Shares of Sanofi /zigman2/quotes/201967021/composite SNY +1.44% /zigman2/quotes/206928357/delayed FR:SAN +1.38% gained 0.4% in premarket trading on Thursday after the drugmaker and GlaxoSmithKline /zigman2/quotes/209463850/composite GSK +0.56% said their COVID-19 vaccine candidate has entered a Phase 1/2 clinical trial. GSK's stock was down 0.3%. The randomized, double-blind, placebo-controlled study plans to enroll 440 healthy participants in the U.S., and the companies said they plan to move their experimental vaccine into Phase 3 trials by the end of the year based on the vaccine's performance in the earlier phases of development. The vaccine has received funding from the Biomedical Advanced Research and Development Authority. Since the start of the year, Sanofi's stock has gained 1.0%, shares of GSK are down 15.9%, and the S&P 500 has rallied 10.8%.