Some investors have made a lot of money this year on the stocks of companies developing coronavirus vaccines. Several of these stocks have increased by hundreds of percentage points in a relatively short period of time. And this has happened before any COVID-19 vaccine has won emergency use authorization or regulatory approval.

There's still plenty of money for investors to make in the coronavirus vaccine race. These three COVID vaccine stocks could even potentially be millionaire-makers.

Man wearing a mask holding a syringe with cash inside it

Image source: Getty Images.

1. Moderna

Moderna (NASDAQ:MRNA) appears to be really close to reaching the market with its COVID-19 vaccine mRNA-1273. The biotech reported fantastic efficacy results for the experimental vaccine a few weeks ago. Assuming there aren't any bumps in the road, Moderna could win emergency use authorization for mRNA-1273 in December.

An influx of cash will likely soon pour into Moderna's coffers. The biotech has supply agreements in hand for more than 330 million doses. While the financial details of all of its supply deals haven't been announced, the combined price tag for these agreements probably tops $6 billion and may be significantly more than that.

Don't expect mRNA-1273 to propel Moderna into a millionaire-maker stock, though. The biotech's market cap already reflects much of the potential for the COVID-19 vaccine. However, success for mRNA-1273 could bode well for the rest of Moderna's pipeline -- and that just might allow investors buying the stock now to become millionaires over time.

Moderna thinks that its messenger RNA (mRNA) technology can be used in vaccines and therapies targeting a myriad of diseases. The company has a dozen mRNA programs in its pipeline now. CEO Stephane Bancel hopes to use the cash that he hopes is on the way from the mRNA-1273 supply deals to expand the pipeline to up to 50 mRNA programs in clinical testing. That's a multiplier effect that could make Moderna shareholders very wealthy if several of these candidates are successful.

2. Novavax

Although Moderna ranks as one of the clear leaders in the coronavirus vaccine race, the biotech stock hasn't delivered the biggest gains so far this year. That honor belongs to Novavax (NASDAQ:NVAX): Its shares have skyrocketed more than 3,000% year to date.

Novavax currently has a late-stage clinical study underway for its COVID-19 vaccine candidate, NVX-CoV2373. The company plans to begin a U.S. late-stage study of the experimental vaccine by the end of November. It has already lined up agreements with Australia, Canada, and the U.K. to supply a combined total of 176 million doses of NVX-CoV2373 if all goes well. In addition, Novavax received $1.6 billion in funding from Operation Warp Speed in an agreement that includes supplying 100 million doses to the U.S.

In addition to its promising coronavirus vaccine, Novavax also has an influenza vaccine candidate, NanoFlu, that could be headed for approval by the Food and Drug Administration. The biotech announced outstanding results earlier this year from a late-stage study of NanoFlu. The main hurdle to jump now is a lot-consistency trial to demonstrate that the company's manufacturing processes are in order.

Despite its huge gain this year, Novavax's market cap stands at around $6 billion. With two potential blockbuster vaccines waiting in the wings, the stock could go a lot higher.

3. Vaxart

Vaxart (NASDAQ:VXRT) isn't nearly as far along in clinical testing of its COVID-19 vaccine candidate, VXA-CoV2-1. However, the stock has been a bigger winner than most, soaring nearly 1,900% so far this year.

The biotech is currently evaluating VXA-CoV2-1 in a phase 1 clinical study. Vaxart completed enrollment in the study earlier this month and could report initial results within the next few weeks.

Could Vaxart be too late to the party to achieve success for VXA-CoV2-1? Not necessarily. There's something unique about VXA-CoV2-1 that could be a game-changer: It's a single-dose tablet. All of the leading coronavirus vaccine candidates are injections, with most of them requiring two doses spaced several weeks apart.

If VXA-CoV2-1 proves to be safe and effective, Vaxart could have great opportunities with oral vaccines targeting other viral diseases. With its market cap below $750 million, investors could still potentially make a fortune with the stock if all goes well in Vaxart's clinical studies.

Don't count your money yet

All three of these COVID vaccine stocks have already delivered massive gains and could have plenty of room to run. But don't count your money just yet.

Vaxart is obviously the riskiest of the group because it's still only in early-stage testing. However, Moderna and Novavax are risky as well. There are still significant hurdles for each biotech to jump.

The potential for any stock to make a lot of money comes joined at the hip with the potential to lose a lot of money. Investors should weigh the risks against the possible rewards before buying any stock -- even those that could be millionaire-makers.

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.