Table of ContentsView AllTable of ContentsWhat Is the Risk?Flu-Like Side EffectsCan I Still Get Flu?Infecting Others

Table of ContentsView All

View All

Table of Contents

What Is the Risk?

Flu-Like Side Effects

Can I Still Get Flu?

Infecting Others

You cannot get the flu from a flu shot because the vaccines are made with killed or weak viruses that cannot cause an infection. With that said, you can experience mild flu-like symptoms a day or two after the injection due to the activation of yourimmune system.

Getting the flu shotdoesn’t mean you won’t get the flu. This is because the annual vaccine only protects against theinfluenza strainspredicted for that season. Even so, flu vaccination may lessen the severity and duration of the flu compared to going unvaccinated.

Reasons for getting sick after a flu shot

Why You Can’t Get the Flu From a Flu Shot

There are three categories of flu vaccines used for this: inactivated vaccines, recombinant vaccines, and live, attenuated vaccines.

Inactivated Flu Vaccines

Inactivated vaccines, also known as whole-killed vaccines, are those made from viruses that have been killed. Although the dead viruses cannot replicate or cause infection, the immune system still regards them as a threat and starts producing defensive antibodies in response.

For most inactivated flu vaccines, the virus is firstincubated(grown) in chicken eggs and then harvested, inactivated (either with formaldehyde or beta-propiolactone), and purified for use in vaccines. The only exception is the Flucelax vaccine which is grown in mammal cells.

Recombinant Flu Vaccine

The recombinant flu vaccine cannot cause flu because it is synthetic and does not contain the influenza virus in any form.

Recombinant vaccinesinvolve a technology in which a small piece of genetic material (DNA) of a virus is inserted into a “host” virus that does not cause disease in humans. The “recombinant” virus then starts churning out harmless proteins that the body regards as a threat when injected into to the body, spurring the production of antibodies.

For the recombinant flu vaccine, the “host” virus is thebaculoviruswhich affects only insects. Chicken eggs are not involved in the manufacturing of the vaccine.

Live-Attenuated Flu Vaccine

Live-attenuated vaccinescontain whole bacteria or viruses that have been weakened (attenuated) so that they trigger an immune response but do not cause disease in healthy people. The version used for flu is called a live-attenuated influenza vaccine, or LAIV.

LAIV is also grown in chicken eggs, but rather than killing the virus after harvesting, the virus is repeatedly passed through a series of chicken eggs (referred to as “passaging”). With each passage, the virus starts to mutate and weaken until it effectively loses itsvirulence(ability to infect).

Why FluMist Cannot Cause the FluEven though live-attenuated vaccines contain weakened viruses, the viruses are still living and, with some vaccines, can potentially cause infection in people with severely weakened immune systems. This is not the case with FluMist.According to the Centers for Disease Control and Prevention (CDC), FluMist will not cause flu, in part because the viruses are cold-adapted, meaning that they only multiply at cooler temperatures within the nose. Once exposed to normal body temperature, the viruses cannot replicate and, without replication, an infection cannot occur.

Why FluMist Cannot Cause the Flu

Even though live-attenuated vaccines contain weakened viruses, the viruses are still living and, with some vaccines, can potentially cause infection in people with severely weakened immune systems. This is not the case with FluMist.According to the Centers for Disease Control and Prevention (CDC), FluMist will not cause flu, in part because the viruses are cold-adapted, meaning that they only multiply at cooler temperatures within the nose. Once exposed to normal body temperature, the viruses cannot replicate and, without replication, an infection cannot occur.

Even though live-attenuated vaccines contain weakened viruses, the viruses are still living and, with some vaccines, can potentially cause infection in people with severely weakened immune systems. This is not the case with FluMist.

According to the Centers for Disease Control and Prevention (CDC), FluMist will not cause flu, in part because the viruses are cold-adapted, meaning that they only multiply at cooler temperatures within the nose. Once exposed to normal body temperature, the viruses cannot replicate and, without replication, an infection cannot occur.

Flu-Like Symptoms After a Flu Shot

Although the flu vaccine cannot give you the flu, it can have side effects that mimicinfluenza. This stems from the sudden activation of your immune system.

When exposed to the vaccine, the immune system will respond as if the actual virus were there. As part of the body’s natural defense, it will release chemicals that spurinflammation.

With flu vaccination, the same can occur, leading to the sudden onset of flu-like symptoms such as:

But, because there is no virus to cause an actual infection, these symptoms are relatively mild and short-lived, with the immune system ramping down its defense within one to two days.

In addition to flu-like symptoms, theFluMist nasal spray can sometimes cause mild, cold-like symptoms, including runny nose, sore throat, and cough.

Getting the Flu After a Flu Shot

The annual flu vaccine is the best way to protect yourself from the flu, but it doesn’t mean you still can’t catch the flu. This may be because you caught the flu before the vaccine was able to afford you immune protection (generally up to two weeks).

At the same time, the flu vaccine is not perfect. In the end, there are many strains of influenza A and B, and the annual vaccine only protects against those most likely to cause illnesses during the next flu season.

In late February or early March of each year, experts from the CDC, World Health Organization (WHO), and others review data from around the world to decide which four vaccine viruses to use in the next quadrivalent vaccines and which three vaccine viruses to use in the next trivalent vaccines.

Vaccinated people who get the flu will sometimes claim that the shot “gave them the flu” when, in fact, the vaccine only affords partial protection. Even with vaccination, other standardpreventive measures(such as routine handwashing and avoiding others who are sick) should be adhered to.

Are Trivalent or Quadrivalent Vaccines Better?Trivalent and quadrivalent flu vaccines are equally effective. The United States is currently transitioning from quadrivalent to trivalent vaccines as one virus type, called influenza B/Yamagata viruses, is no longer actively circulating.

Are Trivalent or Quadrivalent Vaccines Better?

Trivalent and quadrivalent flu vaccines are equally effective. The United States is currently transitioning from quadrivalent to trivalent vaccines as one virus type, called influenza B/Yamagata viruses, is no longer actively circulating.

Remember, too, that the flu shot only protects you from influenza—not other infections like thecommon cold,respiratory syncytial virus(RSV), and other viruses that may cause flu-like symptoms. Adults typically catch two to four colds per year, and young children will get six to eight.

Giving Someone the Flu After Vaccination

FluMist is not recommended forimmunocompromisedpeople, but not because the weakened virus might suddenly reactivate and cause disease.

Another potential risk of FluMist involves people living with aseverelyimmunocompromised person (such as someone going through astem cell transplant).

With FluMist, a vaccinated person will commonly experienceviral sheddingduring which live viruses are released from nasal passages for one to two weeks. These viruses, while unlikely to cause infection in a healthy person, can potentially cause infection in an immunocompromised person.

For this reason, FluMist is contraindicated for use in households with a severely immunocompromised individual. Flu shots should be given instead.

Summary

You cannot get the flu from a flu shot or FluMist (the nasal spray flu vaccine). With that said, it is possible to experience flu-like symptoms a day or two after vaccination due to the activation of the immune system. You can also get the flu after a flu shot as the vaccine is only partially effective in preventing infection.

Even so, annual flu vaccination remains the single best way to avoid getting the flu and potentially serious complications of infection.

14 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Centers for Disease Control and Prevention.Flu vaccine safety information.Centers for Disease Control and Prevention.Key facts about seasonal flu vaccine.Centers for Disease Control and Prevention.How well flu vaccines work.Centers for Disease Control and Prevention.How influenza (flu) vaccines are made.John Hopkins Medicine.Influenza vaccine.Subbarao K.Live attenuated cold-adapted influenza vaccines.Cold Spring Harb Perspect Med. 2021 Sep;11(9):a038653. doi:10.1101/cshperspect.a038653Centers for Disease Control and Prevention.Live attenuated influenza vaccine [LAIV] (the nasal spray flu vaccine).Bennett JM, Reeves G, Billman GE, Sturmberg JP.Inflammation-nature’s way to efficiently respond to all types of challenges: Implications for understanding and managing “the epidemic” of chronic diseases.Front Med (Lausanne).2018 Nov 27;5:316. doi:10.3389/fmed.2018.00316U.S. Food and Drug Administration.It’s a good time to get your flu vaccine.Frutos AM, Price AM, Harker E, et al.Interim estimates of 2023–24 seasonal influenza vaccine effectiveness — United States.MMWR Morb Mortal Wkly Rep. 2024;73:168–174. doi:10.15585/mmwr.mm7308a3Centers for Disease Control and Prevention.Prevent seasonal flu.Centers for Disease Control and Prevention.US will transition to trivalent flu vaccines for 2024–2025.American Lung Association.Facts about the common cold.Li L, Shi N, Xu N, et al.Safety and viral shedding of live attenuated influenza vaccine (LAIV) in Chinese healthy juveniles and adults: a phase Ⅰ randomized, double-blind, placebo-controlled study.Vaccines (Basel).2022 Nov;10(11):1796. doi:10.3390/vaccines10111796

14 Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Centers for Disease Control and Prevention.Flu vaccine safety information.Centers for Disease Control and Prevention.Key facts about seasonal flu vaccine.Centers for Disease Control and Prevention.How well flu vaccines work.Centers for Disease Control and Prevention.How influenza (flu) vaccines are made.John Hopkins Medicine.Influenza vaccine.Subbarao K.Live attenuated cold-adapted influenza vaccines.Cold Spring Harb Perspect Med. 2021 Sep;11(9):a038653. doi:10.1101/cshperspect.a038653Centers for Disease Control and Prevention.Live attenuated influenza vaccine [LAIV] (the nasal spray flu vaccine).Bennett JM, Reeves G, Billman GE, Sturmberg JP.Inflammation-nature’s way to efficiently respond to all types of challenges: Implications for understanding and managing “the epidemic” of chronic diseases.Front Med (Lausanne).2018 Nov 27;5:316. doi:10.3389/fmed.2018.00316U.S. Food and Drug Administration.It’s a good time to get your flu vaccine.Frutos AM, Price AM, Harker E, et al.Interim estimates of 2023–24 seasonal influenza vaccine effectiveness — United States.MMWR Morb Mortal Wkly Rep. 2024;73:168–174. doi:10.15585/mmwr.mm7308a3Centers for Disease Control and Prevention.Prevent seasonal flu.Centers for Disease Control and Prevention.US will transition to trivalent flu vaccines for 2024–2025.American Lung Association.Facts about the common cold.Li L, Shi N, Xu N, et al.Safety and viral shedding of live attenuated influenza vaccine (LAIV) in Chinese healthy juveniles and adults: a phase Ⅰ randomized, double-blind, placebo-controlled study.Vaccines (Basel).2022 Nov;10(11):1796. doi:10.3390/vaccines10111796

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

Centers for Disease Control and Prevention.Flu vaccine safety information.Centers for Disease Control and Prevention.Key facts about seasonal flu vaccine.Centers for Disease Control and Prevention.How well flu vaccines work.Centers for Disease Control and Prevention.How influenza (flu) vaccines are made.John Hopkins Medicine.Influenza vaccine.Subbarao K.Live attenuated cold-adapted influenza vaccines.Cold Spring Harb Perspect Med. 2021 Sep;11(9):a038653. doi:10.1101/cshperspect.a038653Centers for Disease Control and Prevention.Live attenuated influenza vaccine [LAIV] (the nasal spray flu vaccine).Bennett JM, Reeves G, Billman GE, Sturmberg JP.Inflammation-nature’s way to efficiently respond to all types of challenges: Implications for understanding and managing “the epidemic” of chronic diseases.Front Med (Lausanne).2018 Nov 27;5:316. doi:10.3389/fmed.2018.00316U.S. Food and Drug Administration.It’s a good time to get your flu vaccine.Frutos AM, Price AM, Harker E, et al.Interim estimates of 2023–24 seasonal influenza vaccine effectiveness — United States.MMWR Morb Mortal Wkly Rep. 2024;73:168–174. doi:10.15585/mmwr.mm7308a3Centers for Disease Control and Prevention.Prevent seasonal flu.Centers for Disease Control and Prevention.US will transition to trivalent flu vaccines for 2024–2025.American Lung Association.Facts about the common cold.Li L, Shi N, Xu N, et al.Safety and viral shedding of live attenuated influenza vaccine (LAIV) in Chinese healthy juveniles and adults: a phase Ⅰ randomized, double-blind, placebo-controlled study.Vaccines (Basel).2022 Nov;10(11):1796. doi:10.3390/vaccines10111796

Centers for Disease Control and Prevention.Flu vaccine safety information.

Centers for Disease Control and Prevention.Key facts about seasonal flu vaccine.

Centers for Disease Control and Prevention.How well flu vaccines work.

Centers for Disease Control and Prevention.How influenza (flu) vaccines are made.

John Hopkins Medicine.Influenza vaccine.

Subbarao K.Live attenuated cold-adapted influenza vaccines.Cold Spring Harb Perspect Med. 2021 Sep;11(9):a038653. doi:10.1101/cshperspect.a038653

Centers for Disease Control and Prevention.Live attenuated influenza vaccine [LAIV] (the nasal spray flu vaccine).

Bennett JM, Reeves G, Billman GE, Sturmberg JP.Inflammation-nature’s way to efficiently respond to all types of challenges: Implications for understanding and managing “the epidemic” of chronic diseases.Front Med (Lausanne).2018 Nov 27;5:316. doi:10.3389/fmed.2018.00316

U.S. Food and Drug Administration.It’s a good time to get your flu vaccine.

Frutos AM, Price AM, Harker E, et al.Interim estimates of 2023–24 seasonal influenza vaccine effectiveness — United States.MMWR Morb Mortal Wkly Rep. 2024;73:168–174. doi:10.15585/mmwr.mm7308a3

Centers for Disease Control and Prevention.Prevent seasonal flu.

Centers for Disease Control and Prevention.US will transition to trivalent flu vaccines for 2024–2025.

American Lung Association.Facts about the common cold.

Li L, Shi N, Xu N, et al.Safety and viral shedding of live attenuated influenza vaccine (LAIV) in Chinese healthy juveniles and adults: a phase Ⅰ randomized, double-blind, placebo-controlled study.Vaccines (Basel).2022 Nov;10(11):1796. doi:10.3390/vaccines10111796

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