The S&P 500 was on track to open at all-time highs on Tuesday, building on a record-setting rally amid thin trading volumes, with investors unshaken by Omicron-driven travel disruptions and store closures. The S&P 500 and Nasdaq on Monday posted their best four-day rally since November 2020, with the S&P 500 scaling a peak as an upbeat outlook on the U.S. economy helped investors look past thousands of flight cancellations and Apple Inc. shutting its New York stores due to surging cases. The Centers for Disease Control and Prevention (CDC) on Monday shortened the recommended isolation time for Americans with asymptomatic cases of COVID-19 to five days from the previous guidance of 10 days. “This policy change is sending the message that it is becoming more like the flu and less like the variants we saw early on when we had no treatments, no vaccines and it was much more …