Health officials recommend that everyone 6 months and older, with a few exceptions, get a flu shot each year. Yet, the flu vaccine is far from foolproof, according to the Centers for Disease Control and Prevention (CDC). If that’s the case, why should you get it? 

On average, people who get the flu shot are between 40% and 60% less likely to catch the virus than unvaccinated individuals. So, although the flu shot may not prevent all cases of influenza, it helps protect you from severe infection and death and can help reduce the spread of the virus in communities. Each year from 2010 to 2020, between 12,000 and 52,000 people in the U.S. died of flu, and between 140,000 and 710,000 were hospitalized, according to the CDC. The CDC says that 80% of children who die from the flu are unvaccinated, though there isn’t data on vaccination status of adults who die from the flu. 

“Only about half of Americans get an annual flu vaccine,” Katherina Grusich, a spokesperson for the CDC, told Live Science. “Many more people could be protected from flu if more people got vaccinated.”

How does the flu shot work?

The flu vaccine prompts the body to create antibodies against influenza, which primes the immune system to fight the virus the next time it sees it. 

The flu vaccine is available in several forms, some of which work better for specific populations. For example, most flu shots are given as inactivated vaccines, which contain dead virus. The nasal spray vaccine, though, contains a weakened version of a live flu virus, according to the CDC.

“All current flu vaccines in the United States are ‘quadrivalent’ vaccines, which means they protect against four different flu viruses: two influenza A viruses and two influenza B viruses,” Grusich said. “Flu vaccines cause antibodies to develop in the body about two weeks after vaccination. These antibodies provide protection against infection.”

The four flu strains included in the seasonal flu vaccine are chosen based on those that are circulating in the Southern Hemisphere during their seasonal flu outbreaks, according to the CDC.

How effective is the flu shot?

When researchers study vaccines, they’re looking for several types of data that can reveal how well these vaccines work. 

• Efficacy is how well a vaccine works in a controlled clinical trial, looking at how many people who got the vaccine ended up getting sick, compared with those who didn’t get the vaccine. If a vaccine has an 80% efficacy at preventing illness, 80% fewer people in the flu-shot group of the clinical trial will get sick. 
• Effectiveness is how well the virus works in the real world, usually analyzed through observational studies after a given season is over. Real-world populations are much larger and more variable than those included in clinical trials. If a vaccine has an 80% effectiveness at preventing illness, 80% fewer of the people who get the vaccine will have gotten sick that year.

The average effectiveness at preventing laboratory-confirmed cases of influenza between the 2009-2010 season and 2019-2020 season was about 43%, meaning that people who got the flu vaccine over those years were on average 43% less likely to get sick enough with the flu to go to their doctor and get tested. 

Why should you get the flu shot?

Even though the flu shot isn’t the most effective vaccine, it still provides some protection against infection, especially for healthy people. And most importantly, even at its modest effectiveness, the flu vaccine helps to protect against the worst effects of a flu infection: hospitalization or death.

A 2021 review published in the journal Vaccine found that adults who got a flu vaccine but still got sick were 26% less likely to require intensive care, and vaccinated patients who ended up in the hospital were 31% less likely to die from the flu, compared with people who were not vaccinated. 

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The CDC also recommends that people get the flu shot during pregnancy, which changes the immune system, heart and lungs in ways that increase susceptibility to influenza. Between 2010 and 2016, getting a flu shot reduced a pregnant person’s risk of being hospitalized with flu by about 40% compared with unvaccinated pregnant people, according to a 2018 study in the journal Clinical Infectious Diseases. Other studies cited by the CDC suggest that getting a flu shot during pregnancy can also protect  the newborn from catching the flu. Flu vaccines are also important for children. Between 2010 and 2014, flu shots reduced a healthy child’s risk of dying from the flu by 65%, according to a 2017 study in the journal Pediatrics.

Overall, CDC data estimates that during the 2019-2020 flu season in the U.S., the vaccine prevented 7.5 million people from getting sick with the flu, saved 3.7 million people visits to the doctor for the flu, prevented 105,000 people from being hospitalized with the flu, and prevented 6,300 flu-related deaths.

In the United States, flu season usually lasts from December to February, although significant activity can continue until May, according to the CDC. To plan for this, people should get their flu shots by the end of October, Grusich said. 

Originally published on Live Science.

Jennifer Welsh

Jennifer Welsh is a Connecticut-based science writer and editor with several years of bench work in cancer research and anti-viral drug discovery under her belt. She has previously written for Science News, VerywellHealth, The Scientist, Discover Magazine, WIRED Science, and Business Insider.

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