She was scheduled to present the results on Monday in Chicago at a meeting of the American Association for Cancer Research, and they were also published in The New England Journal of Medicine.

Other studies presented at the meeting also highlighted advances in immunotherapy against lung cancer, but were at earlier points in the research and less likely to bring about immediate changes in medical practice.

“If you want to see long-term survival, you’ve got to give immunotherapy as soon as possible,” Dr. Herbst said. “Chemotherapy has limitations. Immunotherapy has the ability to cure. I lead the Yale lung team. We have patients on these immunotherapies alive more than eight years.”

Other studies in lung cancer have involved another checkpoint inhibitor, nivolumab, or Opdivo (made by Bristol-Myers Squibb), which works in a similar way to pembrolizumab. The data are not conclusive, but Dr. Herbst said, “In lung cancer, my suspicion is these drugs are the same, like Coke vs. Pepsi.”

Most patients stay on the drugs for two years, he said. One Yale patient who has survived for eight years took the drug for two years and has remained well ever since. Another had to stop because of side effects after two or three months, but is well two years later.

Dr. Herbst offered several theories about why chemotherapy and immunotherapy could work well together. He said that tumor cells were like bags of hidden proteins that, if exposed, the immune system could use as targets to find and attack cancer. By killing some tumor cells, chemotherapy could pop open the bags, release the contents and help immune cells — unleashed by the checkpoint drugs — to identify their prey. It is also possible, he said, that chemotherapy may kill some immune cells that interfere with the cancer-killing action of other parts of the immune system.

Dr. Gandhi’s study included 616 patients with advanced lung cancer, ages 34 to 84, from medical centers in 16 countries. Their tumors lacked certain mutations that would have made them eligible for other, so-called “targeted” treatments. They were picked at random to receive either chemotherapy plus immunotherapy, or chemotherapy plus a placebo, with two thirds receiving the combination that included immunotherapy.