970s, many experts fear terrorist groups or hostile nations such as Iraq may have acquired the germ and could use it as a weapon. There is no treatment for the highly contagious illness, but vaccination given within several days of exposure often protects people from the virus.

Last year, the advisory committee urged a small-scale, cautious approach to resuming smallpox vaccinations after 30 years. But for the first time in history, the administration overrode those recommendations and unveiled its plan for inoculating millions.

Several groups that have voiced skepticism about the original policy yesterday applauded the panel's cautionary note.

"The lesson for the future is that public officials can't ignore the voice of health care workers on the front lines when they make decisions that affect them and their patients," said Andrew L. Stern, president of the Service Employees International Union. As the nation's largest health care union, SEIU led the fight for compensation of injured workers.