Moderna (MRNA -1.75%) and Pfizer (PFE -1.82%) have generated billions of dollars from their coronavirus vaccines. That was as governments raced to get their populations vaccinated. Today, about 67% of the U.S. population is fully vaccinated.

But vaccine revenue is far from over for these vaccine leaders. That's because the world now is in need of booster shots. Moderna and Pfizer both have asked the U.S. Food and Drug Administration (FDA) to authorize their omicron-specific boosters for the fall vaccination season. So, which stock is the best bet as we head into this new phase? Let's find out.

The case for Moderna

Moderna was among the first to talk about its work on an omicron-specific booster. And the company was the first to share positive data from its clinical trials. Earlier this summer, Moderna said its booster candidate even was effective against omicron subvariants BA.4 and BA.5.

The U.K. already gave the company the nod for its booster targeting both the original coronavirus and omicron. Moderna completed its submission to the FDA last week. It's for use in individuals ages 18 and older. Moderna has been working closely with the agency -- even developing a version of its booster specifically following FDA guidance. And it's clear the FDA is supportive of omicron-specific boosters. In June, the agency recommended boosters cover this now dominant variant.

The U.S. recently ordered 66 million doses of the Moderna booster for $1.74 billion -- with an option to add on 234 million more doses. If demand is high, the booster could represent significant annual revenue.

Moderna has long spoken about its desire and ability to update boosters according to the needs of specific countries. Experts say the coronavirus will stick around. And we've seen the list of variants grow. So, this may be the first of many variant-specific boosters for Moderna.

The case for Pfizer

Pfizer announced its FDA submission one day ahead of Moderna. The company also is ahead in a couple of other areas. Pfizer landed a $3.2 billion order from the U.S. for the delivery of 105 million vaccine doses. This covers the original vaccine as well as the omicron-specific booster. The country has the option to purchase an additional 195 million doses.

The U.S. may eventually buy the same number of doses -- 300 million -- from Moderna and Pfizer. But for now, Pfizer has secured a bigger chunk of the market.

Pfizer's other advantage has to do with the population to be vaccinated. It's requesting authorization for use in individuals ages 12 and older. So Pfizer may continue its leadership in vaccinating teens. (The FDA authorized Pfizer's original vaccine for teens about a year before that of Moderna. This has offered it a significant first-to-market advantage.) Older people have been more vulnerable to coronavirus -- and rival Moderna even said in the past that at-risk populations are the key audience for boosters. But, over time, use in younger groups still can add to revenue.

Like Moderna, Pfizer also has promised the ability to quickly update vaccines and/or boosters as needed. And the mRNA technology used by both companies makes this possible. That's because mRNA vaccines don't need the lengthy processes associated with the making of traditional vaccines.

Moderna or Pfizer?

It looks like Moderna and Pfizer both are set for success in this new vaccine phase. But which one will dominate? When it comes to actually selling vaccines, I think Pfizer will hold onto the lead. The company already is set to deliver a bigger number of doses to the U.S. And it has the ability to immediately target a broader age group.

But Moderna may offer the bigger opportunity for stock market gains. The shares have lost more than those of Pfizer so far this year -- in spite of billion-dollar revenue and profit.

MRNA Chart

MRNA data by YCharts

Evidence that Moderna's success can last beyond initial COVID-19 vaccinations -- and this might begin with a successful booster season -- could lift the stock. Moderna also looks a bit cheaper. The shares trade for 5.1 times forward earnings estimates. That's compared with more than 7 for Pfizer.

That said, Moderna is a riskier bet than Pfizer. Though it has many promising candidates in late-stage development, right now the coronavirus vaccine is its only commercialized product. Pfizer sells several blockbusters across therapeutic areas. All of this means Moderna may be the best bet for aggressive investors. But more cautious investors might prefer to stick with pharmaceutical giant Pfizer.