For the most part, biotech stocks have held up pretty well in the market sell-off resulting from the COVID-19 pandemic. Two of the leading biotech exchange-traded funds (ETFs), iShares Nasdaq Biotechnology ETF and SPDR S&P Biotech ETF, are down year to date but are still outperforming the S&P 500 index.

Several biotechs have delivered solid gains in the midst of the overall stock market plunge. Here are three of these top biotech stocks that still appear to be great picks to buy in April.

Test tubes with increasingly higher levels of green fluid with a green arrow sloping upward in the background

Image source: Getty Images.

1. Vertex Pharmaceuticals

Vertex Pharmaceuticals' (VRTX 0.86%) business hasn't felt any significant impact from the COVID-19 outbreak. The company recently confirmed that its supply chain remains unaffected and its outlook for 2020 hasn't changed at all. And that outlook is quite good: Vertex projects 30% year-over-year revenue growth. 

It's not hard to see why Vertex expects strong growth this year. In recent months, the biotech has secured reimbursement deals for its cystic fibrosis drugs Kalydeco, Orkambi, and Symdeko/Symkevi in key markets. Vertex also won FDA approval for its newest CF drug, Trikafta, and appears likely to gain European approval for the drug soon. I think that Trikfafta will be the company's most successful CF drug yet.

While Vertex dominates the CF market, it's also working hard on expanding beyond CF. The biotech could have especially good chances in developing therapies that target rare diseases that, like CF, are caused by protein misfolding, including alpha-1 antitrypsin deficiency (AATD) and APOL1-mediated kidney diseases.

Vertex is highly profitable with a virtual monopoly in CF. It has strong growth prospects. And the company has an impressive cash position of $3.8 billion that it's likely to use to form partnerships and make acquisitions that boost its growth prospects even more. I can't think of a biotech stock that's as attractive on an overall basis as Vertex is right now.

2. Gilead Sciences

Gilead Sciences (GILD -1.30%) has been a center of attention during the COVID-19 crisis. That's because the big biotech's antiviral drug remdesivir appears to be one of the most promising candidates for treating the novel coronavirus disease. 

I have high hopes for remdesivir, but I have even greater expectations over the long run for another of Gilead's pipeline candidates. The company awaits U.S. and European regulatory approvals for filgotinib in treating rheumatoid arthritis. Filgotinib holds the potential to generate $4 billion or more in peak annual sales.

In the meantime, Gilead's HIV franchise remains a juggernaut. Biktarvy seems likely to become the best-selling HIV drug of all time. Gilead could top Biktarvy's success in the future, though, if clinical studies for experimental long-acting HIV drug GS-6207 go well.

If you like Vertex's cash stockpile, you'll love Gilead's. The company's cash, cash equivalents, and marketable securities totaled $25.8 billion at the end of 2019. Since then, Gilead completed a $4.9 billion acquisition of cancer-focused biotech Forty Seven. However, the company still has plenty of cash to invest in growth plus pay its dividend, which currently yields close to 3.5%.

3. Novavax

Novavax (NVAX 25.15%) is kind of the oddball in this group of biotech stocks to buy in April. Unlike Gilead Sciences and Vertex, Novavax doesn't have any approved products on the market. It doesn't claim a huge cash stockpile. It's not profitable. But Novavax has been the biggest winner of the three so far in 2020 because of two key developments. 

The small biotech captured a lot of attention earlier this year with its progress in developing an experimental COVID-19 vaccine. Novavax's preclinical testing of its vaccine was promising. The company plans to begin clinical testing in humans for its COVID-19 vaccine in May.

An even bigger story for Novavax, though, was its announcement last month of results from a late-stage study evaluating experimental flu vaccine NanoFlu. The nanoparticle-based vaccine beat Sanofi's market-leading flu vaccine Fluzone Quadrivalent in a head-to-head comparison.

Assuming NanoFlu wins FDA approval, Novavax could have a potential blockbuster on its hands in the not-too-distant future. With a market cap of less than $800 million despite the big gains so far this year, I think that Novavax's shares still have plenty of room to run.