This is a big month for coronavirus stocks -- in particular, vaccine stocks. That's because two of the leaders in the coronavirus vaccine race have requested Emergency Use Authorizations (EUA). And many others are advancing in phase 3 trials or expecting key data from earlier trials.

In the coming weeks and months, regulators are likely to authorize vaccines. Afterwards, vaccine makers will begin to generate revenue and carve out market share. Data reports may offer us clues about the next batch of vaccine candidates that enter the market.

But before we look too far into the future, let's look at what's happening just ahead. Here are the top coronavirus stocks to watch in December.

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1. Moderna

I couldn't write this piece without including Moderna (NASDAQ:MRNA). The company was the first to launch an investigational COVID-19 vaccine in a human trial. And, more importantly, Moderna recently requested an EUA in the U.S. and conditional approval in Europe. The U.S. Food and Drug Administration (FDA) set a Dec. 17 meeting with its Vaccines and Related Biological Products Advisory Committee to discuss Moderna's trial data. A decision may come soon afterward.

So, this is clearly the moment to watch Moderna. Is it the time to buy? If you're an aggressive investor and haven't yet purchased shares, Moderna is a stock to consider. An authorization for its coronavirus vaccine -- or full approval later on -- would further lift the shares (even though they've already gained 677% this year). And revenue generated from vaccine sales will drive more growth. This vaccine would be the company's first commercialized product -- a major milestone.

If you're a cautious investor, however, you'll want to watch from the sidelines -- or take just a small position in the stock. That's because the next year or two of revenue and share performance depends on the FDA's upcoming decision.

2. Pfizer

Pfizer (NYSE:PFE) and partner BioNTech (NASDAQ:BNTX) lead the coronavirus vaccine race based on their timeline. The two companies last month became the first to submit their investigational product to the FDA for an EUA. They also applied for emergency use in Europe.

Just this week, the U.K.'s regulatory agency granted them emergency use authorization. The partners say they're ready to deliver doses "immediately." Decisions in the U.S. and Europe may also come this month. The FDA scheduled an advisory committee meeting to discuss the vaccine candidate on Dec. 10; this means a decision on the Pfizer-BioNTech vaccine may come sooner than a decision on Moderna's.

Is this a good time to buy Pfizer shares? The answer is a clear "yes." Share gains may not be as dramatic as those of Moderna following a possible EUA. Why? Because Pfizer, with its large portfolio of products, wouldn't depend on a COVID-19 vaccine for revenue. Still, it's likely to get a bit of a boost. And in the long term, shareholders will benefit from overall revenue growth and dividend payments. In September, Pfizer announced a dividend for the 328th straight quarter.

3. Vaxart

Vaxart (NASDAQ:VXRT) recently completed enrollment in the phase 1 trial of its investigational coronavirus vaccine. Sure, Vaxart lags far behind rivals like Moderna and Pfizer. But something about Vaxart's program makes it stand out from the crowd.

The biotech company is developing a vaccine to be administered orally. It takes the form of a tablet -- just like many everyday medicines. This could be a winner for two reasons. First, most people would be happy to avoid a shot. Second, healthcare systems would save money on transport and storage because the tablet is stable at room temperature; traditional vaccines require refrigeration.

When Vaxart began its phase 1 trial in October, it said it expected initial data in "the next few weeks," so investors are hoping that the company may share some results in December. Is now the time to buy Vaxart shares? That depends on your investment style.

Vaxart is high-risk. The company has no commercialized products: Its most advanced candidate is an influenza vaccine in phase 2 studies. Revenue isn't exactly around the corner. And we don't yet know if the COVID-19 vaccine candidate will be safe and effective in humans. The upcoming data readout will be crucial. For this reason, Vaxart is a stock to watch -- but not one to buy unless you're comfortable with the risk.

Each of these companies faces an important milestone in the coming weeks. But regardless of your investment style, before you invest in any of them, consider the final piece of the puzzle: each company's entire pipeline or portfolio. Don't buy shares of a pharma or biotech company for one program alone. It's much better to invest because you're confident about its product portfolio or pipeline as a whole. With that, there's a higher chance you'll be satisfied with the company's work -- and your investment.

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.