A hazard ratio (HR) compares the rate at which an event, such as a heart attack or death, occurs over the course of a study in one group versus another, usually a treatment group versus placebo. An HR of 1.0 means no difference. An HR of 0.80 means the treated group had events at about 80% of the comparison group's rate, which is a 20% lower relative rate. An HR above 1 means a higher rate. For example, the SELECT trial reported an HR of 0.80 for major cardiovascular events with semaglutide versus placebo.
Always read the HR with its 95% confidence interval: if the interval crosses 1.0, the result isn't statistically significant. Remember too that a hazard ratio is a relative measure. A 20% reduction in a rare event prevents far fewer cases than a 20% reduction in a common one, so look for the absolute numbers as well. See every key GLP-1 clinical trial and the cardiovascular benefits of GLP-1 medications.