potential Covid-19 vaccine, opening mixed on Tuesday.The benchmark Dow Jones Industrial Average was barely changed, adding 0.2 percent to 29,183.59 about 20 minutes into the trading session.But the broader S&P 500 fell 0.5 percent to 3,532.4, while the tech-heavy Nasdaq continued its downward slide, falling 1.3 percent to close at 11,564.12.The indexes blasted through records early Monday after Pfizer and BioNTech announced their vaccine was 90 percent effective in protecting against Covid-19 infection, but then retreated.Uncertainty generated by the US election has receded after Joe Biden defeated President Donald Trump, but analysts said the price swings in recent days represent competing trends fighting each other.Some investors are.