What happened
Shares of Moderna (MRNA -1.28%) climbed 5.5% on Monday after the rollout of a rival vaccine maker's drug was halted.
So what
The South African government had planned to administer AstraZeneca's (AZN 2.34%) COVID-19 vaccine to its front-line healthcare workers. However, that plan was halted after early data from a small clinical trial showed that AstraZeneca's drug provided only minimal protection against the coronavirus strain that's been spreading rapidly in the country.

Investors bid up Moderna's stock on Monday. Image source: Getty Images.
South Africa will likely turn to other pharmaceutical companies to help fill its urgent need for an effective COVID-19 vaccine. Moderna said in January its vaccine provided protection against the coronavirus strain first identified in South Africa. Moderna is also testing whether a booster dose of its vaccine could provide additional protection against the South African variant of the virus.
Now what
Effectiveness against new coronavirus mutations will be a key factor in determining which vaccines emerge as the frontrunners in the global battle against COVID-19. "As we seek to defeat the COVID-19 virus, which has created a worldwide pandemic, we believe it is imperative to be proactive as the virus evolves," Moderna CEO Stephane Bancel said in January. Moderna's success in this area will also have a major impact on the returns it generates for its shareholders in the years ahead.