Flu Shot: How Long It Lasts and Effectiveness

By Terez Malka, MD
Medically reviewed checkmarkMedically reviewed
November 30, 2021

As winter approaches, so does flu season. Short for influenza, the flu is a viral infection that attacks your respiratory system (nose, throat, and lungs).

It’s a highly contagious illness that millions of people get each year.

Since it can cause symptoms of sore throat, runny nose, body aches, fatigue, fever, and chills, the flu can feel like a really bad cold.

It can also cause symptoms you wouldn’t normally experience with a cold, like vomiting and diarrhea.

While getting the flu means staying home sick for about a week for more people, it can also result in more serious illness.

People at higher risk of flu complications include infants and young children, adults 65 and older, and people with chronic health conditions. 

Thankfully, you can protect yourself and those around you from influenza viruses by getting your annual flu vaccine.

Approved by the U.S. Food and Drug Administration (FDA), the flu vaccine—or flu shot—reduces the risk of getting the flu, as well as flu-related hospitalizations and deaths.

While it’s possible to get sick with the flu even if you get the shot, it’s still worthwhile to get a vaccine since it can make your flu symptoms milder if you do get the flu.  

Every flu season is different, so even if you get an influenza vaccine each year, you may still have questions about how the shot works.

In this article, I’ll break down the fundamentals of a flu shot and address common questions I get from my patients, including when the best time to get the flu shot is, how effective it is, and how long it provides protection.

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Flu Shot Basics 

The flu vaccine works by introducing inactive versions of several flu strains to the immune system, so it can develop antibodies to fight against the infection later on.

U.S. flu shots are quadrivalent vaccines, which means they protect against four types of flu viruses: two influenza A viruses (including H1N1) and two influenza B viruses.

The strains included in the shot change each year based on research about which strains will be most likely to circulate that year.

In some years, flu shots are more precise, while in other years, they are less effective. 

The majority of flu vaccines are shots that are administered with a needle, but there is also a needle-free nasal spray vaccine called FluMist Quadrivalent. 

The Centers for Disease Control and Prevention (CDC) recommends that everyone 6 months and older get the flu shot each year, with some rare exceptions.

The flu shot is the best way to protect yourself from flu viruses, and from serious complications of the flu.

After you get vaccinated, your immune system creates antibodies that fight off strains of the virus.

The closer the strains of the virus in the vaccine match the circulating strains, the better equipped you are to fight off the illness. 

So why do you need to get a flu vaccine every year?

In addition to different virus strains circulating each year, the flu shot is effective for about six months.

Over time, protection from the vaccine decreases due to declining antibodies and the changes that occur in the virus as it mutates away from the strains in the vaccine each year. 

Flu Season 

Although strains of the influenza virus are detected year-round in the United States, flu season occurs during the fall and winter, and usually wanes by spring.

While timing varies season to season, flu season typically peaks between December and February.

For the past few decades, February has been the month with the highest cases of influenza virus infections, followed by December, January, and March.

Where to Get a Shot

Flu shots are widely available at a variety of places, including doctor’s offices, flu clinics, urgent care centers, pharmacies, college health centers, and select workplaces and schools. 

What’s in the Shot

Since the influenza virus changes each year, the composition of the flu shot slightly varies every year, too.

This flu season, all flu shots in the U.S. will be quadrivalent vaccines to target four different flu viruses: an influenza A(H1N1) virus, an influenza A(H3N2) virus, and two influenza B viruses. 

Flu shots used to be trivalent vaccines designed to target three flu strains: two influenza A viruses and one influenza B virus.

The first quadrivalent influenza vaccine, which added another B virus, was developed in 2012 to help give wider protection.

There are three types of flu vaccine production technologies approved by the FDA:

  • Egg-based flu vaccines: In egg-based production, flu viruses are grown in chicken eggs. The fluid containing viruses is then extracted from the egg. This is the most common production method and is used for both inactive (killed) vaccines—like the flu shot—and live attenuated (weakened) viruses, which are found in the nasal spray vaccine.
  • Cell-based vaccines: Cell-based vaccine production uses cells instead of eggs to make inactivated flu vaccines, including the flu shot.  
  •  Recombinant flu vaccines: Recombinant flu vaccines don’t require eggs or an egg-grown vaccine virus in the production process; they’re made synthetically with a single protein from the virus.

It’s important to note that the form of the virus in the flu vaccine cannot cause the flu.

It is either dead or inactive, so cannot cause a flu infection.

You may experience mild flu-like symptoms after a flu shot due to your body’s immune response, but these are not due to the shot giving you the flu. 

You also cannot spread flu to others after receiving your vaccine, as there is no active, live flu virus in any form of the vaccine.

When Will a Flu Shot Start to Take Effect? 

After getting vaccinated, it takes two weeks for your immune system to develop antibodies that protect against the virus.

While the flu season doesn’t peak until sometime between December and February, the CDC recommends getting the shot in September and October—preferably by the end of October.

That way you’ll have enough time for the vaccine to take effect before the flu begins to spread in your area.

If you’re not vaccinated by the start of November, you should still get the shot, since different strains can circulate throughout the season. 

It’s also important to avoid getting the vaccine too early, as its effectiveness starts to decline after about six months.

That means if you get your flu shot before September, you risk not being protected through winter and early spring.

In some cases, though, pregnant women in their third trimester should get the vaccine when it first becomes available.

This reduces the risk of their infants getting the flu during their first months when they’re not old enough to get the vaccine. 

How Effective is the Flu Shot? 

The effectiveness of the flu shot varies from year to year, depending on the age, health, and other characteristics of the recipient, in addition to how well the viruses in the vaccine match the viruses in circulation.

If they’re well matched, the vaccine reduces risk of flu illness between 40-60% in the general population, and significantly reduces the risk of serious complications beyond that.  

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When to See a Doctor 

While those who get the flu typically recover after about a week, you should contact your doctor or healthcare provider if you’re experiencing flu symptoms and are at risk of complications.

“High risk” adult populations include:

  • Adults older than age 65
  • People who live in nursing homes
  • Pregnant women
  • People with weakened immune systems
  • People who have chronic illnesses (like asthma, heart disease, diabetes, kidney disease, liver disease, etc)
  • People with a BMI greater than 40
  • Infants and young children

 You should also get medical help if you’re experiencing the following severe symptoms:

Children 5 years and younger are at higher risk for developing serious health complications from the flu.

Some emergency symptoms in children can include:

  • Blue lips or face
  • Chest pain
  • Dehydration 
  • Severe muscle pain
  • Trouble drinking or keeping fluids down
  • Fever or cough that improves but returns or worsens
  • Severe lethargy or fatigue 
  • Seizures
  • Difficulty breathing
  • Ribs pulling in with every breath
  • Worsening of chronic medical conditions 

How K Health Can Help 

If you think you’re coming down with the flu, K Health can help. Did you know you can get affordable primary care with the K Health app?

Download K to check your symptoms, explore conditions and treatments, and if needed text with a doctor in minutes. K Health’s AI-powered app is HIPAA compliant and based on 20 years of clinical data.

Frequently Asked Questions

Should you get a flu shot twice a year?
No. Studies show that getting more than one dose during one flu season does not provide added immunity. The CDC recommends getting one shot every flu season, except for children ages 6 months to 8 years who are getting the flu vaccine for the first time. Children who need two flu shots should get the first dose when the vaccine first becomes available, because they’ll need to wait at least four weeks before getting the second shot- they will need two
What are common flu shot side effects?
Minor side effects you can get from the flu shot include soreness, redness, irritation, or swelling where you got the shot. Other side effects that can occur are mild headaches, nausea, fatigue, and fever.
Can flu shots make you get sicker?
No, flu shots can’t make you sicker or cause you to get sick with the flu. Flu shots administered with a needle are made with inactivated (killed) viruses, or with a single protein. And the nasal vaccine is made with weakened viruses, so it can’t cause illness, either. There are several reasons why you may be experiencing flu symptoms after getting the shot: You may be experiencing the normal side effects of a vaccine, you could be sick with another respiratory virus, you could’ve been exposed to the flu right after getting the shot, before your body had enough time to develop antibodies, or you could have been exposed to a flu virus that differs from the ones in the vaccine. You can also get sick with the flu, even if you’re vaccinated. But the good news is, if you get the shot, you’ll be better equipped to fight off the virus. A recent study showed that compared to unvaccinated adults, those who got the flu vaccine had a 26% lower risk of getting admitted to the ICU and a 31% lower risk of death from the flu.

K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

K Health has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical associations. We avoid using tertiary references.

Terez Malka, MD

Dr. Terez Malka is a board-certified pediatrician and emergency medicine physician.

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