Shares of Moderna (MRNA 0.69%) rose as much as 4.9% on Tuesday before giving up some of its gains, based on data from S&P Global Market Intelligence. The move came after the U.S. Food and Drug Administration (FDA) gave full approval to the company's COVID-19 vaccine in adults ages 18 and older. Is the stock a buy after this milestone?

Ordinarily, an FDA approval would be great news for a biotech. But Moderna's situation is different. The company already markets its COVID-19 vaccine under the FDA's Emergency Use Authorization (EUA) program. The main difference for Moderna now will be that it can use the vaccine's brand name -- Spikevax -- in promoting it in the U.S.

A healthcare professional giving a shot to a person.

Image source: Getty Images.

There are other things for investors to consider with Moderna, though. The company's shares are down more than 60% from their peak last year. Moderna's market cap is at the lowest level since last May. The vaccine stock now trades at only 5.5 times expected earnings.

Some investors could view Moderna as an absolute bargain at that valuation. The catch is that no one knows whether or not the company's vaccine sales and profits will continue anywhere close to current levels after 2023.

If recurring boosters are needed going forward, there's a pretty good case to be made for buying Moderna now. Even if the company's COVID-19 vaccine sales decline by 50%, the stock would still trade at an attractive earnings multiple.

It's also important to keep Moderna's pipeline prospects in mind. The company is currently evaluating a cytomegalovirus (CMV) vaccine in late-stage testing. Moderna thinks the CMV vaccine could generate peak sales of up to $5 billion, if approved. Its pipeline also includes three other candidates in phase 2 testing outside of its COVID-19 program.

There's still considerable uncertainty for the COVID-19 market. However, assuming the widespread need for COVID-19 vaccines won't go away, Moderna is again at a level for long-term investors to consider buying the stock.