Cabinet secretaries and other top officials are gathering at the White House complex Saturday to act out the roles they would play in a smallpox outbreak.

The four-hour drill, designed to test the federal government's smallpox response plans, won't be attended by the president, White House press secretary Scott McClellan said Friday.

Officials conducted a similar "tabletop exercise" in December to examine the nation's readiness for pandemic flu. The drills are kept behind closed doors.

"The president directed his Cabinet to conduct a series of such exercises to assess our response efforts to catastrophic attacks," McClellan said.

The United States ended routine childhood vaccination against smallpox in 1971, and the World Health Organization reported the disease was eradicated in 1980.

But there are fears that smallpox could be used by terrorists as a biological weapon. About 15 months after the Sept. 11, 2001, attacks, the U.S. government ordered certain military personnel vaccinated and recommended shots for front-line health care workers.

The president did not attend the pandemic flu drill, and was to be at Camp David, the presidential retreat in the nearby Maryland mountains, during this one.

After the flu drill, conducted around a C-shaped table for four hours, officials said it showed that saving lives and containing the economic damage of a pandemic flu would require more planning in local communities and increased production of vaccines and medications.