Last year, companies working on coronavirus vaccines led gains in biotech stocks. And this helped the industry outperform the overall market. The iShares Nasdaq Biotechnology ETF climbed 26% in 2020, while the S&P 500 advanced 16%. In the new year, vaccine stocks may extend gains as clinical studies advance for some and revenue rolls in for others. Beyond that theme, I'm looking for companies that posted solid product sales growth last year -- in spite of the coronavirus pandemic. If they weathered that storm, I'm confident they can truly thrive as the situation improves. Considering these elements, here are my top biotech stocks to buy in 2021. Image source: Getty Images. 1. Moderna Moderna (NASDAQ: MRNA) shares climbed more than 430% last year. But there's reason for the stock to gain in 2021, too. That's because the biotech company recently transitioned from developing a coronavirus vaccine to actually selling it. Here's some quick math: Moderna currently has orders for at least 460 million doses. Prices vary according to order size and other factors. But if we use the price paid by the U.S. -- $15 a dose -- revenue could reach at least $6.9 billion just for those orders. The biotech company plans on producing as many as a billion doses by the end of the year. If it reaches that goal, we're looking at revenue of at least $15 billion. Of course, Moderna shares the vaccine market with Pfizer (NYSE: PFE). And the U.S. Food and Drug Administration may grant other rivals Emergency Use Authorization this year. But global demand means that at least five of these players could sell at full capacity and there still wouldn't be enough vaccine for all those who need it. The FDA authorized Moderna's vaccine for adults age 18 and older. Moderna recently launched a phase 2/3 study in teens ages 12 through 17. If all goes smoothly, the company hopes to make the vaccine available to that age group in time for the 2021 back-to-school period. This would be the only coronavirus vaccine authorized for teens so it could be another solid revenue generator for Moderna. 2. Vertex Vertex Pharmaceuticals (NASDAQ: VRTX) is a leader in the cystic fibrosis (CF) treatment market. In fact, the company expects its market leadership to last until at least the late 2030s. This is significant because the indication generates billions of dollars for Vertex. During its most recent earnings report, Vertex increased its guidance for 2020 revenue to the range of $6 billion to $6.2 billion. That's up from $5.7 billion to $5.9 billion. Vertex's star product is Trikafta, a treatment with the potential to treat as many as 90% of those with CF. The FDA approved the drug in late 2019, and it's already generated more than $3 billion in revenue. We can expect Trikafta revenue to increase further as the company aims for label expansions into different age groups and seeks approvals in various countries. Investors sanctioned Vertex stock last year after the company discontinued one of its candidates for alpha-1 antitrypsin deficiency (AATD). The stock lost 20% in one day, and it still hasn't recovered. This represents a buying opportunity. Discontinuing weaker candidates and pursuing stronger ones is all part of drug development -- Vertex emphasized this in its recent earnings call. Along those lines, Vertex continues the trial of another AATD treatment candidate and expects data in the first half. The company also is working on promising candidates to treat two blood disorders: beta thalassemia and sickle cell disease. 3. Seagen A lot is happening at Seagen (NASDAQ: SGEN). Over the past 13 months, the FDA approved two of the oncology company's drugs -- Padcev for the most common form of bladder cancer and Tukysa for metastatic HER2-positive breast cancer. Those two drugs helped drive a 60% increase in product revenue in the third quarter to more than $267 million. Seagen also sells a third product, Adcetris, which is used for Hodgkin lymphoma. We can expect product revenue growth ahead for Seagen's newer products. A European Medicines Agency committee recently recommended the approval of Tukysa in the European Union. The European Commission is now considering the request. As for Padcev, Seagen aims to expand the drug's use to more bladder cancer patients. Positive data from a recent pivotal trial may support a regulatory request, the company said. And finally, pharma giant Merck (NYSE: MRK) is investing in this growing biotech company. This fall, Merck made a $1 billion equity investment in Seagen as part of a collaboration deal. The companies will co-develop and eventually commercialize ladiratuzumab vedotin, an investigational therapy that Seagen has been studying for breast cancer and other solid tumors. Whether the coronavirus pandemic lingers or eases, revenue is set to climb at these three biotech companies. Their products' growth so far, potential in new indications or patient groups, and the global need for their treatments are reasons to be optimistic. So, 2021 is the perfect time to buy -- and hold for the long term. 10 stocks we like better than Vertex PharmaceuticalsWhen investing geniuses David and Tom Gardner have a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* David and Tom just revealed what they believe are the ten best stocks for investors to buy right now... and Vertex Pharmaceuticals wasn't one of them! That's right -- they think these 10 stocks are even better buys. See the 10 stocks *Stock Advisor returns as of November 20, 2020 Adria Cimino owns shares of Vertex Pharmaceuticals. The Motley Fool owns shares of and recommends Seagen Inc. The Motley Fool recommends Vertex Pharmaceuticals. The Motley Fool has a disclosure policy.Source