What happened Shares of BioNTech (NASDAQ: BNTX) were slumping 7.5% lower as of 3:35 p.m. EST on Thursday. This decline might seem surprising considering the good news the company and its partner Pfizer (NYSE: PFE) reported this week. This news notably includes positive efficacy results for coronavirus vaccine candidate BNT162b2 and a supply agreement with the European Union for 200 million doses of the vaccine. Why did the German biotech stock sink? The most likely reason is old-fashioned profit-taking. BioNTech skyrocketed more than 50% in three days after the interim efficacy results were announced for BNT162b2. It's tempting for investors to take profits off the table with a sharp, quick gain like that. Image source: Getty Images. So what The best thing for long-term investors to do when a stock they own tumbles due to others taking profits is... absolutely nothing. Selling too soon is oftentimes the worst mistake an investor can make. BioNTech's growth prospects remain very good even after its huge gains this week. Assuming BNT162b2 wins Emergency Use Authorization (EUA) in the U.S. and approvals in other major countries, BioNTech and Pfizer are poised to make billions of dollars from the COVID-19 vaccine. All signs right now point to the likelihood of the need for COVID-19 vaccination on a yearly basis. If so, BioNTech could be on track for a significant recurring revenue stream. Now what BioNTech and Pfizer should have enough safety data to file for EUA in the U.S. for BNT162b2 within the next week or so. CEO Ugur Sahin said in BioNTech's third-quarter earnings press release, "We believe the prospects for the company have never been brighter." If the safety profile for BNT162b2 looks good, Sahin could be proven right. 10 stocks we like better than BioNTech SEWhen 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 BioNTech SE 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 October 20, 2020 Keith Speights owns shares of Pfizer. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.Source