After the March market crash, some investors scooped up the usually strong stocks that had a difficult spell. They may take a while to rebound, but long-term investors know that's OK, and the purchases will bear fruit in the coming months or years. Others opted for shares that seemed immune to the coronavirus outbreak and had been rising steadily since the start of the year.

And then there were the investors who sought out the best short-term buys or stocks that would likely produce a quick, steep gain. Today, many of these shares happen to be coronavirus vaccine makers. The race to bring a vaccine to market is on, the U.S. government is funding various companies, and many short-term investors are moving from stock to stock according to the day's news.

An investor's hand turns a block marked "yes" on one side and "no" on the other next to blocks spelling out "sell?"

Image source: Getty Images.

That's what often happens on Robinhood, an online trading platform that appeals to young investors. Robinhood is known for its commission-free trades and its offer of free stock when you invite your friends to the platform. Data tracking stocks owned through Robinhood indicates that many Robinhood investors like shares that may be among the riskiest -- for instance, those of businesses that suffered the most during the crash, or shares of coronavirus vaccine makers, which have soared as investors speculate about which company will come up with the winning vaccine.

Which coronavirus stocks are Robinhood favorites?

So, which two coronavirus stocks happen to be the favorites right now among Robinhood investors? Moderna (MRNA 13.67%) and Inovio Pharmaceuticals (INO 2.37%). Through the platform, more than 295,000 people own shares of Moderna, and more than 221,000 own shares of Inovio, according to Robinhood data. That puts them among the top 100 most popular stocks on the platform.

Knowing that many Robinhood investors look for short-term gains, should you be worried if you're a Moderna or Inovio shareholder? Does this mean Moderna and Inovio -- up 318% and 639%, respectively, so far this year -- will continue (maybe) to skyrocket, then crash?

Not necessarily. The fates of Moderna and Inovio depend on data from their coronavirus vaccine trials. In the meantime, the trend on Robinhood is a reflection of what is happening in the market: Moderna and Inovio have become extremely sensitive to coronavirus vaccine news. Robinhood investors know that and want to be in on the day-to-day action.

If either company makes a positive comment about its vaccine development program, the shares climb. If the company stumbles or a rival moves closer to the finish line, the shares slide. For investors with an appetite for risk, money can be made (or lost) very quickly.

Results of vaccine trials

But only the results of the vaccine trials will determine whether, for the long term, Moderna and Inovio shares will extend gains, remain around current levels, or plunge. Right now, both companies are testing their coronavirus vaccines on humans. They both have reported positive interim data from phase 1 trials, though it still is too early to say whether their candidates will make it through phase 3 and to market.

If Moderna and Inovio are successful, they will have established that their technologies work in humans -- and the extra bonus will be a product on the market. So far, both companies remain in the clinical stage, and commercialized products -- besides the coronavirus vaccine -- are likely a few years away.

Successful coronavirus vaccine development could translate into lasting share gains, but if the companies fail, it seems unlikely the share prices will remain at current levels or climb higher.

So, should you sell Moderna or Inovio shares because short-term investors favor them? No -- not for that reason. Instead, look to data from the vaccine trials as a guide, and stick to your comfort level when it comes to risk. You might consider selling shares of these biotech companies if you have doubts about their coronavirus vaccine programs making it to the finish line, and/or if you prefer limiting risk within your portfolio. At the moment, Moderna and Inovio are best left to the most aggressive investors.