The question: Can the flu shot cause the flu?
The science: You may be tempted to skip your flu shot this season because of scheduling issues, a fear of needles or another reason. But misconceptions and misinformation – including concerns that the vaccine actually causes the flu – shouldn’t be one of them, experts said. It is one of the biggest myths you will hear during cold and flu season and one that is particularly vexing to doctors, who do everything they can to counter the concern.
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“This is probably the most common question I get asked about the influenza vaccine. And it is likely one reason why immunization rates against influenza are low,” said Paul Offit, the director of the Vaccine Education Center at Children’s Hospital of Philadelphia. Immunization rates have declined in recent years, with fewer than half of adults getting a flu shot in the 2023-2024 flu season.
Influenza is a respiratory illness mainly caused by two types of influenza viruses, A and B. Each year, these viruses mutate, producing new versions that sweep the globe. The peak time for flu season in North America is from October to May, affecting millions of Americans. In the U.S. during the 2024-2025 season, an estimated 610,000 to 1.3 million people were hospitalized and 27,000 to 130,000 people died.
Scientists gear up for this global sweep months in advance, working to figure out what mutations have popped up and using them to craft the right mix so that the flu shot – which usually becomes available in August or September – can help protect you.
Almost all influenza vaccines work by introducing inactive – or dead – proteins from the surface of the virus, known as antigens, into the body to activate the immune system to protect against future infection.
Our immune system functions as an army with soldiers working to detect and destroy invaders, said Walter Orenstein, a professor emeritus in the Emory School of Medicine.
“You would never send an army into battle if you didn’t have some training. The influenza vaccine shows the immune system what potential components of the virus it should focus its efforts on,” said Orenstein, also a former director of the U.S. Immunization Program at the Centers for Disease Control and Prevention from 1988 to 2004.
But because flu vaccines generally contain snippets of protein, rather than the virus itself, the shot “cannot cause influenza,” he added.
There are a few reasons some people might think otherwise. One is that when your immune system gears up to react to the proteins, your body responds as if it were a real virus, so you can develop temporary side effects that may feel similar to mild flu symptoms.
When you get a vaccine, your immune system releases cell-signaling molecules called cytokines and chemokines. These help activate and stimulate certain cells in your immune system such as B cells, which make antibodies, and T cells, which kill the real virus if you later encounter it, Offit said.
These molecules can cause temporary side effects, including a headache, low-grade fever, fatigue, and muscle and joint pain, he said.
Moreover, one version of the flu vaccine, a nasal mist, uses a live, weakened virus that, while incapable of causing a full-blown infection, can cause a cough and runny nose, among other side effects.
“When people say, ‘I got flu from the flu vaccine,’ that’s not possible. What they got was an immune response to the flu vaccine,” Offit said, noting that the actual flu causes symptoms that are much more severe.
While you can’t get the flu from the vaccine, you can get the flu shortly before or after getting the vaccine.
It takes about two weeks after vaccination to develop enough antibodies to protect against the flu. That means if you get the actual flu three days after getting the shot, for example, it wasn’t because you got the shot; it was because you got exposed to the virus before you developed protective immunity or maybe because the vaccine didn’t offer enough protection.
After two weeks, the vaccine is generally about 40-60 percent effective against infection, meaning you can still get the flu. Also, immunity may last only about six months, some research shows, and you need a new shot every year because the part of the virus that our immune system develops antibodies toward is constantly changing.
Flu shots are considered a success if they reduce hospitalizations and deaths, so while you can still catch the flu, hopefully your symptoms will be a lot milder because of the vaccine.
“That’s the goal – to keep you out of the hospital, out of the intensive care unit and out of a morgue,” Offit said.
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What else you should know
The flu shot is recommended for adults and children ages 6 months and older, with rare exceptions, including a history of a severe allergic reaction called anaphylaxis, the CDC said. There are various types of flu vaccines, however, so speak with your health care provider about which one is best for you.
Here are some of them, and how experts say they differ:
The standard flu vaccine, or trivalent vaccine, protects against three influenza viruses – two influenza A strains and one influenza B strain.
The high-dose flu vaccine contains up to four times the amount of the inactivated antigen, or hemagglutinin, as the standard vaccine and is approved and often preferred for people age 65 and older because they are at higher risk for severe flu and complications such as pneumonia. The vaccine may also be given to organ transplant recipients who are taking immunosuppressive medications.
The adjuvanted flu vaccine also contains the inactivated antigen plus immune system boosters called adjuvants that stimulate cells to mount a stronger immune response. “Just like the high dose gives you more antigen to react to, the adjuvant stimulates your immune system, so it makes it more ready to react to the vaccine antigen and make a protective immune response,” Orenstein said.
The recombinant flu vaccine is also more potent than the standard vaccine but differs from the others in that it does not use the flu virus or chicken eggs in the production process. People who are allergic to eggs can get the standard flu shot as well, though the high-dose vaccines that use eggs are not recommended for those with severe egg allergies, the CDC said.
The nasal spray flu vaccine, FluMist, is the only vaccine that uses a live – but weakened – virus, so the virus will not cause the flu. The needle-free vaccine uses a weakened virus that is engineered in a laboratory to reproduce only in cooler temperatures inside the nose. It can’t survive in the body because core body temperature is warmer, Offit said. “That’s the safeguard of that vaccine,” he said. The nasal spray isn’t recommended for people who are pregnant, immunocompromised or have certain underlying medical conditions.
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The bottom line:While the influenza vaccine cannot cause the flu, it can cause temporary side effects such as a low-grade fever and body aches that may feel similar to flu symptoms. Also, it is possible to get the flu after getting vaccinated.
Are you wondering whether to believe a hyped-up health tip, viral wellness trend or long-standing scientific claim? Email IsItTrue@washpost.com and we may answer your question in a future column.
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