When will the pandemic be over and life return to normal? Moderna (MRNA 1.79%) CEO Stéphane Bancel thinks it could happen in 2022. In an interview with Swiss newspaper Neue Zuercher Zeitung, Bancel expressed his view that COVID-19 vaccines could help usher in a return to normalcy in the second half of next year. 

People across the world will cheer if Bancel's prediction comes true. However, Moderna's shareholders might not join in the celebration. Here's why the vaccine stock could plunge if Bancel is right that the pandemic will be over next year.

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The probability of declining sales 

Moderna expects to rake in $20 billion in sales for its COVID-19 vaccine this year. It could generate similar sales in 2022. But if the pandemic ends in the second half of next year, there's a high probability of declining sales beginning in 2023.

For one thing, Moderna's fortunes thus far have relied upon two doses being given in initial vaccinations. In a post-pandemic world, the best that the company can hope for is for one-shot booster doses to be given on a relatively frequent basis.

Bancel has said in the past that he expects annual boosters will be likely. However, no one knows yet whether or not boosters will be required going forward on an annual basis -- or at all. 

There's also a big question about how high vaccination rates will be once the pandemic is over. It wouldn't be surprising if the rates drop when people think that COVID-19 is no longer a significant concern.

Several companies, including Pfizer and Merck, are developing oral COVID-19 pills that could be used for post-exposure prophylaxis. Assuming one or more of these drugmakers are successful, the availability of these pills could make it even less likely that individuals will want to be vaccinated.

A one-trick pony, for now

Moderna's main challenge in this scenario is that the company is a one-trick pony, for now. It doesn't have any other products on the market except its COVID-19 vaccine. Sure, Moderna has quite a few programs in its pipeline. However, none of them are yet in late-stage testing.

The company plans to advance cytomegalovirus (CMV) vaccine candidate mRNA-1647 into phase 3 clinical studies later this year. But it's going to be a few years before the CMV vaccine might reach the market.

Moderna thinks that mRNA-1647 could generate peak sales of between $2 billion and $5 billion. However, even if the company could somehow magically launch its CMV vaccine in 2023 (which won't happen), it still potentially wouldn't be enough to offset the likely declining sales for its COVID-19 vaccine. 

A post-pandemic plunge?

Wall Street is decidedly bearish about Moderna, primarily because of the uncertainty over the company's recurring COVID-19 vaccine sales. One analyst even projects that the stock could fall close to 80%.

However, a post-pandemic plunge for Moderna isn't a sure thing. It's possible that the company could gain market share with higher efficacy than rivals and increase sales with higher prices even as the overall market shrinks.

Remember, too, that investors are always looking to the future. If Moderna's CMV vaccine or any of its other pipeline candidates appear to be especially promising in clinical studies, it could be enough to keep the stock from falling even if COVID-19 vaccine sales begin to decline.

Finally, Moderna has a large and growing cash stockpile. The company could choose to put that cash to use by acquiring or licensing one or more candidates that are close to reaching the market. Such a move could generate enough excitement to overshadow concerns about sliding COVID-19 vaccine sales after the pandemic ends.

Still, there appears to be a very real chance that Moderna stock could pull back significantly when the pandemic is over. Good news for the world might be bad news for this high-flying stock.