The Durham County Board of Commissioners voted unanimously Monday night to offer Merck, one of the world’s largest pharmaceutical companies, a $2 million development incentive to expand its Treyburn Corporate Park facility.

The company has said it wants to add 150 jobs and invest $300 million in its vaccine-manufacturing facility in Durham.

The county staff had recommended approving the incentive.

Earlier this year, the company completed construction of a $300 million, 235,000-square-foot facility in Treyburn Corporate Park.

The expansion on which the commissioners held a hearing Monday would include a bulk-vaccine manufacturing facility as well as expansions to the utility, warehouse and administrative infrastructures.

Construction would likely begin this year and be completed in 2011, bringing the projected total number of employees at the site to approximately 400.