How the Flu Vaccine Works

Medically Reviewed By William C. Lloyd III, MD, FACS

Health experts say the flu vaccine is our best defense against the influenza virus, but every year, you hear about people who get sick despite getting a flu shot. Some people who hear those stories conclude flu vaccination doesn’t work very well. But while it’s true flu shot effectiveness varies from year to year, it’s also true that the flu vaccine works to prevent the flu, ease symptoms and reduce flu deaths. Knowing how the flu vaccine works (and vaccination in general) will help you understand its benefits.

Flu vaccination gives your immune system a head start.

Imagine preparing for battle: Would you rather have a bunch of weapons piled up by the gate of your castle, ready to go in case of attack, or would you prefer to begin building weapons after the castle gate has been breached?

Most people would rather have weapons on hand, ready to fight. If you wait until you’re attacked to begin building weapons, you’re wasting precious time. Your castle could be taken over before you’ve had a chance to mount a proper defense.

Vaccination allows your body to prepare its defense in advance. When a germ invades your body, your immune system begins building antibodies to target that specific germ. As the germ multiplies and spreads through your body, the immune system churns out more and more of these antibodies. Antibodies bind to germs and stop them from multiplying; they also flag the germ so other cells can come and destroy it.

Vaccination gives your immune system a sneak peek of the enemy. When you receive a flu vaccination, you are exposed to a very tiny amount of the flu viruses that are predicted to be most common that flu season. It’s not enough to make you sick, but it’s enough to kick your immune system into action. Almost immediately, your body begins creating antibodies to target those specific strains of flu virus. In about two weeks, your body is fully prepared for an attack. If a flu virus invades your body, your immune system won’t have to waste any time building weapons because the necessary antibodies are already there, ready to go.

The flu vaccine protects against specific influenza viruses.

Influenza viruses A and B are responsible for seasonal influenza. There are many subtypes and several different strains of each subtype. The flu vaccine isn’t 100% effective, in part because researchers are never 100% sure which strains will cause the most flu infections in any given season. Each year, researchers tweak the composition of the vaccine, which typically provides protection from the 3 to 4 flu strains predicted to be most problematic. Flu shot effectiveness varies from year to year, but according to the Centers for Disease Control and Prevention (CDC), vaccination reduces the risk of illness by 40 to 60% when the flu vaccine is well-matched to circulating flu viruses.

The flu vaccine can protect against influenza A and B viral strains that are identical or closely related to the strains in the vaccine preparation. It does not protect against other influenza viruses or viruses that can cause flu-like symptoms.

Vaccination decreases hospitalization and flu deaths.

Because the flu vaccine gives the immune system a head start against influenza infection, people who get the flu despite vaccination usually don’t get as sick as unvaccinated people.

In fact, the primary purpose of the flu vaccine is to decrease influenza-related complications and deaths. Evidence suggests the flu vaccine works well to prevent serious illness and death. According to a study published in the medical journal Pediatrics, flu vaccines decrease the risk of dying from flu by 65% for healthy children.

The flu vaccine helps adults stay healthy too. According to the CDC, vaccinated adults are 37% less likely to be admitted to an intensive care unit than unvaccinated adults. Unvaccinated adults who are hospitalized with the flu are 2 to 5 times more likely to die than vaccinated adults who are hospitalized with influenza. Vaccinated adults also typically spend less time in the hospital.

Experts recommend a yearly flu vaccination for everyone ages six months and older. For the most protection, get your vaccination in early October, well before peak flu season.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Oct 7
View All Vaccines Articles
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