Pfizer

4. Pfizer
2013 sales: $3.97 billion

Pfizer's ($PFE) got the top-selling vaccine in the world in pneumococcal conjugate Prevnar 13--and not a whole lot else. But even without a vaccine lineup outside of its Prevnar franchise, that jab's top-line power was enough to push Pfizer into the top 5. Prevnar 13 churned out $3.76 billion for Pfizer in 2013, and that's only set to increase, with Evaluate Pharma predicting $6.08 billion in sales for the vaccine come 2020.

But CEO Ian Read has been expressing interest in expanding beyond Prevnar for a while now, and if all goes right for Pfizer, by that time the best-selling shot will have at least one companion in the drug giant's vaccine stable. Last month, Pfizer submitted a Biologics License Application (BLA) for its meningitis B candidate, dubbed rLP2086, to the FDA.

If approved, the vaccine may have some company: The same day Pfizer handed over its BLA, Novartis ($NVS) handed one over for its own vaccine, Bexsero, already on the market in Europe. All things considered, ISI Group analyst Mark Schoenebaum says Pfizer's jab could yield about $225 million in peak sales by 2019, conservatively speaking.

And if Pfizer revives deal talks with AstraZeneca ($AZN)--as some analysts expect it will--that could bring along hopes for padding its vaccine unit even more. AZ boasts FluMist, a nasal spray that protects against seasonal flu; recently a CDC committee recommended the spray over flu shots in children, which should give sales a boost.

For more:
Prevnar use in seniors is cost-effective now, CDC says. But will it last?
Pfizer puts the heat on Novartis with FDA MenB filing
Pfizer teams up to lift adult vaccination rates--and Prevnar sales, too
AstraZeneca merger could bring the vaccines heft Pfizer's been looking for