Shares of Merck & Co. rose on Monday, providing an outsize boost to the Dow Jones Industrial Average in early trading. The stock rose 2.4%, by far the biggest gainer among the blue-chip average's 30 components. The rally came after it released late-stage results showing that a cancer drug combination using its Keytruda reduced patients' risk of death by half, a significant improvement. Health-care stocks were the top-performing sector of the day, and among other components, Johnson & Johnson was up 1.5% and Pfizer Inc. rose 0.7%. The Dow rose 0.6% while the S&P 500 was up 0.5% and the Nasdaq Composite Index rose 0.4%.