By the time the influenza virus circumnavigates the globe each year, it’s become almost an entirely new demon, which is why we need a new vaccine annually. It’s less predictable how much it will change in the span of just a few months. Global travelers can sometimes bring largely unchanged viruses halfway around the world, or a more gradual spread can allow it to mutate more dramatically. As more cases spring up, researchers can sequence the viruses to see which strains are most active, and can compare the virus that the vaccine is based on to those that are actually circulating. Unfortunately, all this happens after most people have (hopefully) been vaccinated, so it’s largely too late to do anything about it except advise everyone to take extra precautions to stay healthy.