There's good news and bad news for Moderna (MRNA 0.60%) investors. The good news is that shares of the messenger RNA (mRNA) vaccine maker have skyrocketed over 1,100% since the beginning of 2020. The bad news is that Moderna has given up a lot of its gains with the stock plunging close to 50% below its high from earlier this year.

But is this beaten-down vaccine stock a buy now? Two Motley Fool contributors have different takes on Moderna.

A masked healthcare professional giving a shot to a masked patient.

Image source: Getty Images.

Bullish take: A household name

Adria Cimino: Moderna became a household name thanks to its coronavirus vaccine. And right now, this biotech company generates billions of dollars in revenue and profit from it. But that same moneymaker is making investors a little bit nervous right now. That's because it's Moderna's only commercialized product. And what if sales stagnate or decline post-pandemic? But here's the good news: Moderna probably won't have to depend on this blockbuster forever. That's because Moderna has filled its pipeline with 37 programs across a broad range of therapeutic areas. And of those, 21 are in clinical trials.

Right now, it's important to look ahead to Moderna's next potential blockbuster. And that brings us to its cytomegalovirus (CMV) vaccine candidate. CMV is a common virus that isn't always harmful. But it can be devastating for those with weakened immune systems and for unborn babies. Today, an approved vaccine doesn't exist. But Moderna has a chance at being first or among the first to market. The company recently launched a phase 3 trial. The next step would be a regulatory submission. Moderna forecasts peak annual sales of $2 billion to $5 billion for a CMV vaccine.

Moderna may be on the path to developing other game-changing products down the road. One to watch is the company's HIV vaccine candidate. Moderna plans to launch phase 1 studies of two mRNA candidates by the end of this year. Of course, many companies have failed in attempts to develop an HIV vaccine. So why should we be optimistic about Moderna? The company's coronavirus vaccine shows that mRNA technology can work in humans. That's a positive sign for other programs in Moderna's pipeline -- even difficult-to-tackle viruses such as HIV. All of this means buying Moderna stock today doesn't have to be a bet on the coronavirus program. But instead, a bet on future blockbusters from this innovative biotech company.

Bearish take: Too many question marks

Keith Speights: Sure, Moderna's pipeline could deliver plenty of blockbuster products down the road. My main concern, though, is that success won't come soon enough to offset potential sales declines for its COVID-19 vaccine. 

Moderna's cut to its full-year 2021 sales guidance for its vaccine didn't bother me. The sales that the company initially expected for the second half of this year will simply shift into next year. However, I think there's considerable uncertainty for Moderna beyond 2022. It's quite possible (and perhaps even probable) that the company's COVID-19 vaccine revenue will drop significantly in a post-pandemic world.

CMV vaccine mRNA-1647 looks promising. But could it be on the market quickly enough to make up for a sharp decline in COVID-19 vaccine sales? I don't think so. Unfortunately for Moderna, the company doesn't have any other late-stage candidates waiting in the wings. It will be years before any of Moderna's other programs could make it to market (and even then, there's no guarantee they'll be successful).

Granted, Moderna could use its quickly growing cash stockpile to make an acquisition or two that give it products with near-term market potential. For now, though, my view is that there are too many question marks about the company's prospects over the next four or five years to make this stock a great pick. 

One area of agreement

While Adria is quite bullish on Moderna and I'm moderately bearish on the stock, we both like the company. Moderna's mRNA technology holds the potential to be a game-changer in developing other vaccines and in developing therapies that target cancer and other diseases.

I could flip to Adria's point of view relatively quickly if certain events unfold. First, if Moderna's market cap falls below $80 billion, I would probably see the stock as a relatively good pick. Second, if Moderna makes a great acquisition that adds a product to its lineup that could be a potential blockbuster this decade, I'd likely become a bull about the stock.