Biotech giant MedImmune has hired 400 of its 800 planned new workers in 2008 as part of an aggressive growth strategy following its acquisition by British pharmaceutical giant AstraZeneca last year.

MedImmune is recruiting from biotech hubs in the United States and in Europe for its mid-Atlantic, West Coast and U.K. facilities. Most of the new hires are in the Gaithersburg and Cambridge, U.K., offices, Copernik said. MedImmune’s facility in Frederick, in the midst of a major expansion, will not be affected by new hires, Copernik said.

‘‘Recruiting is a very important effort for the company this year,” Copernik said. ‘‘It’s an important investment.”

MedImmune develops treatments and vaccines for infectious diseases, cancer and inflammatory diseases. It is best known for its production of FluMist, a nasal-spray influenza vaccine that is manufactured in Philadelphia for now.

Meanwhile, MedImmune recently added a co-general contractor for the next construction phase of its 710,000-square-foot manufacturing facility in Frederick.

Fluor Corp. of Irving, Texas, began work last month, partnering with the original contractor, Parsons, to assist with ‘‘the scope, the scale and the complexity” of the biologics building project, said MedImmune spokesman Tor Constantino.

‘‘The original engineering contractor is continuing to remain on the job,” Constantino said. ‘‘This kind of additional resource deployment with a complex life sciences building is pretty common.”

The Frederick expansion, which broke ground in September 2006 at 636 Research Drive next to the company’s original facility, is on track for occupancy by early 2009 and is still within its roughly $250 million budget, Constantino said.

Coakley & Williams Construction Inc. of Gaithersburg won the contract to perform interior architectural work for the Frederick manufacturing facility. It is Coakley & Williams’ fourth project for MedImmune, which is based in Gaithersburg. The company previously worked on MedImmune’s 25,000-square-foot pilot lab in Gaithersburg and now is under contract to complete the renovation of two more labs.

The new Frederick plant will increase the company’s cell culture production and is expected to create 225 jobs. The expansion will support the company’s $170 million, five-year award from the U.S. Department of Health and Human Services to develop influenza vaccines for pandemic and seasonal purposes.

MedImmune’s original 150,000-square-foot Frederick building was built from 1996 to 1998 for production of the company’s biggest money-maker, Synagis, which is designed to prevent respiratory infection in babies. In the first quarter of 2007, the last period for which sales data are available, Synagis sales totaled $507 million.

The general contractor for the first building — a $60 million project — was Riparius Construction Inc. and the construction manager, architect and designer was Fluor Daniel, according to MedImmune’s Web site. Fluor Daniel is the predecessor of Fluor Corp.

MedImmune plans for the Frederick site to be its primary manufacturing facility and for its Gaithersburg building to house its research and development operations and corporate offices. There are 1,000 employees in the two offices: 225 in Frederick and 775 in Gaithersburg.

Since its acquisition by AstraZeneca, MedImmune has doubled its pipeline and increased productivity targets. MedImmune plans to represent 25 percent of AstraZeneca’s late-phase development projects by 2010, according to its statement.