Vaccines can be cash cows for pharmaceutical companies like Pfizer (NYSE: PFE), GlaxoSmithKline (NYSE: GSK), Sanofi (NYSE: SNY), and Novartis (NYSE: NVS). If a company can get on the preferred list with government agencies, sales will come pouring in.

But then what? There are only so many short people needing vaccinations. Except for hoping for a baby boom, there aren't many options for increasing sales.

Pfizer's Prevnar 13 is an exception because it prevents infections by bacteria that cause pneumonia, which old people get, too. The vaccine, which is already a multibillion-dollar product, could see an instant sales boost if it can get approved for the added indication.

Data released yesterday certainly seems to support an approval. The vaccine looks good compared with Merck's (NYSE: MRK) Pneumovax, which is already approved to treat adults. Of the 13 strains that Prevnar 13 covers, it promoted a stronger immune reaction in nine of them and the same in four others.

In a second study, Prevnar 13 appears to work well as a booster shot, meaning it can be used earlier than Pneumovax. Adults become more susceptible to pneumonia starting at age 50, but doctors don't like to use Pneumovax before 65 because the vaccine wears off and a booster shot isn't that effective.

Of course, these trials only measured an immune response. The true test is whether Prevnar 13 prevents infections, but that requires a large study -- one that's still under way. The Food and Drug Administration will likely accept the early data as verification that the vaccine works and then require Pfizer to finish the trial if it wants to continue marketing it for adults.

We'll know soon enough if Pfizer can make money at both ends of the life cycle; the FDA is currently reviewing the marketing application.

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Pfizer is a Motley Fool Inside Value recommendation. GlaxoSmithKline and Novartis are Motley Fool Global Gains selections. The Fool owns shares of GlaxoSmithKline. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Motley Fool has a disclosure policy.