30,000-patient trial of Moderna?s?coronavirus?vaccine candidate, expected to start next week, has been delayed, a potential hurdle in the company?s ambitious effort to deliver key data by Thanksgiving. Moderna is making changes to the trial plan, called a protocol, which has pushed back the expected start date of the Phase 3 study, according to investigators. The investigators, who spoke on condition of anonymity, emphasized that protocol changes are common but said it?s not clear how long the start will be delayed. ?My understanding was that they wanted to get the first vaccines given in July, and they say they?re still committed to do that,? one investigator said. ?As best I can tell, they?re close to being on target for that.? nvestigators at the University of Illinois at Chicago had previously said Moderna?s trial would?begin July 9. On Thursday, NIH Director Francis Collins also told lawmakers in Washington that the study would begin this month. Moderna did not respond to multiple emails asking about how long the delay will last, the nature of the protocol changes, or whether they have anything to do with the vaccine?s safety or manufacturing. After publication, CEO St?phane Bancel?told CNBC?that Moderna still intends to start the trial in July. In a statement?posted to Twitter?on Thursday afternoon, the company said it has ?worked closely? with the National Institutes of Health, which is funding the Phase 3 study, ?to align the final protocol in order to begin the trial on time.? The intense focus on the exact timing of the trial stems from the tight nature of the?race to develop a vaccine?for the novel coronavirus ? and the fact that any delay could imperil Moderna?s pole position. Pfizer, working with the German firm BioNTech, plans to start a 30,000-patient study of its own later this month. AstraZeneca and Oxford University are slated to begin a similarly sized trial in August, followed by Johnson & Johnson in September. All of the companies are working at unprecedented speed to advance their vaccines, and Moderna may not be the last to see its timeline delayed. Developing and manufacturing vaccines at scale is always a challenging endeavor, with unexpected hitches the norm rather than the exception. Candidate vaccines that looked promising in animals can fail to deliver the same results in people. Production problems arise. During the 2009 H1N1 flu pandemic, U.S. officials confidently predicted there would be a vaccine in the early autumn, in time to fend off an expected second wave of infections. But the new virus grew poorly in eggs, the substrate used in influenza vaccine production. By the time mass amounts of vaccines were ready for distribution, the peak of infection in the country had already passed. Matthew Herper and Helen Branswell contributed reporting.