NOT NEARLY AS MUCH AS WE’D LIKE: How Effective Are Flu Shots? “Here’s the thing: The efficacy of the shot varies drastically from year to year, ranging from 10% to 60%. It all depends on whether the vaccine ends up aligning with the strains of the virus that appear.” 60% as a best-case scenario isn’t great. But there’s also this: “The vaccine can decrease the length of having the flu, decrease the risk of complications and the severity of the flu, and decreases the risk of spreading it to others.”