FORT JACKSON, S.C. — Thousands of Army recruits in training must line up at least once more before heading home for the holidays, this time for mass inoculations by the hundreds against swine flu.

The Army's largest training camp, just outside Columbia, S.C., and other posts are hurrying to finish the shots before the year-end break. More than 40,000 soldiers in advanced and basic training across the country head home over the next two weeks in a massive troop movement known as "block leave," Army officials said.

"We have been very aggressive in trying to assure the safety of our soldiers," said Maj. Soo Hee Kim-Delio, the Army physician in charge of the inoculations at Fort Jackson. "Our basic training population is at particularly high risk."

Swine flu, also known as 2009 H1N1, has proved to be similar to seasonal flu but a bigger threat to children and young adults. Many of the trainees are in their late teens and early 20s.

The soldiers are also vulnerable because of the physical stress of basic training and from living in close quarters with hundreds of other recruits, Kim-Delio said. When they return home, they may rub shoulders with a host of relatives or be around children fresh from exposure at school.

Fort Jackson recorded the Army's first death from complications of swine flu when Spc. Christopher Hogg of Deltona, Fla., died Sept. 10 from pneumonia due to H1N1 influenza, authorities said.

Fort Jackson started the shots about two weeks ago and expected to vaccinate as many as 9,000 soldiers by the end of last week and another 2,000 by the middle of this week. The trainers who remain on post, the Army's medical work force, family members, pregnant women and those with certain medical conditions such as asthma also will get the shots for a total of about 15,000 doses, Kim-Delio said.

Overall, the Department of Defense purchased 2.7 million doses of the H1N1 vaccine, which it is trying to deliver to soldiers, sailors, airmen and Marines around the globe before the end of the year, department spokesman Col. Wayne Hachey said.

At Fort Knox, Ky., home of the Army's Armor School, about 13,000 soldiers have been vaccinated for the swine flu, but the post won't get another shipment of doses until January. Capt. Franklin Hogue, who is in charge of the flu program there, said they have vaccinated approximately 70 percent of their troops.

At the Army's sprawling Fort Campbell, on the Kentucky-Tennessee line, more than 7,000 soldiers have been vaccinated, according to Laura Boyd, a spokeswoman for Blanchfield Army Community Hospital. Another 5,000 doses have been given to families, retirees, schoolchildren and others in the community.

Flu shots are mandatory for all active duty men and women unless they have a medical condition such as an egg allergy, officials said.

"It's just one more shot in a long list of shots," Staff Sgt. David Gavula said after rolling up his sleeve for a swine flu shot at this post's vaccination clinic.

Gavula, of North Kingstown, R.I., also got a seasonal flu shot several months ago. But if left to his own devices, "I probably wouldn't bother to do it," the 26-year-old infantry soldier said with a shrug.