When you think of Pfizer (PFE -2.03%) and Moderna (MRNA -9.07%) these days, you probably think of the coronavirus vaccine. These companies were first to market -- and have generated billions in revenue. That revenue is far from over. Pfizer and Moderna both predict billion-dollar vaccine sales this year. And they are set to introduce strain-specific boosters that may lead to recurrent revenue well into the future.

But Pfizer and Moderna aren't only competing in the world of coronavirus vaccines. In fact, the two are developing vaccine candidates for another potentially billion-dollar market. They're tackling respiratory syncytial virus (RSV). And Pfizer has just taken a major step forward. Is the big pharma company set to beat biotech Moderna? Let's find out.

A common respiratory virus

First, a bit about RSV. It's a common respiratory virus that usually isn't harmful to adults and older children. Symptoms may be a lot like those of the common cold. But RSV can be very serious for infants, the elderly, and people with weakened immune systems. It may lead to bronchiolitis or pneumonia.

Today, an approved vaccine for RSV doesn't exist. And the market opportunity is big. SVB Leerink has predicted the RSV vaccine market may reach $10 billion by the end of this decade.

Pfizer and Moderna both are studying vaccine candidates for older adults in phase 3 trials. But Pfizer recently reported trial results -- and plans to file for regulatory approval this fall. Moderna, in its most recent earnings report, said its phase 3 trial is "ongoing."

Even if Moderna reports data in the coming weeks or months, it's clear Pfizer is a step ahead from a timeline perspective. If all goes smoothly during the U.S. Food and Drug Administration's review process, the Pfizer vaccine probably will reach commercialization first.

This is great news for Pfizer. It would gain the first-to-market advantage. If Pfizer successfully follows through with vaccine quality, manufacturing, and delivery, it could hold onto market leadership -- as it's done with the coronavirus vaccine.

Blockbuster RSV vaccine sales also may at least partially compensate for declines in the sales of older drugs. Some of Pfizer's biggest products today will face competition later on this decade.

Pfizer also remains farther ahead in its work to prevent RSV in infants. The company is vaccinating pregnant people in a phase 3 trial. Moderna's pediatric RSV trial is in phase 1.

Bad news for Moderna?

So, is all of this bad news for Moderna? Not necessarily. As mentioned above, the RSV market is big. There is room for more than one player. In the coronavirus vaccine market, Pfizer and Moderna have both been able to carve out share -- and generate blockbuster revenues. The same thing can happen in the RSV market.

Second, Moderna is working on an RSV candidate that may stand out farther down the road. Moderna plans on bringing a combination vaccine candidate covering flu, coronavirus, and RSV into clinical trials this year. If it's successful, this program could be big for the company.

Here's why. No one likes the idea of getting multiple vaccines every fall. And even if an individual does this once or twice, chances are that person will eventually tire of it -- and drop one or more of the vaccines or boosters. A combination vaccine solves that problem. Clearly, it makes vaccination easier for patients and healthcare providers. So Moderna could win over the long term with this sort of vaccine.

Let's get back to the original question: Could Pfizer beat Moderna in the RSV market? In the near term, it's very possible. But down the road, Moderna is on the right track to make its mark in this new billion-dollar market. And a combination vaccine could even push Moderna into the lead.

So, even if Pfizer scores the first approval for a RSV vaccine, Moderna investors shouldn't worry. RSV still represents a huge revenue opportunity for this innovative biotech player. And potential success here could help lift Moderna shares over the long term.