I bought quite a few stocks in March and April. Am I now going to follow the old investing adage "sell in May and go away"? Absolutely not.

However, there are still a lot of stocks that I like. Several tech stocks, dividend stocks, and financial stocks are on my watchlist. But my top pick doesn't fall into any of these categories. Instead, it's a biotech stock that's already up by a solid double-digit percentage this year and has plenty of room to run. Here are three reasons my top stock to buy in May is...Vertex Pharmaceuticals (NASDAQ:VRTX).

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Some investors are concerned that the recent stock market resurgence could fizzle. Although I don't pretend to know what will happen next with the market, I do share those concerns. There's a lot of pent-up desire among many to return to normal life quickly. Count me in that group. But I also realize that the COVID-19 outbreak could continue to be a problem for longer than any of us would like. And the economic toll could be greater and last longer than some expect as well.

These factors weighed on my decision in picking Vertex as my top stock to buy in May. I think that Vertex is practically coronavirus-proof and recession-proof. The company markets drugs that treat the underlying genetic cause of cystic fibrosis (CF).

Because CF can impact lung function, patients really don't want to be diagnosed with COVID-19, which also affects the lungs. The likelihood that Vertex's product sales will fall off significantly because of the pandemic is therefore quite small.

An economic recession won't hurt the biotech's revenue much, either. Insurers will continue to pay for Vertex's drugs and patients will continue taking the drugs regardless of whether the economy is good or bad.

Vertex reported its first-quarter results on Wednesday. Those results included $895 million in sales for a new CF drug, Trikafta.

There are two things that are remarkable about that sales figure. First, the huge sales were generated in the drug's first full quarter on the market. Second, the FDA's approval decision was originally scheduled for March 20, 2020, which would have meant only minimal sales in Q1. But the FDA instead approved Trikafta five months early on Oct. 21, 2019 -- something that's nearly unheard of in the drug industry.

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Trikafta's commercial success is another key reason why I think so highly of Vertex's prospects. I'm not expecting massive near-term sales growth in the U.S. after the fantastic launch. Vertex CEO ReshmaKewalramani noted in the company's Q1 call that the majority of eligible patients in the U.S. are already taking Trikafta. However, Vertex should win European approval for the drug later this year -- and that will pave the way for tremendous growth.

Also, Vertex plans to file for FDA approval of Trikafta in treating younger CF patients between the ages of six and 11 in the second half of this year. Although this indication won't likely be approved until 2021, it sets the stage for even more growth.

While I think Vertex is a great stock to buy in May, it's not just because of what will happen for the biotech in the coming months or even the next year or two. My view is that Vertex has several exciting longer-term potential catalysts.

The company is leveraging its expertise in CF to target other genetic diseases that are also caused by protein folding issues. Its most promising pipeline candidate in this category is VX-814, which is being evaluated in a phase 2 study for treating alpha-1 antitrypsin deficiency (AATD), a rare genetic disease that affects a similar number of patients worldwide that CF does.

Another of Vertex's clinical programs that appears to be really promising is its collaboration with CRISPR Therapeutics on CTX001. The gene-editing therapy is currently in an early stage study targeting rare blood diseases beta-thalassemia and sickle cell disease. If CTX001 is successful, it could potentially cure both of these diseases.

Speaking of potential cures, Vertex also hopes to advance into clinical testing later this year or at least by early 2021 a gene-editing therapy just might cure type 1 diabetes. Vertex picked up the program with its acquisition of Semma Therapeutics last year. It's still really early, but this is a candidate that couldbe a game-changer.

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I'm unabashedly a fan of Vertex, but I'll admit that there are two main knocks against the stock. One is its valuation. Shares trade at more than 33 times expected earnings. The other is that the biotech doesn't have any late-stage pipeline candidates. Neither of these issues concerns me all that much, though.

Although Vertex's forward earnings multiple is high, my view is that the company's longer-term growth prospects need to be taken into consideration. The stock's price-to-earnings-to-growth (PEG) ratio, which factors in five years of projected growth, stands at only 0.59. A PEG ratio below 1.0 is usually considered to be an attractive valuation.

Vertex's lack of late-stage programs might be worrisome if the company didn't have a clear pathway to growth over the next several years. But it does have such a pathway. As Trikafta wins additional approvals around the world and for treating younger patients, it should enable Vertex to treat roughly 90% of all CF patients. Winning these approvals and launching the drug for new approved indications will give Vertex plenty of room for growth while its pipeline programs advance.

It's also important to note that Vertex sits on a cash stockpile of $4.2 billion. The company's management team has stated repeatedly that more business development deals are on the way to bolster the pipeline. I'm confident that Vertex will have more growth drivers in place when the time comes that its CF franchise revenue growth begins to taper off.

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Here's My Top Stock to Buy in May - The Motley Fool