What happened Shares of Moderna (NASDAQ: MRNA) leapt 7.4% on Tuesday after the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) recommended a temporary halt to Johnson & Johnson's (NYSE: JNJ) COVID-19 vaccine rollout. So what The CDC and FDA are asking states to pause immunizations with J&J's drug while they investigate six reported U.S. cases of blood clots in people who received the vaccine. The regulatory agencies noted that these incidents were "extremely rare," as more than 6.8 million doses of J&J's vaccine have been administered. Moderna's stock price rose sharply on Tuesday. Image source: Getty Images. Additionally, J&J said that it decided to "proactively delay the rollout" of its vaccine in Europe. "We have been working closely with medical experts and health authorities, and we strongly support the open communication of this information to healthcare professionals and the public," the company said in a statement. Now what Moderna and fellow COVID-19 drugmaker Pfizer are expected to help make up for a potential shortfall in J&J's vaccine supply if the pause in its rollout is extended, though it will likely take some time for them to ramp up production. Moderna said after the market close that it remains on track to deliver a total of 300 million doses to the U.S. government by end of July 2021. 10 stocks we like better than Moderna Inc.When investing geniuses David and Tom Gardner have 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.* David and Tom 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 February 24, 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