A real-world test of Pfizer's Covid-19 vaccine in more than a million people confirms that it's very effective at preventing serious illness or death, even after one dose. The vaccine proved 92 percent effective in a real world study involved 1.2 million people in Israel. Results published on Wednesday showed the vaccine was 92% effective at preventing severe disease after two shots and 62% after one. Its estimated effectiveness for preventing death was 72% two to three weeks after the first shot. It seemed as effective in patients over 70 as in younger people.