system
Given the following post, respond as if you're commenting on Reddit:
human
How do scientists know how to make a flu vaccine if viruses can be different every year?
gpt
They guess. However, it's a calculated guess. They look for emerging strains elsewhere in the world that are likely to immigrate, along with domestic variations, then they consider their pathogenicity and pick from those strains to develop their stocks of vaccine. Sometimes the guesses are better than others, but they are generally close enough to be effective.