You probably hear a lot about the leaders in the coronavirus vaccine race. A couple of them could even potentially be available to at least some Americans by the end of this year. It's important to understand, though, that the first wave of coronavirus vaccines could just be the beginning.

Other drugmakers are hard at work developing experimental COVID-19 vaccines that might ultimately be even more successful than the current leaders. Investors who recognized the potential for these stocks early on could make a fortune if that happens. With that in mind, here are two under-the-radar coronavirus vaccine stocks you'll want to especially watch.

Stock chart in front of a globe with coronavirus inside it

Image source: Getty Images.

Arcturus Therapeutics

Most of those COVID-19 vaccine candidates that you hear so much about require two doses, typically administered several weeks apart. Although this isn't necessarily a huge issue, it could lead to many people not receiving full immunization because of the hassle of going back for a second dose.

Arcturus Therapeutics (NASDAQ:ARCT) is tackling this potential problem head-on with its experimental coronavirus vaccine ARCT-021 (also known as LUNAR-COV19). ARCT-021 only requires one dose. 

The company began a phase 1/2 clinical study evaluating ARCT-021 in August. This study followed Arcturus' promising preclinical testing of the vaccine candidate that showed 100% production of neutralizing antibodies (which hold the potential to prevent infection by the coronavirus) after a single administration of a super-low 2-microgram dose. Those preclinical results also demonstrated promising induction of T cells, which are important in the body's immune response.

Arcturus' market cap currently stands at close to $1.3 billion. The biotech stock has nearly quintupled so far this year, thanks largely to excitement about its COVID-19 vaccine program. Arcturus expects to report initial results from its phase 1/2 study of ARCT-021 later this year. If those results look as encouraging as the preclinical data did, look for Arcturus' shares to soar even more.

Vaxart

The requirement for multiple doses isn't the only challenge for several of the leading COVID-19 vaccine candidates. Some of them also must be stored at sub-freezing temperatures. Even those that don't have this drawback have another downside: Most people simply don't like to be stuck with a needle.

Enter Vaxart (NASDAQ:VXRT). The company focuses on developing oral vaccines. No needles involved -- just swallow a tablet or two.

Earlier this month, Vaxart dosed its first participant in a phase 1 study of oral COVID-19 vaccine VXA-CoV2-1. Up to 48 healthy adult participants will receive two doses of the oral tablet 28 days apart, with some participants receiving a higher dose than others. It won't take long to know how well VXA-CoV2-1 is working: Vaxart expects enrollment to be completed in early November, with initial results coming in a few weeks.

The preclinical data for VXA-CoV2-1 looked very good. Vaxart also recently announced results from a study where hamsters received two doses of the oral vaccine and then were exposed to SARS-CoV-2, the virus that causes COVID-19. The results showed that all hamsters receiving VXA-CoV2-1 had no systemic weight loss -- a key indicator of protection against viral infection for hamsters.

Vaxart's shares have skyrocketed close to 1,400% year to date, but its market cap is still under $600 million. Positive news for VXA-CoV2-1 in the phase 1 study could pour more fuel on the fire of this hot coronavirus vaccine stock.

Don't overlook the risks

Both Arcturus and Vaxart could be huge winners if their respective COVID-19 vaccines continue to chalk up success in clinical studies. However, investors shouldn't overlook the risks with these small biotech stocks.

It's quite common for experimental vaccines that seem promising in preclinical testing to stumble when they advance to clinical studies in humans. Early-stage clinical studies can also sometimes raise expectations that ultimately aren't met in larger later-stage studies. 

Arcturus and and Vaxart are absolutely worthy of investors' attention. But my view is that it's still too soon for most investors to seriously consider buying these stocks.