Image source: Pfizer. 

If you're a shareholder in pharmaceutical giant Pfizer (NYSE:PFE), it might feel as if it's been a rather ho-hum year. The stock has traded in a fairly tight range (not that Pfizer is all that volatile to begin with), but it's managed to tick slightly higher in a year where broad market indexes and many of its peers are currently trading marginally lower.

Pfizer turns the corner
The reason? Pfizer appears to have finally turned the corner and put the worst of its patent exclusivity issues in the rearview mirror. The loss of key patents covering cholesterol-fighting juggernaut Lipitor and anti-inflammatory drug Celebrex packed some unwanted punishment to Pfizer's top and bottom lines. However, in each of the past three quarters, Pfizer has delivered operational growth once the effects of currency fluctuations, divestments, and acquisitions are accounted for.

The big reason Pfizer was able to grow its business was the outstanding performance of a few key drugs. The launch of metastatic breast cancer drug Ibrance in February following its approval by the Food and Drug Administration was a big spark. Sales of Ibrance totaled $230 million in the third quarter -- just its second full quarter of sales. The pill, which essentially doubled progression-free survival for advanced breast cancer patients who are HER2-negative and estrogen-receptor positive and added more than four months to median survival relative to the placebo, looks well on its way to blockbuster status in 2016.

Nerve pain treatment Lyrica has been another source of impressive growth for Pfizer in 2015. Although Lyrica's exclusivity will be drawing to a close here in the coming years, Pfizer has had no trouble raising its price for the drug.

Image source: Army Medicine via Flickr.

Pfizer's top-selling drug in 2015
However, if we want Pfizer's best drug in 2015, we need to look no further than the top-selling vaccine in the world: Prevnar 13. The Prevnar family of vaccines tallied $1.58 billion in global sales in the third quarter, up 44% from the previous year. U.S. sales soared 77%, while sales of the vaccine overseas jumped a more modest 10%. 

Prevnar 13 is a vaccine that's designed to prevent infections caused by pneumococcal bacteria, such as pneumonia. As a combination, influenza and pneumonia are the eighth-leading cause of death in the United States based on data from 2013.

The reason Prevnar 13 sales exploded higher in 2015 can be traced back to the recommendation of the Advisory Committee on Immunization Practices in Sept. 2014. The ACIP, which is a division of the Centers for Disease Control and Prevention that advises it on vaccines, recommended all adults ages 65 and over get the Prevnar 13 vaccines in series with Pneumovax 23, a vaccine developed by Merck & Co. (NYSE:MRK). Prevnar 13 is also approved for children under the age of five, and people ages six and up who have certain risk factors. Out of 45 million eligible people, Pfizer is estimating that it's reached about 25% to 30%.

Image source: Flickr user

Pfizer's management has suggested that it's reached all of the elderly persons who were already getting a pneumococcal vaccination, and that reaching the remainder of the eligible population could prove more challenging. However, that doesn't mean investors shouldn't expect modest growth out of the top-selling global vaccine moving forward as it expands internationally and attracts elderly Americans, albeit at a slower pace than in 2015.

One up-and-coming drug to watch
Aside from Ibrance, the up-and-coming drug that shareholders should be keeping their eyes on that has a shot at becoming Pfizer's best drug of the future is avelumab.

Avelumab is a cancer immunotherapy being developed with Merck KGaA (OTC:MKGAY). Instead of going after metastatic melanoma, which has been the early game plan for cancer immunotherapies, avelumab is being targeted at advanced non-small cell lung cancer and a number of other solid tumors. The advanced NSCLC trial is currently in phase 3, while most of its solid tumor studies are still in the early stages.

Image source: Merck.

Why go gung-ho over cancer immunotherapies? Look no further than Keytruda for the answer. This drug was developed by Merck & Co. (not to be confused with Merck KGaA) and was the first checkpoint inhibitor approved by the FDA for metastatic melanoma. It's since been also approved to treat advanced NSCLC in patients who express PD-L1 at high levels. It's also the drug that former President Jimmy Carter took that appears to have cleared up the 91-year-old's metastatic melanoma spots on his liver and brain.

Sales of Merck & Co.'s immunotherapy totaled $159 million in the third quarter, and it's possible with the expansion into advanced NSCLC in October that the drug could make a run at $1 billion in sales by as early as 2016. Pfizer realizes the potential for immunotherapies is huge based on Keytruda's initial response, and it and Merck KGaA could benefit in a big way from avelumab if it succeeds in clinical studies.

Time to buy Pfizer?
Pfizer has successfully delivered three quarters in a row of operational growth, but does that mean its stock is worth adding to your portfolio?

In my opinion, the answer to this question will depend on your investment goals and time frame. If you're looking for higher-growth investment opportunities or anything where you'll potentially need your money over the next one to three years, I don't believe Pfizer is the stock for you. Pfizer still has to contend with Lyrica's imminent exclusivity loss, and it's not as if the company's top line is growing at a breakneck pace.

However, if you're an income-seeking investor and targeting low volatility, then Pfizer might be perfect for you. Its merger with Allergan should result in substantial cost savings, but it'll take a good two to four years before we see the benefits of this combination. As such, I do believe Pfizer can be a winner for certain investors over the long run, but they need to understand that this is a long-tail growth opportunity and not a flash-in-the-pan investment.