Science Corner

Can the Flu Vaccine Give You the Flu?

Influenza virus diagram showing surface proteins that are the targets in the flu vaccine
Hi Eun Jung and Heung Kyu Lee, CC BY 4.0 via Wikimedia Commons

It’s flu vaccine time of the year again.  Whether you choose to get the vaccine or not, is that choice based on accurate scientific information?  While there may not be one “right” choice, making a choice based on myths is a mistake.

How do vaccines work?

When someone receives a vaccine, the body generates an immune response, producing antibodies that are specific to the infectious agent.  If that person is subsequently exposed to that infectious agent, antibodies will quickly be produced so the antigen can be cleared from the body quickly without causing an infection.  

There are two main types of vaccines: live attenuated and inactivated.  Live attenuated vaccines contain a weakened version of the infectious agent, while inactivated vaccines contain an infectious agent that has either been split or inactivated by some other means so it is not capable of replicating.  Live vaccines generally aren’t recommended for people with compromised immune systems, as it can potentially produce a mild infection.

The influenza virus and the flu vaccine

The diagram of an influenza virus above shows the green squiggles of genetic material (RNA) contained within a capsule made of proteins and lipids.  Infection happens when the virus enters host cells and hijacks their cellular machinery to mass reproduce itself.  Variations in two of the surface proteins, hemagglutinin and neuroaminidase, are used to name influenza subtypes, hence the H1N1 type of nomenclature.

Every year, typically by February, the World Health Organization predicts the 3-4 strains of the influenza virus that are likely to be predominant in the upcoming flu season.  These are used to produce the vaccine, which is cultured in chicken eggs, so that there will be a supply available for the next flu season.

Can the flu vaccine give me the flu?

The injectable flu vaccine is inactivated, so there’s no replicating virus present to possibly cause an infection.  Depending on the formulation, this contains either “split virus” or isolated protein subunits.  Because the vaccine does not contain replicating virus, it is not biologically capable of causing an influenza infection.

The nasal spray version of the vaccine, however, contains live attenuated virus.  The weakened virus cam cause a mild influenza infection in individuals with weakened immune systems, so it isn’t recommended for that population.

Why is the flu vaccine only good for a year?

The strains of influenza can vary considerably from year to year through a process known as antigenic drift.  Changes in the genetic structure will produce different variations in virus elements including H and N proteins.  With current vaccine technology, it’s not possible to produce a vaccine that will be effective against all strains.

What are the possible side effects?

You may feel mildly unwell as your body mounts the expected immune response.  People who receive the live vaccine may have mild flu-like symptoms.  There may be pain or redness at the injection site.  This is more likely with vaccine formulations that have an “adjuvant” to generate a stronger immune response.

There is a low possibility of serious side effects.  There may be an anaphylactic reaction, and the health care provider administering the vaccine will always have epinephrine on hand in case of this.  Other rare side effects include the neurological condition Guillain-Barre syndrome (in an estimated 1 in a million vaccinations) and oculo-respiratory syndrome.  It’s important to consider these rare but serious effects in the context of the potential risks of an influenza infection and make a balanced decision about what is right for a given individual.

I got sick afterwards – is it the flu?

There are a few possibilities.  You may have already had an influenza infection but hadn’t become symptomatic yet when you got the vaccine.  You may have been exposed to an influenza strain that was not included in the vaccine.  Or you may have become infected with a bacteria (e.g. Streptococcus) or a virus other than influenza that targets the respiratory tract.  The “stomach flu” is a misnomer and is not in any way related to the influenza virus.

It’s also possible that what feels like being sick is actually your immune system doing what it’s supposed to do, mounting an immune response to the viral components in the vaccine. Developing a mild fever, for example, is a sign that your immune system is working hard at developing immunity.

The effectiveness of the vaccine also varies from year to year.  It typically protects 60-80% of those immunized; the elderly and other high risk groups tend to be on the lower end of that range.  However, getting the vaccine doesn’t increase the risk of getting the flu. If enough people surrounding those high risk groups are vaccinated, there can be an added element of protection through some degree of herd immunity, which slows the transmission of an infectious agent through a population.


So, what’s the take-home message here?  Because the influenza virus changes so much and so often, the flu vaccine is nowhere near as effective as many other vaccines.  However, it can be an important lifesaving measure for those who are at high risk of developing complications of influenza.

Since the vaccine is not perfect, it’s also important for those who have close contact with these at-risk individuals to reduce the chance that they will expose that person to the virus, especially since the virus is contagious before a person even develops flu symptoms.  The influenza vaccine that most people receive contains inactivated virus, which makes it biologically impossible for it to cause an influenza infection.

As a nurse who will come into contact with high-risk individuals, I’ve already gotten my flu vaccine this year.  Will you?  Why or why not?


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14 thoughts on “Can the Flu Vaccine Give You the Flu?”

  1. When I had Guillian Beret the first thing that asked if I got a flu shot.

    I was
    Shocked they thought you could get this disease from a flu shot

    Now I am never allowed to get a flu shot

    I was
    Paralyzed for three

      1. One in a 100,000 get it

        Supposed to be temporary but some never recover and my toes and fingers still feel weird.

        It attacks your peripheral nervous system.

        I had to learn everything over. Walking, eating etc

        Humbling but I had the skill set to overcome it

        Some of the things I thought were horrible, or awful in life were bad judgments.

        Being paralyzed was a great challenge. My mindfulness practice would be severely tested.

        We either accept our situation, then we can work on getting better.

  2. i did
    and always do
    used to get a middle ear infection
    every november
    when i was in my twenties
    my eardrums have been perforated sooo many times
    and now with heart failure
    any viral
    disease could be the end of me
    now words
    and remember
    i consider you a friend!
    oh happy day!

  3. “They” demand that people in my particular group get the flu shot. Diabetics (apparently, from how I understand it) have compromised immune systems and catching the flu means we get sicker from it than regular folk. Still. I have noticed that whether or not I participate, I usually get really sick in the winter, at least a couple of times. Your information was timely and great! Thanks! 🙂 And “yes” I will. Best idea.

    1. Now if only they could come up with a combination vaccine that could target all the nasty winter beasties, then we’d really be getting somewhere 🙂

  4. It’s certainly an odd one that so many people seem to get the flu within 3 days of having the jab, having gone all year without a flu. This happened to my mum two years ago and I’d never seen her so poorly with the flu before. I’m all for the jab, however, and had it last year without any problems. This year, within 3 days I started to feel unwell and it got worse from there, so poorly and now this is the end of week 4 with the flu. While the jab can’t ‘give’ you the flu, there is a small part of me that wonders whether there’s something else at play that they haven’t discovered yet (similar to opening a back door to letting a virus in, weakening the immune system in some way they haven’t pinpointed or noticed yet). Great post! x

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