Many Robinhood investors seem to like coronavirus stocks. Eight of the 100 most popular stocks on the trading platform right now are of companies that are developing COVID-19 therapies or vaccines.

Some of these popular coronavirus stocks on Robinhood appear to be great picks, but not all of them. Here are two of the stocks to buy -- and one to avoid.

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Two to buy

Investors who prefer not to take on too much risk might be comfortable with Pfizer (NYSE:PFE). The big drugmaker jumped into the thick of the coronavirus vaccine race earlier this year by partnering with German biotech BioNTech (NASDAQ:BNTX). Now Pfizer and BioNTech appear to be in the lead position in that race.

The two partners expect to announce preliminary results from a late-stage clinical study of COVID-19 vaccine candidate BNT162b2 later this month. If all goes well, BNT162b2 could be the first coronavirus vaccine to win Emergency Use Authorization from the Food and Drug Administration. Pfizer and BioNTech stand to receive at least $1.95 billion from a deal with the U.S. government to supply 100 million doses of the vaccine if it's authorized for use.

But Pfizer should be in pretty good shape even if everything doesn't work out so well for BNT162b2. The company has several other growth drivers in its lineup, and a loaded pipeline. Pfizer's growth prospects should improve once its merger of Upjohn with Mylan closes, which will probably happen within the next two months.

For more aggressive investors, Novavax (NASDAQ:NVAX) looks like a popular Robinhood stock to consider buying. Even though the biotech is much smaller than other drugmakers that have late-stage COVID-19 vaccine candidates, Novavax is beating its bigger rivals in terms of stock performance so far this year. It's also holding its own in lining up supply deals with governments.

Some analysts view Novavax's NVX-CoV2373 as potentially best-in-class among the leading COVID-19 vaccines in development. The company is a little behind some of its rivals in that it has only started a phase 3 study of its coronavirus vaccine candidate in the U.K., but not in the U.S. yet. However, Novavax should soon begin a late-stage study of NVX-CoV2373 in the U.S. as well.

The biotech also has another potential big winner waiting in the wings. Novavax is moving forward with plans to prepare a filing for FDA approval of influenza vaccine candidate NanoFlu, after reporting overwhelmingly positive results earlier this year. It's also exploring the possibility of developing a combination influenza/COVID-19 vaccine, with NanoFlu and NVX-CoV2373, for use after the pandemic ends.

One to avoid

While now seems to be a great time to buy shares of Pfizer and Novavax, I think investors would be better off staying away from Sorrento Therapeutics (NASDAQ:SRNE) for the time being. Sure, the biotech stock has soared more than 200% so far this year. However, my chief concern with Sorrento is that it has limited financial resources that are spread very thin.

Sorrento reported a net loss of nearly $85 million in its latest quarter. Its cash position totaled only $24.3 million as of June 30. That's not a strong financial position to be in with a pipeline that includes 12 clinical-stage programs.

Granted, some of Sorrento's pipeline candidates could have potential. Several of its experimental antibody therapies, for example, have shown promise in neutralizing the novel coronavirus that causes COVID-19 in preclinical testing. But most of Sorrento's coronavirus-related programs haven't advanced to clinical studies yet, which means their risks for investors remain quite high.

I think that Sorrento could be an attractive stock to buy at some point, should its pipeline candidates perform well in clinical testing. However, the company will need a lot more money to fund those clinical studies. It's also spending cash on bolstering its pipeline. My suspicion is that it's just a matter of time before Sorrento will conduct a stock offering to raise additional capital, with the resulting dilution causing its share price to sink.