The Moderna vaccine is the most effective of the three available, according to a Centers for Disease Control and Prevention study released Friday. Moderna’s jab was 93 percent effective at preventing hospitalization from COVID-19 among 3,600 healthy Americans from March to August, researchers found, while Pfizer’s was 88 percent effective and J&J’s 71 percent. Immunity given by the Pfizer vaccine falls faster than by Moderna, beginning to decline about four months after the second shot, which accounts for the difference in efficacy, according to the study.