Moderna (MRNA 2.57%) once delivered rapid multibagger returns -- and was the stock everyone wanted to own. That was back in the pandemic's early days. The coronavirus vaccine maker rose by more than 400% in 2020 and soared even more in the first half of 2021. And it went on to generate billions of dollars in revenue and profits.

But over the past couple of years, investors have turned their backs on Moderna stock. That's because its coronavirus vaccine, Spikevax, is its only approved product -- and investors worry about what its revenue will look like in a post-COVID-crisis world. Wall Street analysts, as a group, are a lot more optimistic about its prospects. The average share price forecast calls for Moderna to soar 78% in the coming 12 months.

Today's revenue picture

Today, Moderna trades for about $120 a share. While analysts' share price forecasts range from a low of $82 to a high of $430, the average comes in at $215.93, according to Yahoo Finance. Now, let's look at Moderna's revenue picture today and its future prospects. It was a growth machine when people were initially getting inoculated against COVID. But with about two-thirds of the world's population fully vaccinated, demand for COVID-19 vaccines has dropped -- and so have Moderna's earnings. For example, in the most recent quarter, its product sales tumbled 69% year over year to $1.8 billion. And net income fell 97% to $79 million.

Analysts expect the company's revenue to drop to about $7.4 billion this year from more than $19 billion last year.

While this isn't great news, it's not disastrous either. First of all, it was expected. A pandemic situation is temporary, so it was clear Moderna's vaccine sales wouldn't continue at peak levels forever. Second, the revenue opportunity from Spikevax isn't disappearing. Moderna may continue to generate billions of dollars from its updated vaccines annually.

The company predicts the market for COVID vaccines will mirror that for flu shots, with many people opting to get annual updates. About half of the U.S. population lines up for a yearly flu shot -- and members of that group may opt for yearly coronavirus boosters, too.

A respiratory vaccine portfolio

All of this is positive. And there's even more good news. Moderna has a portfolio of respiratory illness vaccines that are moving closer to market -- one that could produce annual free cash flow in the range of $4 billion to $9 billion.

The company aims to launch respiratory syncytial virus (RSV) and flu vaccines next year. With those and an updated coronavirus booster, Moderna expects to capture an $8 billion to $15 billion share of the respiratory vaccines market by 2027.

And this is just its respiratory vaccine portfolio. Moderna has many other programs that could add to its revenues -- and eventually, its profits. For example, the biotech recently reported positive data from a trial of its personalized cancer vaccine combined with Merck's Keytruda. The companies plan to launch a phase 3 trial of the combination in high-risk melanoma patients this year.

Even if it takes time for its revenues to return to their pandemic highs, this company's growth story is just beginning. And if Moderna reaches those sales levels again, it will be thanks to several products and outside of a crisis situation -- so it will be easier to maintain those revenue levels.

Too much optimism?

Considering all of this, are Wall Street analysts on target with their consensus forecast for a 78% share price gain -- or are they too optimistic?

I'm bullish on Moderna over the long term, and I would buy its shares today. But I wouldn't expect them to surge by more than 70% in the next 12 months. Even if Moderna wins approval for its RSV and flu candidates, it will take some time for revenues from them to start rolling in. And speaking of Moderna's revenue, analysts expect it to fall even more next year -- to $6.4 billion.

So there really isn't a catalyst that could drive the stock to such a gain in a short period of time. That said, Moderna's shares -- in the doldrums today -- represent a great buying opportunity for patient investors who will buy and hold on for at least five years. That's because Moderna's potential to become a respiratory vaccine powerhouse and launch other products as well could drive lasting gains over time.