"Since 2001, the threat of terrorism has been evident and we've been worried about bio-terrorism the last few years," Thomason said.
He said the purpose of the "no blue flu" exercise is to test the department's capacity to immunize its first responders during an outbreak such as pandemic flu.
Department employees have the opportunity to receive a flu vaccination as part of the simulation.
Participation in the exercise is mandatory for all of the department's staff, including civilian employees. The flu vaccination is voluntary.
Thomason said 900 employees have gone through the exercise so far and 400 of them have elected to get a flu vaccination.
The drill began Monday and will end at 6 p.m. today.
Joining Thomason at the drill site at Oakland police headquarters, Zerlyn Ladua, the public health emergency preparedness coordinator for the Alameda County Public Health Department, said the drill is part of her department's preparation for terrorist events.
Ladua said that in the event of a bio-terrorism attack, the Public Health Department's goal is to get medicine to all of the county's 1.5 million residents within 48 hours.