How does the flu shot work? How it protects against the flu and why it changes every year

The flu shot works by introducing your body to antigens, which produces antibodies that can help your immune system fight off a flu infection.
The flu shot is made of inactivated flu viruses, so it can help you build immunity without actually making you sick.
Flu viruses are constantly mutating, so the vaccine is updated annually based on researchers’ best guess of which virus strains will be circulating among people in a given season.
This article was medically reviewed by Jason R. McKnight, MD, MS, a family medicine physician and clinical assistant professor at Texas A&M College of Medicine.
This story is part of Insider’s Flu Shot Guide.

While the world continues to figure out how to get the COVID-19 pandemic in check, another viral menace is just around the corner: influenza season. Luckily, scientists have a much better handle on how the flu works, and vaccination greatly reduces your chances of getting sick or spreading it to others.

How the flu vaccine works

The flu shot works by introducing your body to antigens, which produces antibodies that can help your immune system fight off a flu infection. Because the flu shot is made up of inactivated flu viruses, they can help you build immunity without getting you sick.

Introducing inactive flu viruses to your system is like showing your body pictures of enemies. As a reaction to that exposure, your immune system produces antibodies, which are like soldiers who remember what those enemies look like and are prepared to fight them.

If you’re later exposed to the flu, these antibodies help your body “remember” the flu is an invader, and kick into gear, creating more cells and antibodies to combat the virus. But building this army in your body takes time, so the vaccine won’t protect you from infection immediately.

“You do need about two to four weeks after getting vaccinated to sort of maximize your protection,” says Nicole Bouvier, MD, an associate professor of infectious diseases at the Icahn School of Medicine at Mount Sinai.

Flu season doesn't typically appear in the Northern Hemisphere until December, January, or even February. But sometimes it starts early, so Bouvier usually recommends her patients get vaccinated in October. "It's better to be prepared," she advises.

Several factors determine how well the flu vaccine will protect an individual, according to the CDC: the age and health of the recipient; the particular type of vaccine received; and the similarity between the types of virus in the vaccine and the types currently circulating among people in a given location and year. This brings us to why the vaccine changes every year.

Why the flu vaccine changes

Flu viruses are constantly mutating through a process called "antigenic drift." This happens because the viruses have poor "proofreading" capabilities, meaning they are unable to correct subtle mistakes that occur in their generic material as they're trying to replicate themselves. These mutations add up and create genetically distinct flu strains, and these new strains are called "clades."

Small drifts might not affect the efficacy of the targeted specific flu clade for that year's vaccine. But over time, the viruses can mutate enough that the antibodies created by the vaccine no longer recognize the flu and thus fail to guard against that strain.

Bouvier compares it to a bouncer kicking someone out of a bar. "The bouncer is going to remember what that person looks like. But if that person puts on a wig and maybe some dark glasses and changes their clothes, they're not going to look the same, and your immune system is kind of the same way," she says. "So it doesn't actually realize, 'Hey, I've seen you before and you don't belong here.'"

Organizations like the WHO, the CDC, and the Food and Drug Administration monitor which clades are appearing in different regions at any given time, and based on that, try to predict what strains will be most prevalent the following flu season. But they don't always predict perfectly, so the efficacy of the vaccine can vary from year to year.

Still, even if one of the circulating viruses has changed somewhat compared to what's in the vaccine, there is still some possible benefit from the vaccination, and other flu types may still be well matched, providing more protection overall.

Benefits of the flu shot

Numerous studies have shown that getting an annual flu vaccine creates far-reaching protection for individuals and communities as a whole.

The flu shot can prevent you from getting the flu, or from having to go to the doctor if you do get sick. The CDC estimates that during the 2018-2019 flu season, vaccination prevented about 4.4 million flu cases, 2.3 million doctor visits, almost 60,000 hospitalizations, and 3,500 deaths in the US.
The vaccination also can reduce the severity of the flu if you do catch it. A 2018 study found that people who had gotten the flu shot and still had to go to the hospital because of the flu were 59% less likely to be admitted to the ICU compared to those who were not vaccinated.
Annual vaccination against the flu saves children's lives. A 2017 study showed that vaccination for healthy children reduced the risk of dying from flu by almost two thirds. For children with underlying high-risk medical conditions, the risk of death was cut in half.
Vaccination is especially beneficial for people with chronic health conditions. Vaccination has been associated with fewer hospitalizations for people with heart disease, chronic lung disease, and diabetes.
Vaccination may prevent the spread of flu to others around you. This is especially important to protect those more vulnerable to the virus — like the elderly, young children, and those with chronic health conditions.

The bottom line

In 2019 in the US, more than 35 million people contracted the flu, and almost 35,000 people died from the virus, according to the CDC. But only about half of Americans get a flu shot every year, despite the proven benefits.

As COVID-19 continues to strain the American healthcare system, flu vaccination is as important as ever, Bouvier says. "When you start to add that [flu] death toll on top of what we're seeing from COVID, we just would prefer not to see that happen. So the more people who get vaccinated [against flu] to make it harder for the virus to circulate, the better," she says.