Where can you find the most explosive growth? With small-cap stocks. That's true with nearly any sector. And it's certainly true when it comes to investing in companies focused on fighting COVID-19.

There are many small biotechs developing experimental COVID-19 therapies and vaccine candidates. Which could deliver the biggest gains in a relatively short period of time? Here are three small-cap coronavirus stocks that could skyrocket.

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1. Arcturus Therapeutics

At the end of 2019, Arcturus Therapeutics' (NASDAQ:ARCT) market cap stood at only $165 million. The biotech stock has soared close to 380% so far this year. Arcturus' market cap now tops $1 billion. And it could very well move much higher.

Arcturus announced in March that it was teaming up with Duke-NUS Medical School to develop a COVID-19 vaccine. In late April, the company reported positive preclinical results for its LUNAR-COV19 candidate. Arcturus received a green light on July 21 from regulators in Singapore to begin a phase 1/2 clinical study evaluating the experimental vaccine and expects to start dosing patients soon. 

Even though Arcturus has a long way to go with LUNAR-COV19, two countries have made deals with the company to reserve doses of the vaccine -- Singapore and Israel. If clinical testing goes well, it won't be surprising if Arcturus adds to that list. 

In the meantime, the company's pipeline includes a promising experimental treatment for ornithine transcarbamylase deficiency, a genetic disease where ammonia accumulates in the blood. Arcturus expects to announce phase 1 data for the drug candidate in the fourth quarter of 2020. 

2. iBio Pharma

iBio Pharma (NYSEMKT:IBIO) began the year a lot smaller than Arcturus with its market cap standing below $14 million. But after its shares racked up gains of more than 1,700%, iBio now sports a market cap of over $530 million.

On March 18, 2020, iBio announced that it planned to develop COVID-19 vaccine candidates and filed four provisional patent applications for its approach. The company began preclinical testing of its first candidate the following week. In June, iBio reported that a second COVID-19 vaccine candidate had advanced into preclinical testing. 

The company thinks that its vaccine candidates could potentially achieve a more durable immune response with fewer required doses. If iBio succeeds in this goal, it could have a key advantage over leading COVID-19 vaccine candidates that are farther along in development. 

3. Vaxart

Vaxart's (NASDAQ:VXRT) story is similar to iBio's. At the beginning of 2020, Vaxart's market cap stood at only $17 million. Now, its market cap is over $1 billion after Vaxart's shares skyrocketed more than 2,600%.

The biotech announced on Jan. 31, 2020, that it kicked off a program to develop an oral vaccine against the novel coronavirus. By late March, Vaxart reported that it had five COVID-19 vaccine candidates in preclinical testing. The company announced encouraging preclinical results in April.

However, the biggest news of all for Vaxart came on June 26. Operation Warp Speed, the U.S. government's initiative to accelerate COVID-19 vaccine development, picked its oral vaccine candidate to be included in a non-human primate challenge study. While Vaxart's participation in this study isn't the same thing as Operation Warp Speed funding the biotech's vaccine program, it's still a big honor -- especially since no other oral COVID-19 vaccine candidates were chosen. 

High potential rewards, high risks

Each of these stocks could become even bigger winners than they've already been if their COVID-19 programs are successful. The potential for Arcturus, iBio, and Vaxart to make investors a boatload of money is real. However, the possibility that any or all of these three companies could stumble is also real. 

The probability of a vaccine candidate that enters early stage testing (as Arcturus' LUNAR-COV19 is about to do) will go on to win regulatory approval is low. Candidates that are still in preclinical testing, such as those belonging to iBio and Vaxart, face even more daunting prospects.

Because of these high risk levels, none of these coronavirus stocks are well-suited for most investors. However, more aggressive investors might want to keep Arcturus, iBio, and Vaxart on their radars. Positive clinical results in the months ahead could dramatically improve the companies' chances.