But the company – which has never produced an approved vaccine or run a large trial – has squabbled with government scientists over the process, delayed delivering trial protocols and resisted experts’ advice on how to run the study, according to three sources familiar with the vaccine project. The sources said those tensions, which have not been previously reported, have contributed to a delay of more than two weeks in launching the trial of the Moderna’s vaccine candidate, now expected in late July.

Moderna “could be on schedule if they were more cooperative,” one of the sources told Reuters.

Some of the disagreements have stoked concerns over the young biotech firm’s relative inexperience and what the sources described as its lack of staff and expertise to oversee the most critical phase of human trials. The U.S. government is not facing similar problems with established drugmakers, such as AstraZeneca Plc and Johnson & Johnson, working on other leading vaccine candidates, the sources said.