ATLANTA (Reuters) Mar 26 - U.S. health officials said on Tuesday night they were investigating whether the smallpox vaccine had contributed to the death last weekend of a Maryland nurse and serious side effects in six other people recently inoculated against the virus.

The U.S. Centers for Disease Control and Prevention, which has been spearheading a campaign to vaccinate almost half a million front line health care workers and technicians, said it was recommending that people with heart disease not be vaccinated until an investigation was completed.

"We're adding a temporary deferral for the smallpox vaccine for persons who have been diagnosed with a history of heart disease," CDC spokeswoman Karen Hunter said. Hunter added that the unidentified Maryland woman had heart disease before being vaccinated.

Earlier this month, top U.S. health officials said that reports of side-effects linked to the current smallpox program were overblown.

The United States stopped routine smallpox vaccinations in 1972, but decided to resume them for select groups last year as fears grew that the virus could be used as a weapon by radical groups or countries like Iraq.

When administered in the past, the vaccine caused fatal complications in between one to two out of every million people inoculated and caused others to suffer brain damage. But it has never before been linked to heart problems.

Maryland state health department spokesman J.B. Hanson told Reuters a preliminary autopsy of the Maryland nurse, who died on Sunday after receiving a smallpox vaccination on March 18, showed the cause of death to be "some sort of heart condition" although that was not yet certain.

"I can assure you they are vigorously looking into this," Hanson said, noting the Department of Health of the neighboring state of Virginia, where the woman died, was assisting Maryland and the CDC.

"At this point there's no correlation between the vaccination and her death," Hanson said.

He could not give the nurse's name or age, but said she worked at a private Maryland hospital and was one of 414 state health workers who had been vaccinated so far as part of the national vaccination effort.

If confirmed, however, it could be the first smallpox vaccine-related death since the start of the national vaccination campaign in January.

President Bush ordered the vaccination program late last year amid growing fears that smallpox and other deadly pathogens could be used in a bioterrorist attack. But the federal government's smallpox vaccination plans have been marred by concerns about side-effects as well as a sharp dispute over who should pay for those who become sick after being vaccinated.

The CDC says about 20,000 civilian health workers have been vaccinated across the country so far against smallpox, so they would be ready to vaccinate others and treat patients in case of an attack.

Some 500,000 troops are being vaccinated and the U.S. Health and Human Services Department hopes to eventually vaccinate 450,000 health care workers in the first round of the program.

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