Novavax (NASDAQ:NVAX) stock plunged 89% in 2019. Most of that steep decline occurred in February after the biotech reported disappointing results from a late-stage study of respiratory syncytial virus (RSV) vaccine ResVax in maternal immunization of infants.

But so far in 2020, shares of Novavax have skyrocketed more than 400%. Could once beaten-down Novavax possibly even be a millionaire-maker stock?

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Why Novavax stock is hot

The biggest reason why Novavax has zoomed higher so far this year is the clinical success for experimental influenza vaccine NanoFlu. In March, the biotech reported overwhelmingly positive results for the vaccine in a late-stage study comparing it against Sanofi's (NASDAQ:SNY) approved flu vaccine FluZone Quadrivalent.

NanoFlu achieved non-inferiority immunogenicity versus FluZone Quadrivalent across four different influenza strains. It also clearly beat Sanofi's flu vaccine in key secondary endpoints of the study. In addition, Novavax's nanoparticle-based flu vaccine demonstrated a similar safety profile as FluZone, albeit with what Novavax referred to as a "modest increase" in local adverse events.

But while NanoFlu is the biggest story for Novavax in 2020, it hasn't been the only catalyst for the biotech stock. Novavax announced in late February that it was evaluating multiple COVID-19 vaccine candidates to select the most promising one to advance to clinical testing.

The company selected that candidate earlier this month. It plans to begin a phase 1 clinical trial in May. Although it's far too soon to know how successful the vaccine will be, Novavax's preclinical testing was encouraging, producing high levels of antibodies in animals that hold the potential to neutralize the novel coronavirus.

Crunching the numbers

Novavax plans to seek an accelerated approval from the U.S. Food and Drug Administration (FDA) for NanoFlu based on its positive late-stage study results. Although FDA approval isn't a sure thing, the company's chances of success appear to be pretty good.

But how much revenue could NanoFlu generate? Some predict the vaccine could make around $550 million per year at its peak. Others are more optimistic, predicting peak sales in the ballpark of $1.7 billion.

Let's use the higher number just for the fun of it. Novavax's market cap currently stands at close to $1 billion. Clearly, the company would be worth a lot more if NanoFlu rakes in $1.7 billion annually. It's not uncommon for biotech stocks to trade at six or seven times sales. A price-to-sales ratio of seven would make Novavax worth more than nearly $12 billion.

It's also possible that Novavax's COVID-19 vaccine could be successful. Estimating how much money the company could make from the vaccine, though, is challenging. There's certainly a massive market -- the entire world. However, there will also be a lot of pressure on companies to price COVID-19 vaccines so that everyone can be immunized. Multiple drugmakers could market COVID-19 vaccines as well, limiting the market size for any one company.

With all of this in mind, let's assume that Novavax is still able to make close to the same amount with a COVID-19 vaccine as it could with NanoFlu. That would double the company's potential market cap to nearly $24 billion.

And don't forget ResVax. Novavax still hasn't given up hope for winning approval for the RSV vaccine in immunizing infants. Before the clinical setback last year, one analyst projected that ResVax could achieve peak sales of more than $2 billion. That level isn't likely to be reached even if Novavax manages to win regulatory approval for the vaccine. But let's estimate that it could make half of that amount. If so, it could push Novavax's potential market cap up to over $30 billion.