The U.S. Centers for Disease Control and Prevention is investigating the cases. For now, it recommends that people with heart disease not be vaccinated until the investigation is completed, according to Reuters news service.

Earlier this month, 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 terrorists.

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

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

Maryland state health department spokesman J.B. Hanson told Reuters a preliminary autopsy of the Maryland nurse, who died last 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.

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

The federal government's smallpox vaccination plans have been marred by concerns about side effects as well as a dispute over who should pay for those who become sick after being vaccinated.

The vaccine being used comes from 30-year-old stocks. It uses a live virus called vaccinia that is related to smallpox and can have severe side effects, mostly in young children and those with weak immune systems.