What happened

Shares of Moderna (MRNA 0.80%) 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 (JNJ -0.12%) 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. 

A person is pointing to an upwardly sloping digital stock chart.

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.