3 Reasons You Might Catch the Flu Despite Getting a Flu Shot

Have you (or someone you know) sworn off getting a flu shot because it didn’t work? Sometimes, flu shots aren’t 100 percent effective, but they are considered the best method of flu prevention, lowering your risk of getting sick between 40 and 60 percent. Why aren’t flu shots completely reliable? Here are three possible reasons.

Reason 1: Your Immune System May Need Some Boosting

St. Jude Children’s Research Hospital has been studying the immune response to flu vaccines. Last year, they found that protection from severe flu symptoms lies in having a more diverse set of immune cells, particularly helper T-cells, as opposed to having higher levels of influenza antibodies, as historically thought, according to a study published in Nature Immunology.

Research conducted this year found that a subset of helper T cells called T follicular helper (Tfh) cells, which help produce antibodies to fight pathogens and play a critical role in your body’s response to flu vaccines. Researchers believe that Tfh cells indirectly control the anti-influenza response because they often “see” the wrong parts of the virus, leading to less effective immunity, according to a study published in Nature Immunology.    

“The results from the Tfh study might lead to improving flu vaccine design,” says Bernard Kaminetsky, MD, medical director, MDVIP. “However, results from the helper T cell study suggest that you should try boosting your helper T cells, which includes drinking green tea, eating a healthy diet, cooking with garlic, avoiding heavily processed foods and getting plenty of exercise. I also recommend managing stress and sleeping seven to eight hours every night.”

Reason 2: You Might Catch the A(H3N2) Strain

Vaccines for the 2024-2025 flu season are formulated to protect against three flu viruses that are expected to be predominant this season - A(H1N1), A(H3N2) and B/Victoria viruses. Historically, vaccines tend to work better against A(H1N1) and B strains of influenza, while being less effective in fighting the A(H3N2) strain. Some flu seasons, such as 2018-2019 and 2002-2003, H3N2 become the dominant strain, leading to more serious symptoms and hospitalizations compared the years in which H1N1 was the dominant strain.

“You can help lower your risk for more serious symptoms by strengthening your immune system and raising the number of helper T cells,” Kaminetsky says. “This is especially important if you have a weakened immune system, a BMI of 40 or higher, a nervous system condition or chronic disease.”

Reason 3: You Might Be Exposed to a Strain that’s Not Included in the Vaccine

There are many influenza strains that circulate each year. Scientists collect viral surveillance data that help detect new strains, predict predominant strains for the upcoming season, track how quickly the strains spread and what type of patients are being at risk. Scientists send private manufactures viral information for them to create vaccine that can kill the identified strains for that flu season. Sometimes, scientists can’t figure out how to kill a strain safely so that it can be included in the vaccine. This means you may catch a strain that was not included in the vaccine.

“This is why strengthening your immune system, washing your hands regularly, avoiding crowds, disinfecting commonly used surfaces and covering coughs and sneezes is so important,” says Kaminetsky.  

“I always encouraged my patients to get a flu shot, especially if they are at high risk for developing complications because for most people, it’s their best protection against the flu and its complications,” says Kaminetsky. “However, I strongly suggest you discuss getting one with your primary care physician if you have a history of flu shot reactions, an egg allergy, a bleeding disorder or Guillain-Barrè syndrome.”

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