- You can still get the flu after being vaccinated. If you get the shot too early in the year, its effectiveness could wane, according to the Center for Disease Control and Prevention.
- The flu vaccine can take the average healthy adult’s immune system up to two weeks to build up enough antibodies for protection, which means you still have a chance of getting the flu within those two weeks.
- If you have a weakened or impaired immune system your chances of getting the flu are higher, even if you’ve been vaccinated.
- Don’t confuse the side effects of a flu shot for the flu itself.
- This article was reviewed by Tania Elliott, MD, who specializes in infectious diseases related to allergies and immunology for internal medicine at NYU Langone Health.
- Visit Insider’s homepage for more stories.
Yes, you can still get the flu after a flu shot. For example, if you get the shot too early in the year, its effectiveness could wane, according to the Center for Disease Control and Prevention. Or sometimes it might just be bad luck, since the flu vaccine is not 100% effective.
Here are the primary ways you can still get the flu after a flu vaccine.
You got the flu vaccine too early
Getting your flu vaccine is one instance when being overly prepared in life can have downsides.
The CDC recommends that adults get vaccinated by the end of October each year for the best protection against the flu. If you received your vaccine earlier, say in July or August, the CDC says your immunity to the influenza viruses will be “suboptimal” by the end of flu season, which could increase your risk of getting sick.
For children who need two doses of the flu vaccine, they should receive their first vaccine earlier because they need to wait at least four weeks for the second dose. The second dose should be administered no later than the end of October.
The flu vaccine is not 100% effective
Even if you get your flu vaccine on time, it’s not 100% effective. So there’s still a chance you can get the flu even after you’ve been vaccinated.
That’s because scientists don’t always correctly match the viruses in the vaccine with those that pop up during the flu season. This is what’s called a mismatch.
The CDC typically recommends that vaccine manufacturers include virus particles or weakened viruses that have been circulating in the Southern Hemisphere six months or so before the upcoming flu season in the Northern Hemisphere. However, what infects the Northern Hemisphere isn’t always the same virus as what popped up in the Southern Hemisphere.
When there’s a mismatch, that can make the flu vaccine less effective and increase your chances of getting sick from influenza. In this case, CDC experts warn the public that the shot might not offer as much protection as it has other years. So far this flu season, it’s too early to tell how effective this year’s vaccine will be against the flu.
A weakened immune system increases your risk
If you have a weakened or impaired immune system, it increases your chances of getting the flu, even if you’ve been vaccinated. People with an impaired immune system include infants, children, and the elderly.
The reason people with impaired immune systems are at greater risk of getting sick, even after vaccination, is because the flu vaccine relies on the strength of your immune system to work.
After you receive the flu vaccine, the viral strains come into contact with your body’s B cells, a special type of cell whose job is to recognize foreign invaders and prepare your body to fight them. So when your B cells spot a full-strength virus, they produce antibodies that attach themselves to the virus and, eventually, another type of immune system cell called phagocytes overwhelm and destroy the virus.
But people with impaired immune systems don’t produce as many antibodies and aren’t as protected.
For example, a 2019 study published in the journal Cell Host and Microbe showed that elderly participants who got a flu vaccine had fewer antibodies to target the flu virus than younger participants, and that made it harder for them to fight infection.
That’s why the CDC offers a high dose vaccine for certain people over the age of 65. The high dose vaccine contains four times as many inactive viruses as the regular flu shot and has been proven to trigger a stronger immune response for greater protection.
The flu vaccine doesn’t work immediately
After getting your flu vaccine, it can take the average healthy adult’s immune system up to two weeks to build up enough antibodies for protection.
“So if you got a flu shot on Monday and you come across someone on Tuesday with the flu, you’re not going to be completely protected,” says Rhonda Conner-Warren, an assistant professor in the College of Nursing at Michigan State University.
Getting the flu shot is your best chance at protection against the flu, even in years where scientists don’t get it completely right. Your best bet is to also make sure to practice proper hand hygiene and respiratory etiquette – like using tissues and coughing or sneezing into your elbow instead of your hands – to improve your chances of avoiding the flu.
Related stories about the flu:
- How to prevent the flu, according to doctors
- Yes, the flu is contagious. Here’s when you’re most likely to spread the virus
- You can’t get the flu from the flu shot, but there are side effects
- How long the flu should last and when you should see a doctor
- The flu vaccine is not 100% effective but you should still get it every year