Most people know: If you got a flu shot this year, next year you’ll need it again.

This is because the virus changes, usually rendering the previous year’s vaccine partly or totally useless. And it’s no secret that the flu vaccine’s effectiveness falls well short of what scientists and public health officials would like to see. Yes, it reduces the severity of influenza infections and prevents thousands of deaths and hospitalizations every year, but nowhere near other recommended vaccines.

But why does the virus change so much every year? Why does the vaccine’s effectiveness vary so much? Here’s the explainer you’ve been waiting for.

The big picture

First, a primer on how vaccines work: Vaccines include pieces of the pathogens — the viruses or bacteria — that cause disease. Some vaccines contain the whole pathogen, but whether it’s the whole thing or just pieces, the pathogen in the vaccine has been killed or modified so it can’t cause illness. (That’s why the flu vaccine can’t give you the flu.) What it can do is get a healthy immune system’s attention.

The body’s immune response to a vaccine resembles a military drill for fighting a specific enemy. When a vaccine enters the body, the immune system recognizes the deactivated pathogen as an intruder. Among other things, the body builds an army of antibodies, proteins in the blood specialized to fight that specific pathogen. Even though the vaccine’s components don’t cause infection, the body is primed for battle nonetheless, with antibodies on standby. If the body ever encounters the real pathogen, the antibody army wipes it out before it causes disease.