By Lisa Richwine

WASHINGTON (Reuters) – The US government on Tuesday offered vaccination and extra antibiotics to people who were exposed to inhalational anthrax in recent mail attacks as an added precaution to prevent the development of the infection.

The US Department of Health and Human Services will make the vaccine available to individuals who were exposed to the bacterium and are finishing up a 60-course of prophylactic treatment. About 10,000 people, including congressional employees and postal workers, could be eligible for vaccination as part of a federal study, although many fewer are expected to opt for immunization.

Receiving the vaccine after exposure to anthrax is considered experimental because such treatment has not been approved by the US Food and Drug Administration. Critics also say the vaccine may cause serious side effects. The first shots could be given as early as Wednesday, officials said.

Experts are concerned that some anthrax spores may survive for longer than 60 days in the lungs of people who inhaled the highest doses and they hope the vaccine will boost immunity. So far, no known anthrax cases have developed in people who recently were exposed and prescribed treatment with antibiotics for 60 days.

But studies in monkeys found traces of anthrax spores in the animals' lungs up to 100 days after exposure, raising the question of whether the same thing could happen in people.

If infection did develop, health officials believe it could be treated successfully, as were other inhaled anthrax cases that were caught early, according to an HHS statement.

Still, officials felt it was necessary to offer people additional options "out of an abundance of caution," HHS Secretary Tommy Thompson told reporters.

Anthrax-tainted letters were sent to two US senators and to media outlets by an unknown perpetrator in the aftermath of the September 11 attacks on the US. Five people have died from anthrax since early October, and 13 others have been infected.

HHS officials said the approximately 10,000 people who have been placed on the 60-day regimen have three options.

They can elect to be vaccinated, getting three vaccine doses over a four-week period, plus 40 additional days of antibiotics. They can skip the vaccine but get the 40 additional days of antibiotics. Or they can skip the vaccine and end their antibiotic treatment after the original 60-day regimen, but closely monitor for signs of illness.

HHS officials said people have to make their own decisions about whether to have the vaccine after consulting with a physician. Officials said people who elect to take the vaccine would be informed of risks and required to sign a consent form.

"The data that's supporting giving the vaccine are very slim indeed," said Dr. D. A. Henderson, head of the HHS Office of Public Health Preparedness. "We do not have a lot of information about this. Anthrax is a rare disease, and this would represent a different way of using the vaccine."

Officials said about 3000 people would be considered at the highest risk for developing anthrax because they were close to where a contaminated letter was opened or worked near someone who became ill.

This may include workers at the Brentwood central postal facility in Washington and staff members for Senate Majority Leader Tom Daschle, whose office in the Hart Senate Office Building received an anthrax-laced letter.

Speaking on condition of anonymity, a congressional aide who had worked in that building before it was closed for decontamination said, "Some folks will say, 'OK, we will take it [the vaccine]. But I think a lot of others won't." The aide added, "Members of the armed services have been reluctant to take this, and if they have been reluctant we will be, too."

Military troops who were vaccinated to protect them from a battlefield attack with the germ warfare agent have said the shots caused serious, chronic health problems. Health officials say the vaccine has not been linked to long-term problems but that studies are ongoing. The most common side effects are local reactions such as redness and swelling, they said.

Daschle said he supported the decision to offer the vaccine. "I think it is a wise course of action, simply because of the unknowns, because of the uncertainty," he told reporters.

The US Postal Service was "disappointed" it did not have more advance notice that the vaccines would be offered, spokeswoman Kristin Krathwohl said. She said thousands of postal workers may be interested in the vaccine.

"Logistically this is a challenge, but we will make it possible for our employees to get the medical care they desire," Krathwohl said.

Maker Bioport Corp., based in Lansing, Michigan, has had manufacturing problems that have delayed regulatory approval for its new plant.

HHS officials said it has obtained 10,000 doses from the Defense Department. The doses are from a lot made recently at Bioport, but they have not received full regulatory approval because the plant lacks clearance.