What happened Shares of Inovio Pharmaceuticals (NASDAQ: INO) were slipping 5.7% lower as of 2:58 p.m. EDT on Tuesday. The biotech didn't make any announcements that would cause its stock to slide. Instead, we're seeing a continuation of a sell-off that began in July. So what It's more important to understand why Inovio's shares have been falling for several weeks than it is to delve into one day's movement. The bottom line for Inovio is that confidence about the prospects for its coronavirus vaccine candidate, INO-4800, has decreased. Image source: Getty Images. But didn't the company announce positive results from a phase 1 study for INO-4800 in late June? Yes. The problem is that the details from that study haven't yet been released. Some investors are likely wondering if the actual data was as good as Inovio indicated. Another issue is that Inovio still hasn't received a green light to begin a phase 2/3 study of its COVID-19 vaccine candidate. The company had said that it expected to initiate the study this summer, but summer will soon end. Now what Inovio could easily reverse its slide by getting a Food and Drug Administration go-ahead for its phase 2/3 study of INO-4800. It could also reassure investors with solid (and detailed) data from the phase 1 study of its vaccine candidate. In the meantime, the biotech stock is likely to remain highly volatile. 10 stocks we like better than Inovio 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 Inovio 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 August 1, 2020 Keith Speights has no position in any of the stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.Source