Read this excerpt from “Herd about the Flu .” We can’t easily predict what people will do. Some people oppose taking any vaccine. Others don’t get around to it. Finally, some people may not take it because they know it doesn’t always work. Unlike vaccines against measles, polio, or smallpox, today’s flu shot doesn’t give full immunity, or resistance, to the flu.

What is the point of a flu shot if it doesn’t work, and why doesn’t it always work? The flu is complex and strains change quickly. People need to be vaccinated with a new batch of vaccine every year. Vaccines must be produced long before the actual flu virus, or the strain of virus, starts to work its way through the community. Some years, scientists do a better job of predicting which strains will be common that year. Sometimes, even if the scientists predict the right strains, the vaccine may not create a strong immune response.