The recent authorization of AstraZeneca's (NASDAQ:AZN) coronavirus vaccine candidate for emergency supply in the U.K. is getting a lot of attention from investors everywhere.

While there are plenty of good reasons to buy AstraZeneca stock right now, you might be surprised to learn that the company's COVID-19 vaccine didn't even make the list. Here are four of the most important reasons this top pharma stock deserves a place in your portfolio.

Doctors looking at something.

Image source: Getty Images.

1. AstraZeneca is already successful 

AstraZeneca doesn't need its coronavirus vaccine candidate to succeed to report healthy growth. Despite all the pressure the pandemic placed on commercial drug launches during the first nine months of 2020, sales of new medicines drove total product sales 9% higher year over year.

Over the past year, AstraZeneca generated around $1.9 billion in free cash flow. This is essentially a profit the company can distribute to shareholders as a dividend, use to pay debts, or spend on acquiring new sources of revenue. 

During the first nine months of 2020, AstraZeneca reported core earnings that rose 13% year over year, but the company's dividend has been frozen for years at a level that currently yields 2.8%.

2. It's adding a blockbuster rare-disease franchise 

AstraZeneca raised a lot of eyebrows recently with an offer to acquire Alexion Pharmaceuticals (NASDAQ:ALXN) for $39 billion in a combination of cash and AstraZeneca stock. This deal probably won't close until the second half of 2021, but AstraZeneca will gain the Soliris/Ultomiris franchise when it does. 

Soliris is a complement inhibitor launched in 2007 to treat paroxysmal nocturnal hemoglobinuria (PNH), a rare disorder that causes some patients' immune systems to attack healthy blood cells. Over the years, Alexion funneled a lot of Soliris profits into expanding its addressable patient population and succeeded. At its current growth rate, Soliris franchise sales will reach $6 billion next year.

In 2018, Alexion successfully launched a longer-lasting version of its lead drug called Ultomiris, which requires a maintenance dose delivered through intravenous infusion every other month. While less expensive biosimilar versions of Soliris could become available several years from now, its maintenance dose needs to be infused every other week.

A drug that doesn't stick around at therapeutic concentrations very long isn't just an annoyance, there's increased potential for treatment gaps when people need to reschedule appointments. As a big pharma with more resources than Alexion had to work with, AstraZeneca could squeeze decades of healthy returns from its acquisition of Alexion.

Biotech laboratory

Image source: Getty Images.

3. Tagrisso 

Tagrisso is an easy-to-swallow tablet the Food and Drug Administration approved in 2015 to treat lung cancer patients with tumors driven by specific gene mutations. The FDA recently expanded Tagrisso's addressable patient population to include newly diagnosed patients who just had tumors removed surgically. This adjuvant setting could be a lucrative one because these patients tend to remain on therapy for a long time.

AstraZeneca won't have trouble getting oncologists to notice Tagrisso's value. Clinical trial results show it can reduce the risk of disease recurrence or death by 80% for their patients.

Third-quarter Tagrisso sales grew 30% year over year to an annualized $4.6 billion, and new sales in the adjuvant setting could allow the blockbuster to grow at this pace for years to come. 

4. More oncology blockbusters

During the first nine months of 2020, AstraZeneca reported sales of cancer therapies that soared 23% year over year to $8.2 billion. Tagrisso helped, but it isn't the only growth driver in this company's oncology lineup. 

Lynparza is a targeted therapy initially approved to treat ovarian cancer patients in 2014 that has since been expanded to treat pancreatic, breast, and prostate cancer. Third-quarter Lynparza sales bounded 42% higher year over year to an annualized $1.9 billion, and an approval in May to treat prostate cancer patients will provide more lift.

Last year, AstraZeneca paid Daiichi Sankyo (OTC:DSNKY) $1.35 billion up front to jointly develop and commercialize a new cancer treatment called Enhertu. This treatment has since earned approval to treat HER2-positive breast cancer patients who relapse or fail to respond to standard care.

Enhertu's also under review for the treatment of advanced-stage stomach cancer, with a great shot at approval. The treatment reduced stomach cancer patients' risk of death by 41% compared to standard chemotherapy.

A diversified pharma for the long haul

Adding rare-disease drug sales from Alexion Pharmaceuticals to AstraZeneca's quickly growing oncology segment will create a big pharma stock positioned to deliver gains regardless of what happens to the broader economy. 

Enhertu could go on to generate more than $5 billion annually at its peak, but it could also fizzle out in the face of unforeseen competition. Continued success for all of AstraZeneca's growth drivers might lead to market-crushing gains, but investors who buy the stock now don't need to get lucky to realize a healthy return. That makes it a great stock to buy now and hold for the long run.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.