What happened Pfizer (NYSE: PFE) announced Monday that its vaccine candidate BNT162b2 appears to be even more effective at preventing COVID-19 than anyone had hoped -- on the order of 90%! Stock markets loved that news, and today the Dow Jones Industrial Average is soaring nearly 5% as investors place their bets on a recovery. But not all stocks are faring so well. It looks like shares of Stamps.com (NASDAQ: STMP), iRobot (NASDAQ: IRBT), and 2U (NASDAQ: TWOU) are getting simply devastated by today's news, down 9.9%, 10.5%, and 16.2%, respectively, in 12:45 p.m. EST trading. Image source: Getty Images. So what Why are these stocks down so much when seemingly everything else is up? To understand what's going on, consider the businesses they're in, how those businesses benefited from the pandemic and widespread stay-at-home orders ... and what's likely to happen when those stay-at-home orders go away and everything gets back to normal. Stamps.com is most closely associated with the sale of postage over the internet, especially to small businesses and solo operators selling and mailing stuff from their homes. This is an area of the economy that flourished despite the recession -- sales were up 42% last quarter alone -- because so many people, having lost their day jobs, started new careers working from home. One notable segment of the workforce impacted by the pandemic was housekeeping service providers, as homeowners sought safer alternatives to allowing other people into their homes to clean. iRobot shares benefited mightily -- and sales were up 43% last quarter -- as they bought up Roomba vacuums and Braava mops. Finally, 2U sales boomed (a bit less than the others, up only 31% last quarter) as colleges scrambled to find ways to put their courses online during the pandemic. It's not necessarily a "work from home" stock, but it's clearly a beneficiary of pandemic fears regardless. Now what Now what happens if those concerns go away? What happens when a coronavirus vaccine that's 90% effective at preventing COVID-19 infections gets widely distributed so that (a) employers begin to rehire their employees, (b) homeowners feel comfortable allowing others to enter their homes to clean, and (c) colleges reopen to fully in-person education? At the very least, growth rates are going to slow at Stamps.com, iRobot, and 2U. Sales will slow, and could even decline, as customers return to the ways they used to do things and abandon the alternatives and stopgaps that -- while useful and indeed critical in enabling them to survive the pandemic -- are no longer so essential in its aftermath. That's what investors are afraid of. That's why Stamps, iRobot, and 2U stocks are all down today. 10 stocks we like better than Stamps.comWhen 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 Stamps.com 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 Rich Smith has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends 2U and iRobot. The Motley Fool recommends Stamps.com. The Motley Fool has a disclosure policy.Source