I can easily name two good arguments for buying shares of Pfizer (NYSE:PFE). First, the stock is a bargain with its forward earnings multiple of 11.5. Second, Pfizer offers an attractive dividend that currently yields north of 3.7%.

Granted, those arguments might not be compelling enough for some investors. If you need more than just a relatively low valuation and a fantastic dividend, there are now 200 million more reasons to buy Pfizer stock.

Vaccine vials forming the shape of a dollar sign.

Image source: Getty Images.

Keep 'em coming

Pfizer and its partner BioNTech (NASDAQ:BNTX) announced last week that the U.S. government has bought an additional 200 million doses of its COVID-19 vaccine. These doses will likely be delivered beginning in October 2021 through April 2022.

What really makes this latest supply deal striking is the context. The U.S. government initially placed an order for 200 million doses of the Pfizer-BioNTech vaccine in 2020. In February of this year, another 100 million doses were ordered.

Now, Pfizer and BioNTech have sold 500 million doses in total targeted for the vaccination of U.S. residents. That amount doesn't include another 500 million doses that the U.S. government has purchased for donation to poor countries.

Only Moderna (NASDAQ:MRNA) comes close to rivaling Pfizer and BioNTech when it comes to supply deals with the U.S. government. The company has sold a total of 500 million doses of its vaccine to Uncle Sam. 

A few billion here, a few billion there

So why does this latest supply agreement make Pfizer stock more attractive? It's simple: The company is now guaranteed to make a lot more money.

Pfizer and BioNTech charge $19.50 per dose to the U.S. government for their vaccine. Those extra 200 million doses amount to another $3.9 billion in sales for the two companies to split equally.

The late Sen. Everett Dirksen from Illinois reportedly once said, "A billion here, a billion there, and pretty soon you're talking real money." Even for a pharmaceutical giant like Pfizer, pocketing an additional $2 billion qualifies as real money.

We seem to be seeing a duopoly of sorts shaping up in the COVID-19 vaccine market, at least in the major developed countries, with Pfizer-BioNTech and Moderna. The more this dynamic solidifies, the higher these vaccine stocks should move.

The big unanswered question

There's one big unanswered question for investors, though: How long will the large vaccine orders continue? If the Pfizer-BioNTech vaccine provides protection for years, the impressive sales we're seeing now will plunge in the not-too-distant future.

However, the rise of new coronavirus strains including the delta variant could make this an unlikely scenario. Both Pfizer and Moderna believe that booster doses of their vaccines will be needed. Israel has already begun administering third booster shots to some immunocompromised individuals. 

It's possible that Pfizer will be able to count on supply deals with the U.S. and other major nations for hundreds of millions of doses annually from now on. If so, the stock's current price could look like an absolute steal in hindsight within the next couple of years.

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.