Merck executives dropped hints last week that the New Jersey drug maker's vaccine manufacturing plant in Durham has a good chance of attracting more jobs.

Projected to fill the first vials of vaccine for commercial use next year, the plant is already undergoing two expansions. Merck has committed to investing up to $750 million to complete construction by 2011 and create about 400 jobs.

Within the next year or two, Merck is expected to decide where to make two new vaccines it is still testing -- a combination pediatric vaccine and a vaccine to protect against staph infections. The Durham plant, which was dedicated as the Maurice R. Hilleman Center for Vaccine Manufacturing Oct. 15, will compete with Merck's two other U.S. vaccine production plants for the work.

"I'm not about to make an announcement," said CEO Richard Clark. "But I have a suspicion that we haven't stopped at $750 million [in Durham]."

When Merck selected the 262-acre site four years ago, executives said expansions were part of the master plan. In 2006, Merck announced the first expansion. The second expansion, a facility to make vaccine in bulk, was announced in June.

Further expansions would be necessary if the Durham plant is picked as the production site for additional vaccines, said Willie Deese, who heads Merck's manufacturing division.

Merck stands to receive as much as $45 million in state and local tax breaks and other incentives to build the facility, based partly on total jobs created.

Merck's continued expansion also would strengthen the Triangle's reputation as center for vaccine makers.