Investors have gotten used to Moderna (MRNA -0.74%) delivering earnings growth. Thanks to its coronavirus vaccine, the biotech has reported billions of dollars in revenue and profit over the past two years. And though share performance has cooled in recent times, the stock still has put a smile on the faces of long-term investors, climbing almost 400% over the past three years.

But things are changing for Moderna. Vaccine demand is declining as we move closer to a post-pandemic situation. And that means a significant drop in revenue for Moderna -- since the vaccine is the company's only commercialized product.

Does this mean we should forget about investing in Moderna? Before making any decisions, let's consider both the bear and bull cases for the stock.

The bear case

Moderna generated more than $36 billion in vaccine revenue over the past two years. This year, the company has signed contracts to deliver $5 billion in vaccines. That's a far cry from earlier vaccine revenue.

Of course, that figure could move higher in the coming months. But considering the decline in vaccine demand, this year's vaccine revenue won't be a repeat performance of 2021 or 2022.

At the same time, Moderna doesn't have other products to compensate for this lost revenue. So, in the near term, Moderna is likely to report declines in product revenue.

And Moderna's costs may move higher. The vaccine market is shifting to a private one. This means Moderna will sell vaccine doses directly to healthcare providers instead of selling to governments. The company's logistics costs will rise as it sells to many customers rather than just one in each country.

Finally, some investors may find it difficult to determine whether Moderna stock is reasonably priced today. That's because we don't yet have a solid idea of what vaccine revenue will look like this year and in the coming years. What will determine that? How many people actually go out and get annual boosters?

Considering all of this, Moderna's shares may stagnate until we know more about the post-pandemic vaccine market -- and/or Moderna launches another product.

The bull case

Moderna may be a coronavirus vaccine company now -- but a lot more lies ahead. The company has 38 candidates in clinical studies right now across treatment areas. Three in particular stand out because they represent potential revenue opportunities in the not-too-distant future.

I'm talking about vaccine candidates for respiratory syncytial virus (RSV), flu, and cytomegalovirus (CMV). The company recently reported solid data for the RSV candidate and expects to file for regulatory approval in the first half of this year.

Moderna may face competition from Pfizer and GSK -- regulators now are reviewing the Pfizer and GSK candidates. But with a market opportunity of more than $10 billion, Moderna should be able to carve out significant share.

The flu and CMV markets also represent billion-dollar opportunities. These vaccine candidates are in phase 3 trials, so if all goes well, we might see products hit the market in the next few years.

All of this means Moderna soon may be a multiproduct company. It was great seeing Moderna report enormous levels of revenue over the past couple of years, but relying on one product is risky -- and one product serving a pandemic situation is even riskier. Moderna makes an even better -- and safer -- investment today and down the road with a potential portfolio of products.

Bear or bull?

Moderna's shares may not soar overnight. And this year might be a bumpy one as Moderna shifts into a post-pandemic vaccine market. But I still think the reasons to buy Moderna outweigh the reasons to sell or avoid the stock.

If even only a few pipeline candidates make it to commercialization in the next few years, Moderna could deliver significant revenue. And areas like flu and coronavirus vaccination represent recurrent revenue opportunities.

Moderna's expertise in messenger RNA technology also make it a company to watch. It's already showed us this mRNA technology works -- thanks to the coronavirus vaccine. And Moderna uses this technology throughout its pipeline. All of this means right now is a great time to be bullish on Moderna stock.