Demand for Moderna's (MRNA 1.69%) one and only product is on the decline. I'm talking about the coronavirus vaccine. As we approach a post-pandemic situation, Moderna already has started to feel the impact. In the first-quarter earnings report, vaccine sales dropped 69% year over year. And Moderna so far has orders for about $5 billion in vaccine sales for 2023.

That's a stark contrast to the last two years, when Moderna generated more than $17 billion in annual product sales. People worldwide rushed to get vaccinated during these earlier stages of the pandemic -- and even followed up with boosters. Does today's drop in vaccine sales mean it's too late to buy Moderna stock? Let's find out.

From zero to billions

It's true that, on a first glance, it may look as if Moderna's better days are in the past. The company went from zero product revenue to billions in earnings in just a few months. Moderna's vaccine revenue also helped it build up an enormous level of cash -- that can help it advance candidates through the pipeline. Moderna reported more than $18 billion in cash at the end of last year.

But the negative point about Moderna's story so far is that it depends on one product. That's fine for a short period of time -- but not an ideal long-term situation.

Here's what makes me optimistic about the biotech moving forward. Moderna is on its way to becoming a multiple-product company. And it's set out precise goals. The company has its eye on the respiratory vaccines market. This includes vaccines for coronavirus, flu, and respiratory syncytial virus (RSV).

Moderna, of course, already sells a coronavirus vaccine -- but the company aims to update it as the virus evolves and eventually hopes to launch a combined coronavirus/flu shot. That candidate is involved in a phase 1/2 trial right now. As for RSV, Moderna plans to file for approval of that candidate shortly -- with a goal of launching next year. And the company aims on commercializing its flu vaccine candidate then too.

A free cash flow goal

What does this mean for Moderna financially? The company expects to win $8 billion to $15 billion of this respiratory vaccine market by 2027. It predicts this respiratory vaccine business could lead to $4 billion to $9 billion in free cash flow annually -- and that may move higher over time.

Now, you might say this still represents a lower level of product revenue than that of the past couple of years. But there are three things to keep in mind. First, this is just Moderna's respiratory portfolio. The company has more than 40 programs in development across treatment areas. If even a few are successful, this could significantly add to revenue.

It's also important to remember that Moderna's respiratory franchise could bring in recurrent revenue. For example, the company expects the post-pandemic coronavirus market to follow that of the flu vaccine market.

Finally, Moderna's situation over the past two years was exceptional: It was selling a product to address a pandemic. So, we can't expect that product or another product to deliver those levels of revenue during normal times. What I mean here is we have to give Moderna an opportunity to start afresh rather than comparing future revenue to that of pandemic times.

Moderna's valuation

Now, let's consider Moderna's valuation. The stock has dropped 28% so far this year -- after already declining last year. Today, Moderna trades for about 8 times forward earnings estimates. That seems very reasonable considering the depth of Moderna's pipeline, and the potential products close to commercialization.

The stock may not recover overnight -- investors may continue to shun Moderna shares as the company transitions from the highly lucrative early pandemic market to a post-pandemic situation.

But, over time, Moderna has a solid product and potential products that should drive earnings growth. The company also has the resources to get to the commercialization finish line. And that's why it's not too late to get in on this innovative biotech player.