Famed hedge fund manager Bill Ackman had his best trade of all-time earlier this year when he bet against the market, and he's placing a similar bet again despite the market optimism surrounding the development of a COVID-19 vaccine.What Happened: Business Insider reports that Bill Ackman placed a new hedge against the market Monday.The news came on the same day Pfizer Inc (NYSE: PFE) and BioNTech SE (NASDAQ: BNTX) announced a 90% efficacy rate for their COVID-19 vaccine candidate.