Moderna (MRNA -0.15%) isn't the high-flyer that it once was. Sure, the stock is still up nearly 170% year to date -- but it's also more than 40% below its high set this summer.

Don't rule out the possibility that investors could again flock to this beaten-down vaccine stock. Here's what it would take to make Moderna a screaming buy.

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Image source: Getty Images.

A big boost from boosters

Moderna's share price rose around 5% last Friday after the U.S. Food and Drug Administration (FDA) expanded its authorization of boosters to include all adults ages 18 and older. However, a much bigger boost from boosters is needed for Moderna to return to again wowing investors.

The company probably can't count on increased revenue from boosters in the U.S. over the near term. The U.S. government has already bought more-than-enough COVID-19 vaccine doses to fully meet demand, including boosters for everyone.

Moderna's best hope lies in other countries. Singapore's health minister, Ong Ye Kung, publicly recommended that individuals who initially received two doses of the Pfizer-BioNTech vaccine consider getting the Moderna booster. He said that the mix-and-match approach gives "a slight edge" in reducing the likelihood of infection.

If other countries follow in Singapore's footsteps, it could lead to Moderna selling more doses of its COVID-19 vaccine in the coming months. It already has advanced purchase agreements in place worth $17 billion to deliver doses in 2022.

Significant forward-supply deals...

Securing supply deals for 2023 and beyond, though, is what would most move the needle for Moderna. The main reason the stock only trades at 11 times expected earnings is that investors aren't confident about the ongoing demand for COVID-19 vaccines.

So far, Moderna has achieved only limited progress in signing supply agreements for 2023 or later. The company inked a deal with Canada to deliver 20 million doses in 2023 with an option for an additional 15 million doses. Canada also has an option to buy up to 35 million doses in 2024. Taiwan agreed to purchase 15 million doses of Moderna's vaccine in 2023.

But Moderna hasn't secured supply deals beyond 2022 with the U.S. or the European Union. Should the company land major advanced-purchase agreements in these big markets for 2023 and 2024, the likelihood of a major rebound for the stock will increase quite a bit.

The ideal scenario for Moderna would be for the FDA and European Medicines Agency to determine that annual COVID-19 boosters are needed for everyone. This would potentially lead to additional supply deals throughout the world that extend well into the future.

... or another steep decline?

There's one other way that Moderna stock could become a screaming buy. The biotech's shares could experience another steep decline and fall to a level that makes Moderna too tempting to ignore. How far the stock would have to fall depends on the demand for COVID-19 vaccines in a post-pandemic world.

The worst case for Moderna is that a consensus emerges that COVID-19 vaccines will only be needed on a regular basis in the future for individuals at high risk of hospitalization or death. This scenario would make Moderna's pipeline much more important to its stock valuation.

As of now, the company has three other programs in late-stage testing: a cytomegalovirus (CMV) vaccine candidate, an experimental flu vaccine, and a respiratory syncytial virus (RSV) vaccine candidate. These programs hold significant potential, but not nearly enough to justify Moderna's current market cap of close to $115 billion.

The bottom line is that Moderna either needs something really good or something really bad to happen for its stock to become a compelling buy. And we'll probably have to wait at least until the second half of next year to know which outcome is more likely.