In a fierce rivalry to prove cancer drug combinations work better for advanced lung cancer, Merck & Co. drew ahead of rivals on Monday, releasing late-stage results showing that a cancer drug combination using its Keytruda reduced patients’ risk of death by half, a significant improvement. Merck shares rose 3% in heavy Monday trade. Bristol-Myers Squibb also released results Monday morning for a cancer drug combination including its drugs Opdivo and Yervoy, finding that the combination improved progression-free survival with patients in a particular subgroup who had a high tumor mutational burden. Progression-free survival, which looks at how long it takes before a patient’s cancer worsens, is one measure of a cancer therapy’s success but is considered less important than overall survival, which the Merck trial measured. The odds of a patient recovering faster with the Merck treatment was 0.49, referring specifically to overall survival, while the odds for a patient on the Bristol-Myers treatment was 0.58, referring specifically to progression-free survival. Bristol-Myers shares dropped 5.7% in Monday trade, while Astrazeneca , another drugmaker working in the space, had shares fall 1.7%. The Merck results are the latest evidence supporting a trend of pairing immuno-oncology drugs, which harness the immune system to fight cancer, with other cancer treatments, including other immuno-oncology drugs, chemotherapy and radiation. Drugmakers have made big investments in this area, but experts fear it could cause the already-high prices of cancer drugs to double or triple. Merck shares have surged 8.3% month-to-date, compared with a 12.7% drop in Bristol-Myers shares and a 1.2% rise in the S&P 500 .