Middle-aged Americans probably remember it as little more than a ho-hum trip to the family doctor, but the resumption of large-scale smallpox vaccinations after a 30-year hiatus promises to be anything but simple.

The Bush administration is expected today to set in motion plans to inoculate 500,000 military personnel and as many as 500,000 health care workers nationwide -- the front-line defenders in any biological warfare attack against the United States.

As more vaccine becomes available, federal health officials say the "Phase I" program will soon expand into a "Phase II" to include 10 million to12 million additional Americans -- virtually the nation's entire contingent of police, firefighters and other first responders. The vaccine would probably be offered to the general population by 2004.

Sixty-two states, cities and territories have been getting ready for Phase I for months, calling for volunteers, educating them on the vaccine's dangers and preparing to monitor its effects. Wyoming conducted drills to determine how long it took to vaccinate 100 people.

"We are extremely pleased and quite impressed with the plans that have been submitted so far," Health and Human Services Secretary Tommy G. Thompson said yesterday. "It is obvious the state and local health departments and hospitals have stepped up to the plate under an extremely tight timetable."

The military plan, unlike the civilian version, is mandatory, according to a key member of Congress, but in other respects will largely mimic civilian procedures -- vaccinating first-line medical personnel before immunizing soldiers in high-risk areas such as Iraq and the Middle East. The Pentagon did not respond to requests for information on its plan.

Despite administration enthusiasm for the vaccinations, many questions remain unresolved.

"This is not something that anyone can take lightly," said Diane Sosne, a registered nurse and leader of the 16,000 health care workers in the Washington state branch of the Service Employees International Union. "We want to make sure this doesn't pose more risks than it is supposed to address."

The smallpox vaccine, used throughout the United States until 1972 and in the U.S. armed forces until the 1980s, can have extreme side effects. For every million people vaccinated, as many as 52 will suffer serious illness, including prolonged fever, and one or two will die.

At the least, people who receive the vaccine -- usually in a series of pinpricks on the upper arm -- can expect swelling, redness, itching and a large, lingering scab. A few days of fever or malaise is common.

Such adverse reactions were deemed more than acceptable when smallpox was a world scourge, but there has not been a reported case anywhere in the world since 1977.

Doctors warn that the vaccine poses unacceptable risks for babies, people with a variety of skin conditions and anyone with a weakened immune system.

"In the old days, we didn't have a lot of the medications we have today, so those patients would die," said Laurene Mascola, who is in charge of the vaccine program for Los Angeles County, Calif. "Today there are many more cancer patients living, more people on steroids, and more people living with HIV."

The Service Employees International Union, the nation's largest health care workers union with 1.5 million members, criticized the administration earlier this month for failing to provide compensation for volunteers who may miss work, become ill or even die from taking the vaccine.

"This is an economic decision as well as a health care decision," said SEIU President Andrew L. Stern. "Suppose I'm a per diem nurse, and I'm out for a couple of days or I go to the hospital. That has to come out of my pocket. This should be a no-fault situation."

On Capitol Hill, Rep. Christopher Shays (R-Conn.) questioned whether the armed forces will adequately screen personnel likely to suffer adverse reactions:

"It's mandatory, so people will take this vaccine against their will," said Shays, a longtime critic of Pentagon vaccine policy. "Since they will have to comply, it's important to determine who should get it."

Shays said medical workers in the military must not cede control of the vaccination process to field commanders with little expertise: "The record of the military in following private-sector procedures is basically nonexistent," Shays said.

Despite these misgivings, the public climate for the vaccination program appears to be markedly positive. A Robert Wood Johnson Foundation poll released Wednesday found 65 percent of respondents are willing to get a vaccination.

"I thought we'd be having a lot of our people saying this is a bad idea," said Wyoming state epidemiologist Karl Musgrave, in charge of vaccinations in the least populous state. "But more questions came from people who wanted to do it and were asking for a waiver -- one nurse was on chemo, another was taking steroids and another had diabetes. No matter what, they want to take it."

In Wisconsin, Herb Bostrom, the director of the Bureau of Communicable Diseases, found the same enthusiasm, "even though significant questions remain" about compensation and medical care. "People in hospitals and public health recognize they are supposed to do this, and they will," Bostrom said.

The U.S. Centers for Disease Control and Prevention gave states and local governments until Dec. 9 to submit plans for conducting vaccinations both before and after a smallpox outbreak. The CDC said yesterday that plans it has examined so far envision inoculating almost 450,000 people.

Musgrave said Wyoming's plan calls for "about 1,000 people" to receive the vaccine in Phase I, a task he said could probably be completed in a few hours. He said eight communities practiced vaccinating 100 people this summer and managed the job "in a couple of hours."

Elsewhere, authorities appeared less inclined to move that quickly: "We'll start slowly, do one site first and see what happens," said Georges C. Benjamin, former Maryland secretary of health and mental hygiene, who said his state expects to vaccinate between 6,000 and 8,000 people during the initial phase. (Benjamin resigned Sunday to take another position.) He said Maryland could complete the vaccinations "within a week," but is in no hurry: "There is no emergency. We're going to look at the experience, and see if our criteria are too rigid or too loose. There's a learning curve."

In Los Angeles County, Mascola will vaccinate as many as 12,000 people but is "not interested in doing it until January," when the worst of the flu season is over.

Staff writer Ceci Connolly contributed to this report.