What happened 

Shares of Moderna (MRNA -2.45%) 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 5.38%) 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. 

A person points to an upwardly sloping stock chart.

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.