A vaccine is Pfizer's best-selling drug

Over the past 4 years, the product that generated the most sales for Pfizer wasn't one of its household names, like Lyrica, Lipitor or Viagra. It was Prevnar 13, a vaccine that protects against pneumonia and other infections.

The bottom line: Pfizer has collected $23.4 billion in Prevnar 13 sales since 2015, including $5.8 billion last year. The pharmaceutical giant got a big revenue boost in 2015, after the federal government recommended the vaccine for people older than 65 and required insurance to cover it.

Details: Before 2015, most of Pfizer's Prevnar 13 revenue came from international markets.

• But the seal of approval from the Centers for Disease Control and Prevention in late 2014 prompted U.S. seniors to get the vaccine, which most commonly was given to children younger than 2.
• Pfizer commands higher prices for the drug in the U.S. The price of a single Prevnar 13 shot this year, before taking out rebates and discounts, is about $195 — a 5% increase from 2018 and 79% higher than its list price from 2010.
• The patent for Prevnar 13 doesn't expire until 2026, several years after some of Pfizer's other growing medications, like Ibrance and Xeljanz, will have lost their patent protection. (Lyrica, Lipitor and Viagra are already off-patent.)

Go deeper: The rising price of the pneumococcal vaccine is angering doctors, Kaiser Health News reports.