Shares of Moderna Inc. /zigman2/quotes/205619834/composite MRNA -3.97% declined 6.2% in trading on Thursday after people familiar with the matter told Stat News the start date for the company's Phase 3 clinical trial for its experimental COVID-19 vaccine has been delayed. The sources attributed the delay to protocol changes - a common issue with clinical studies - not a problem with the vaccine candidate. Following publication of the story, Moderna tweeted that the Phase 3 trial is still set to begin in July. "We have worked closely with [the National Institutes of Health/Operation Warp Speed] to align on the final protocol in order to begin the trial on time," it said . Moderna's stock has soared 197.7% year-to-date, while the S&P 500 /zigman2/quotes/210599714/realtime SPX -2.14% is down 3.5%.