Moderna (NASDAQ:MRNA) stock might look a bit expensive these days. The shares have climbed more than 200% this year. And while the forward price-to-earnings ratio of 11 isn't too excessive -- it's much higher than its level of less than four earlier this year. At the same time, some investors wonder whether Moderna may have reached peak sales of its coronavirus vaccine. That's as more and more people get vaccinated and coronavirus cases decline.

Right now, it's too early to predict if and when coronavirus vaccine sales will slow. Experts say the virus will stick around. And we'll need protection. That means ongoing need for vaccines. So, it doesn't seem likely sales will drop off a cliff. But even if COVID-19 vaccine sales stagnate at some point in the future, Moderna investors should look ahead with optimism. Here's why I think Moderna's growth is just getting started...

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Moderna's entire future

Moderna's work on the coronavirus vaccine offered it a big opportunity. I'm not talking about the opportunity to make billions of dollars in revenue on one product. That's great. But I'm talking about an opportunity that will impact its entire future. And that was the opportunity to prove its technology works. Moderna's entire pipeline is based on this messenger RNA (mRNA) technology.

The company employs mRNA to deliver specific instructions to the body's cells. The body then uses these instructions to produce proteins to prevent or treat disease. Until we saw the performance of Moderna's mRNA vaccine for COVID-19, it wasn't clear if this approach actually worked in humans.

But the COVID-19 vaccine demonstrated that the technique is efficacious -- and safe. Moderna's vaccine has resulted in very few side effects. And efficacy has topped 93%.

Moderna even says on its website something similar to what I've been thinking: "mRNA is like software for the cell. And if it works for one disease, it has the potential to work for many diseases."

That's a very exciting statement. Especially if we consider what Moderna has on the horizon.

Of course, Moderna is working on various coronavirus booster candidates and even a next-generation COVID-19 vaccine candidate. But let's look beyond the coronavirus program.

Next on the agenda

And there, next up from a timeline perspective, is Moderna's investigational cytomegalovirus (CMV) vaccine. CMV is a common virus that's most dangerous for unborn babies or people with compromised immune systems. Today, there isn't a vaccine to prevent CMV.

Moderna plans to start a phase 3 trial for its CMV candidate this year. The company has said it expects peak annual sales for a potential product in the range of $2 billion and $5 billion. So, this could be another blockbuster for the biotech company. Big pharma Merck also has a CMV vaccine candidate in phase 2 studies. But most other players are farther behind in development. This opens the door to massive market share for Moderna.

But Moderna isn't stopping there. The company has 22 candidates in clinical trials in various disease areas that could equal billions of dollars in revenue in the future. That's because these particular areas represent a health risk for the masses or already impact a broad percentage of the population. Moderna plans to launch phase 1 trials for two HIV vaccine candidates this year, for example. And Moderna is testing a flu vaccine candidate in phase 1 and a combined candidate for flu and coronavirus in preclinical studies. The company also has an investigational personalized cancer vaccine in phase 2 studies.

What does this mean for investors?

I wouldn't be scared away by Moderna's valuation today or by fluctuation in the stock price. I wouldn't even be scared away if sales of the coronavirus vaccine slow at some point in the future. (As long as any potential slowdown isn't linked to a problem with Moderna's vaccine or processes.)

My focus now is on Moderna's pipeline: the candidates' performance and Moderna's ability to bring these potential products through studies to commercialization in an organized manner. The performance of the COVID-19 vaccine doesn't guarantee the success of every candidate. But it's a strong sign that Moderna's success is far from over.

All of this means Moderna still is a buy today -- if you're willing to ignore bumps along the road and hold on for the long term.

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.