How do we decide what goes in next year's flu shot?

Every year, health officials face a critical question: what should go in the next flu shot? They have to predict which strains of the flu virus will be most common months in advance, so vaccine manufacturers have time to produce enough doses. This report describes the process of tracking flu activity and deciding on the composition for the Northern Hemisphere's 2022-23 influenza vaccine. The details of the specific findings, the population studied, and any safety observations from this tracking are not reported here. This is a routine, essential part of public health planning, but the ultimate test of how well the vaccine works comes when flu season arrives.