As many as 3,000 people who were exposed to anthrax bacteria, including several hundred Washington postal employees and Capitol Hill workers, may be asked to take an anthrax vaccine this week because of fears they still harbor the deadly spores in their bodies.
The vaccination program would be intended for people recently exposed to anthrax-laden letters and who are considered at high risk even after taking antibiotics for 60 days. Although the anthrax vaccine is said to be safe, federal officials said any vaccinations would take place on an "experimental" basis because the vaccine has not received full government approval.
Senior scientists will make a final recommendation about the plan to Health and Human Services Secretary Tommy G. Thompson early this week. The first vaccinations could take place within the week, officials said.
The plan promises to be controversial: Local health officials, who would be central to the implementation of any program, said they were unconvinced the vaccinations are necessary. A representative from a postal workers association expressed concern about giving the experimental vaccine to mail carriers, many of whom he said are middle-aged and had other health problems. Critics of the anthrax vaccine also pointed out that it has not passed all of the government's licensing inspections.
At a meeting in downtown Washington yesterday, top federal scientists said the measure was prompted by fears that anthrax spores could lie dormant in people beyond 60 days, before erupting in a fatal burst.
"The risk of infection . . . may be as long as 90 days," said D.A. Henderson, director of the Office of Public Health Preparedness at the Department of Health and Human Services. "That we don't have definitive answers is frustrating for all of us."
Many local workers were started on antibiotics in early October -- and some have completed the 60-day treatment.
Bradley Perkins, an anthrax expert at the Centers for Disease Control and Prevention, said that workers still thought to be at highest risk were those in buildings where the letters were opened, those who worked in facilities where others had fallen sick with inhalation anthrax and those shown to have inhaled large numbers of spores.
Nasal swabs from some Senate staffers showed more spores than were found in entire buildings elsewhere, said Greg Martin, a Walter Reed infectious diseases expert. Martin has told 75 Senate workers who are at highest risk to continue taking antibiotics past 60 days as a decision about vaccinations is being made.
Apart from Capitol Hill staffers, workers at the Brentwood postal facility in Washington topped the list of those at greatest risk, said Perkins, followed by those at the State Department annex where spores were found and New Jersey postal workers.
Another CDC official, Nancy Rosenstein, said 2,510 Washington postal workers and 95 staffers on Capitol Hill may still be at high risk. Many, but not all, would be counted among the 3,000 potential candidates for vaccination.
If the vaccination plan is approved, doctors will administer three injections at two-week intervals. Workers will be told to continue antibiotics until the final dose.
Alternative treatment options could include simple monitoring for anthrax symptoms or an extension of antibiotics without vaccination. Doctors are uncomfortable with the first choice because anthrax symptoms are easily confused with the flu. They are also concerned that the second choice may not offer enough protection.
Data on every aspect of human anthrax is limited by the disease's rarity -- only 18 cases of inhalation anthrax have occurred in the United States in the 20th century, Henderson said. But studies in animals exposed to anthrax spores show that although antibiotics hold the disease at bay, some animals develop the disease after the antibiotics are stopped. Other studies have shown that 2 percent of anthrax spores inside animals are viable after 50 days, 1 percent after 75 days and trace amounts after 100 days.
When animals are given a combination of an anthrax vaccine and antibiotics, they have better survival rates than animals given antibiotics alone. Scientists believe the vaccine helps the immune system build defenses that are still effective when the antibiotics are stopped.
One human study found that the vaccine protected mill workers from cutaneous anthrax. But all previous studies tested the vaccine before people or animals were exposed to spores -- one reason that giving vaccines after exposure is considered experimental.
The other reason is that the vaccine stocks that are most likely to be used have not been licensed. The Food and Drug Administration says the vaccine has passed muster, but its licensing is awaiting final evaluations at BioPort Corp., the Lansing, Mich., company that makes the vaccine.
Since that could take weeks, and because workers completing their 60-day course of antibiotics should, ideally, be vaccinated promptly, the program would be classified as experimental. All participants would be informed of the risks, and participation would be voluntary. 
Washington's top health officials, Ivan Walks and Larry Siegel, cautioned that any plan must be tailored to meet the needs of local postal workers. Local mail workers, two of whom died from anthrax, have complained that scant attention was paid to them even as doctors rushed to look after staffers on Capitol Hill.
Many mail workers have also stopped taking antibiotics because of side effects. Since they have not developed anthrax, there is some question of whether they were really at risk. 
"I am personally unconvinced that I would recommend vaccines to an individual on antibiotics knowing there is not a great risk of getting inhalation anthrax," said Siegel, Washington's deputy health director. 
Initial reaction among local postal workers was mixed: Lenster Banks said the antibiotic Cipro made him dizzy and gave him diarrhea but he would be willing to take a vaccine.
"I'll be the first in line," said Robert Dawson Jr. of Forestville. "If they don't know if the drugs are enough to kill the spores, I don't think we have any choice."
Cassandra Woodson of Temple Hills was adamantly opposed: "I took the Cipro and I took the doxycycline and they made me sick," she said. "I'm not taking nothing else."
Staff writer Tracey Reeves contributed to this report.