WASHINGTON (Reuters) - A Maryland nurse who died after getting a smallpox vaccine suffered a heart attack, which suggests the shot did not kill her, health officials said on Wednesday.

But officials at the Centers for Disease Control and Prevention put out a warning saying people with heart disease should not get the smallpox vaccine, just in case.

The case drew attention because the U.S. government, fearful that smallpox could be used as a biological warfare agent, has begun a controversial effort to vaccinate nearly 1.5 million troops and health-care workers against the disease.

The nurse, who worked at a private Maryland hospital, died on Sunday in Virginia. She got her smallpox immunization on March 18.

"The autopsy was conducted on Tuesday and it indicates a heart attack was the cause of death," Lucy Caldwell of the Virginia Department of Health said in a telephone interview.

This, said Caldwell, suggests but does not prove that her death was not a side effect of the vaccine.

The nurse was one of seven female health-care workers, all in their 50s, who developed heart symptoms after being vaccinated, said CDC head Dr. Julie Gerberding.

"We promised to closely monitor this program and to put safety first, so we are exercising exceptional caution," Gerberding said in a statement.

"We have seven patients with what looks like coronary artery disease and two additional patients that have inflammatory conditions that have affected the heart."

Gerberding said she doubted any of the cases were caused by the smallpox vaccine but the CDC was being extra careful. She noted all the patients had heart disease to begin with.

CASES IN MILITARY, TOO

But there have also been 10 cases of inflammatory heart conditions such as myocarditis among troops vaccinated for smallpox, said California Democrat Henry Waxman, ranking minority member on the House Committee on Government Reform,

"According to the Department of Defense, approximately 350,000 individuals in total have been vaccinated," Waxman wrote in a letter to Gerberding on Wednesday.

"This means there have been 10 cases of myocarditis or pericarditis among the 250,000 who are first-time vaccinees."

All the patients had recovered, but Waxman asked the CDC to publicize the information.

The CDC says more than 25,000 civilian heath workers have received the smallpox vaccine. The U.S. government hopes to vaccinate 450,000 health workers in this first phase of the program. Half a million troops are also due to be vaccinated.

Smallpox was eradicated in 1979 but the government fears Iraq and other countries have developed smallpox for use as a biological weapon.

The vaccine can have severe side effects. In the past, it killed between one and two people per million who got the shot and made 52 severely ill.

Dr. David Waagner of the University of Texas Health Sciences Center in Lubbock said the vaccine was never linked in the past with heart conditions. "But it was predominantly given to children who didn't have risks for coronary heart disease," he added in a telephone interview.

"We are vaccinating older persons now than we used to. You are talking people in their 40s and 50s who have more of a risk of heart disease."

Congress is working on a plan to compensate volunteers or their families should they die or become seriously ill from the vaccine.