U.S. stocks faced their steepest decline since Election Day, with the S&P 500 dropping 1.3% as fears over President-elect Trump's nomination of vaccine skeptic Robert F. Kennedy Jr. to head HHS impacted vaccine manufacturers and the broader market. Vaccine stocks plunged, contributing to a general market downturn, particularly in technology and biotech sectors. This follows a post-election rally that has now faded, raising concerns about potential policy shifts and their effect on interest rates. Comments from Federal Reserve officials about a cautious approach to rate cuts further influenced investor sentiment, leading to increased volatility in stock trading.