What happened Shares of Moderna (NASDAQ: MRNA) climbed 3% on Monday following reports that Food and Drug Administration officials are considering a new safety warning for Johnson & Johnson's (NYSE: JNJ) COVID-19 vaccine. So what The FDA plans to add a warning that Johnson & Johnson's vaccine has been linked in extremely rare cases to Guillain-Barre syndrome, according to The Washington Post. Roughly 100 cases of the autoimmune condition have been reported among the more than 12 million people who have received that vaccine. No similar link has been found to that illness for the Moderna or Pfizer vaccines. Image source: Getty Images. Johnson & Johnson previously suffered a setback in May when the Centers for Disease Control and Prevention (CDC) said that 28 people who had received its COVID-19 vaccine had experienced a rare blood clotting issue. The CDC determined that the benefits of the vaccine outweighed the risks, but the incident added to the hesitancy among many people who had been considering getting the single-shot inoculation. Now what Should the FDA issue a new safety warning on Johnson & Johnson's vaccine, it would likely further heighten these concerns. And that, in turn, could drive more people to choose to get Moderna's or Pfizer's coronavirus vaccines instead. Those two drugmakers have emerged as the leaders in the COVID-19 vaccine space thanks in part to their shots' high efficacy rates and impressive safety profiles in both clinical and real-world settings. 10 stocks we like better than Moderna Inc.When our award-winning analyst team has a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* They just revealed what they believe are the ten best stocks for investors to buy right now... and Moderna Inc. wasn't one of them! That's right -- they think these 10 stocks are even better buys. See the 10 stocks *Stock Advisor returns as of June 7, 2021 Joe Tenebruso has no position in any of the stocks mentioned. The Motley Fool recommends Johnson & Johnson and Moderna Inc. The Motley Fool has a disclosure policy.Source