Pfizer (NYSE:PFE) and Moderna (NASDAQ:MRNA) will almost certainly enjoy huge revenue growth this year. Their respective coronavirus vaccines have already been given to millions of people in the U.S. and other countries. Billions of dollars will soon flow into the drugmakers' coffers.

Both of the stocks could deliver solid gains in the coming months. However, I think there are two COVID-19 vaccine stocks that could be much-bigger winners. Here's why Novavax (NASDAQ:NVAX) and Vaxart (NASDAQ:VXRT) could trounce both Pfizer and Moderna in 2021. 

Syringe and vaccine vial labeled COVID-19,  on top of a stock chart

Image source: Getty Images.

Powerful potential catalysts

Experienced investors understand that the market is always looking to the future. Hopes and fears about what might happen are the biggest drivers for biotech stocks especially.

The near-term prospects for Pfizer's and Moderna's vaccines don't require much speculation. Both BNT162b2 and mRNA-1273 have won key regulatory authorizations. Both Pfizer (along with its partner BioNTech) and Moderna have secured major supply deals. It's a foregone conclusion that the companies are at the cusp of making a lot of money with their COVID vaccines.

Novavax and Vaxart, on the other hand, have more uncertainty about their prospects. And that uncertainty means that pipeline updates from the companies could serve as powerful catalysts for both stocks.

It could only be a matter of days or weeks before Novavax announces interim results from a late-stage study of its coronavirus vaccine NVX-CoV2373 conducted in the U.K. If those results show a high efficacy and a solid safety profile, look for Novavax's shares to soar. Good news from the U.K. study would also bode well for the prospects of NVX-CoV2373 in a late-stage study in the U.S. and Mexico that kicked off at the end of 2020.

Vaxart could have big news soon as well. The biotech is currently evaluating its experimental COVID-19 vaccine VXA-CoV2-1 in a phase 1 study. Results from this study are expected this month. If all goes well, Vaxart hopes to advance its vaccine candidate into phase 2 testing in the first quarter.

Although Vaxart is lagging well behind in the coronavirus vaccine race, it's still one to closely watch. Unlike the vaccines from Pfizer, Moderna, Novavax, or other leaders, VXA-CoV2-1 is a single-dose tablet. If it's successful in clinical testing, Vaxart's vaccine could have significant competitive advantages including convenience, lower cost, and fewer logistical challenges.

Market caps matter

There's another key reason why Novavax and Vaxart could deliver much-higher gains than Pfizer and Moderna this year: Market caps matter. It's more difficult for a stock with a larger market cap to double or triple than it is for a stock with a smaller market cap.

For example, Pfizer's market cap now stands at more than $200 billion. It's taken a decade for the big pharma stock to double. Moderna isn't nearly as big as Pfizer, with a market cap of a little under $50 billion. However, after Moderna's shares soared more than 430% last year, it doesn't have nearly as much room to run.

Novavax ended 2020 as the biggest coronavirus vaccine winner of all with a staggering gain of 2,700%. Even after this jaw-dropping performance, though, the company's market cap is still only around $8 billion. If NVX-CoV2373 wins emergency use authorizations, the vaccine could make around $3.8 billion this year, according to Bernstein analyst Ronny Gal. 

Vaxart was also one of the most successful COVID vaccine stocks last year. Its shares skyrocketed over 1,500%. Despite this huge gain, the biotech's market cap remains below $800 million.

Rolling the dice

To be sure, investors who buy Novavax and Vaxart are rolling the dice. There's a real possibility that the companies' coronavirus vaccines won't meet expectations. That's a risk particularly with Vaxart, since its vaccine is not nearly as far along in clinical testing.

Keep in mind, though, that both biotechs have other pipeline candidates waiting in the wings. Novavax reported positive results last year for experimental flu vaccine NanoFlu. Vaxart also has a promising oral flu vaccine in early stage clinical testing. Even if their COVID-19 vaccines prove to be disappointing, it wouldn't be game over for either of these companies.

Vaxart could be the biggest winner if VXA-CoV2-1 is successful in further clinical studies. However, I think that Novavax has a more attractive risk-reward profile. With a little luck, though, both stocks could handily outperform both Pfizer and Moderna this year.

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.