What happened

Shares of Moderna (NASDAQ:MRNA) and Novavax (NASDAQ:NVAX) were jumping 3.7% and 4.4%, respectively, as of 11:23 a.m. EDT on Thursday. Neither company reported any news. However, the gains are likely related to increasing concerns about COVID-19. The major market indexes were sinking Thursday morning after Japan declared a state of emergency for the upcoming Olympic games due to increased COVID-19 cases in Tokyo.

So what

There's a direct connection between the fortunes of vaccine stocks and concerns about COVID-19. When worries increase, the shares of top vaccine makers tend to move higher as well. The longer COVID-19 is a serious issue, the longer the demand will continue for safe and effective vaccines.

Image of the earth inside the coronavirus.

Image source: Getty Images.

Moderna already ranks as a leading supplier of COVID-19 vaccines across the world. Novavax hasn't won approvals or authorizations for its COVID-19 vaccine yet. However, the company has reported positive late-stage results and expects to soon file for authorizations in the U.S., Europe, and U.K. 

Why didn't other vaccine makers, particularly Pfizer (NYSE:PFE) and BioNTech (NASDAQ:BNTX), also enjoy solid upswings today? Probably because of recent data from Israel's Health Ministry indicating that the Pfizer-BioNTech vaccine is significantly less effective against the quickly spreading coronavirus delta variant.

Now what

The key thing to watch with Novavax is the upcoming regulatory submissions for its COVID-19 vaccine. Investors will also want to monitor any updates related to how well Moderna's and Novavax's vaccines provide protection against the delta variant and other emerging coronavirus strains.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.