What happened Altimmune (NASDAQ: ALT) was plummeting in late trading on Wednesday, with the shares in free fall by nearly 40%. That was no surprise given the company's revelation the previous afternoon. So what Just after market close on Tuesday, Altimmune officially threw in the towel on the development of its COVID-19 vaccine, AdCOVID. It's also halting enrollment on a phase 1/2 clinical trial of its T-COVID drug, which had been under way. Image source: Getty Images. AdCOVID, a nasal spray, disappointed in its phase 1 trial. While the vaccine appeared to be well tolerated, it produced lower-than-expected immune responses. The number and magnitude of those responses were also "substantially lower" than rival vaccines already approved for emergency use, Altimmune said. As for T-COVID, although it was well tolerated with no observed serious adverse events, the biotech was having difficulty enrolling patients for the third of three planned cohorts. This is due to the significantly declining incidences of the disease throughout the U.S., which is rapidly lowering the chances of drafting participants. Altimmune is therefore terminating further enrollment; it said it will consider its options once it evaluates the available data and consults with its T-COVID development partners. Now what Although investors are punishing Altimmune severely for these decisions, they're sensible given AdCOVID's weak performance, and the fact that the coronavirus is fading in many areas of the U.S. Altimmune has other vaccines and drugs in its pipeline, so perhaps this sell-off is an opportunity to buy a still-promising biotech on the cheap. 10 stocks we like better than ALTIMMUNE INCWhen our award-winning analyst team has 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.* They just revealed what they believe are the ten best stocks for investors to buy right now... and ALTIMMUNE INC 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 June 7, 2021 Eric Volkman 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