Influenza Vaccination FAQs

Influenza Vaccination FAQs

What is the flu? And is it really a big deal?

The flu is a contagious respiratory illness caused by the influenza virus. There are 4 main types: A, B, C, and D. These types can each be further broken down into subtypes or strains.1 The CDC estimates that since 2010, the collective flu has resulted in between 9.2 and 35.6 million illnesses, between 140,000 and 710,000 hospitalizations, and between 12,000 and 56,000 deaths annually.2

What does the flu vaccine protect me against exactly?

The flu vaccine can offer protection against both A and B types. Type C is believed to cause a milder form of illness and Type D is not thought to affect humans.1 The challenge is because the influenza virus is always evolving, vaccinations are constantly having to evolve as well. Each year, vaccinations are tailored to target the perceived biggest threats for the upcoming flu season. For the 2018-2019 season, strains covered by vaccination in the U.S. (and northern hemisphere) include:

  • A/Michigan/45/2015 (H1N1)pdm09-like virus
  • A/Singapore/INFIMH-16-0019/2016 (H3N2)-like virus
  • B/Colorado/06/2017-like virus (Victoria lineage)
  • B/Phuket/3073/2013-like virus (Yamagata lineage).3,4

The nomenclature is complicated, and is based on the host, region of origin, number of lineage, year of isolation, and protein antigen type. 5(Don’t worry, there will not be a quiz at the end).

How does the vaccine work? Is it effective?

In simplistic terms, the flu vaccine works by introducing an antigen (a molecule that elicits an immune response) to your body, which causes your white blood cells to respond by producing antibodies (proteins that defend your body against a specific antigen). In about 2 weeks, these antibodies should be able to mount a big enough defense to keep you from developing the flu if you are exposed to the virus. Essentially, your immune system becomes more prepared and less likely to be overwhelmed by the virus.6

Here is the unfortunate part – the flu vaccine is not 100% effective. In fact, research estimates that it is probably only reduces the risk of illness by about 40-60% in the general population, and this is when the prevalent subtypes/strains match the vaccine for that year.7

Can the influenza vaccine give me the flu?

The short answer is no. You or someone you know may have taken the flu shot and gotten flu-like symptoms immediately following or sometime later after receiving the flu shot. Now, depending on when you receive the vaccine, it is possible that you could contract the virus right before vaccination but not develop symptoms until after your vaccination, which might make it appear like the vaccine gave you the flu. It is also possible that the vaccination may elicit a response from your body’s immune system – that is its job after all – and cause mild flu-like symptoms, which can be mistaken for the flu. Additionally, there are many other similar viruses that mimic the flu. Finally, as mentioned above, you may contract another subtype not covered by that year’s vaccination.

It should be noted that a possible exception is immunocompromised patients. It is recommended that these individuals should not receive a live version of the vaccine. Although the CDC clearly states that live viruses cannot cause influenza, ACIP cautions that there is an “uncertain but biologically plausible risk for disease” in immunocompromised populations.3 Similarly, the Infectious Diseases Society of America (IDSA) states that there live AIV is contraindicated in immunocompromised patients because the risks are “unknown.”8

Are they safe?

Generally, flu vaccines are very well tolerated. However, as with any medication, there is always the possibility of an allergic reaction or adverse reaction. Reported symptoms following administration have ranged from mild pain at the injection site and low-grade fever, to more serious conditions such as febrile seizures. There have even been some very rare reports (1-2 cases per 100,000) of Guillain-Barré Syndrome, a disease mediated by the immune system that involves neuropathy and paralysis, following administration.9 It is important to note that although there may be an association between the vaccine and illness, this is not the same as causation, meaning that there is no proof that the vaccination causes Guillian-Barré. In fact, although it is theorized that infection may precipitate this disorder, the exact mechanism of the process is not certain.10

There are also specific populations and conditions which may incur greater risk, so please see the section below on contraindications/precautions.

Who should get vaccinated?

According to the CDC and Advisory Committee on Immunization Practices (ACIP), routine annual vaccination is recommended for all people > 6 months of age (excluding those with contraindications). Immunization is especially important in the following populations:

  • Children 6 – 59 months
  • Adults > 50 years
  • People with diabetes and those with chronic disease of the lungs (asthma, COPD), heart, kidneys, etc.
  • Pregnant/soon-to-be pregnant women
  • Residents in nursing homes/long-term care/assisted living facilities
  • Caretakers/healthcare provider
  • People who are immunocompromised
  • People with a BMI > 40
  • American Indians/Alaska Natives
  • Children/adolescents on aspirin/salicylate medication (due to risk of Reye syndrome following influenza)
  • Household contacts of any of the above at risk3

In addition to reducing your own risk of developing the flu, it may help others as well. This is according to the concept of “herd immunity,” in which vaccination of a significant percentage of the population results in a protective effect on the population as a whole. Those who are vaccinated are less likely to develop the disease, which means they are less likely to serve as a host and pass it to others in their community.

Are there different types of vaccines?

Yes. Traditionally flu vaccines come in a muscular injection form, but can be administered in a nasal mist if appropriate. Some are referred to as trivalent, providing immunity against 3 types of influenza, and some are quadrivalent, which protects against an additional 4th type. You will also hear the terms inactivated vaccines vs live attenuated vaccines. Inactivated refers to a “dead” form of the virus (the debate about whether or not viruses are “alive” in the first place is ongoing, but that is beyond the scope of this article). Live attenuated refers to a form of the virus which still has the ability to be active, but has basically been weakened (this is the one that can be given nasally); these are contraindicated in pregnant and immunocompromised individuals, and children under 2 years of age.3,11-12 In addition to inactivated and live attenuated forms, there is also newer version termed recombinant – basically, scientists use gene-splicing technology to make a hybrid virus from which a special protein can ultimately be extracted.13

It is very important that you speak to your provider and/or pharmacist regarding which is best for you.

What are some of the major contraindications/precautions I should consider?

  • History of allergic reaction to a vaccine or component (i.e. eggs)
  • Aspirin or salicylate medication in children
  • Asthma or recent episode of wheezing in small children
  • Immunocompromised states (HIV, chemotherapy)
  • Pregnancy
  • Recent antiviral use
  • Close contact with immunocompromised people
  • Any moderate to severe illness
  • History of Guillain-Barré syndrome within past 6 weeks
  • Chronic illness (diabetes, renal disease, cardiovascular disease, etc.) 3,11

Again, it is crucial that you bring any of these issues to the attention of the healthcare provider to help you decide on appropriate vaccination.

When should I get my flu shot?

Become influenza tends to peak in the winter months of January and February. Unfortunately, there is no strong consensus as to the optimal time to vaccinate. While some studies indicate that vaccinating too early may result in waning immunity to influenza, there is mixed research. On the other end of the spectrum, there is concern that waiting too long may result in missed attempts to vaccinate, and may not confer immunity in time. Therefore, it is currently recommended that vaccination be offered by the end of the October, but technically can be given any time during the flu season. Children requiring a 2-dose series should ideally receive the first dose earlier, so that after the minimum 4 week waiting period, their second dose can be administered before the end of October as well.3


1. CDC (2017, September 27). Types of influenza viruses. Retrieved from

2. CDC (2018, May 22). Disease burden of influenza. Retrieved from

3. Grohskopf, L.A., Sokolow, L.Z., Broder, K.R., Walter, E.B., Fry, A.M., & Jernigan, D.B. (2018). Prevention and control of seasonal influenza with vaccines: Recommendations of the advisory committee on immunization practices – United States, 2018-2019 influenza season. MMWR Recommendations and Reports, 67(3), 1-20. DOI:

4. World Health Organization (2018, February). Recommended composition of influenza virus vaccine for use in the 2018-2019 northern hemisphere influenza season. Retrieved from

5. Lamarino, A. (2009, September 20). Influenza A (H1N1) blog. Virtual Health Library. Retrieved from

6. CDC (2018, September 6). Key facts about seasonal flu vaccine. Retrieved from

7. CDC (2018, September 6). Vaccine effectiveness – how well does the flu vaccine work? Retrieved from

8. Rubin, L.G., Levin, M.J., Ljungman, P., Davie, E.G., Avery, R., Tomblyn, M., Bousvaros, A., Dhanireddy, S., Sung, L., Keyserling, H., & Kang, I. (2014). 2013 IDSA clinical practice guideline for vaccination of the immunocompromised host. Clinical Infectious Disease, 58(3), e44-e100. Retrieved from

9. CDC (2017, October 3). Seasonal influence vaccine safety: A summary for clinicians. Retrieved from

10. Vriesendorp, F.J. (2018, September 25). Gullain-Barré syndrome: Pathogenesis. In J.M Shefner, I.N. Targoff, and A.F. Eichler (Eds), UpToDate. Retrieved from

11. Hibberd, P.L. (2018, September 4). In Seasonal influenza vaccination in adults. In M.S. Hirsch & A.R. Thorner (Eds.), UpToDate. Retrieved from

12. U.S. Department of Health and Human Services (2017, December). “Vaccine types.” Retrieved from

13. CDC (2018, September 24). How influenza (flu) vaccines are made. Retrieved from

I’m a Family Nurse Practitioner by licensure, with clinical expertise and a Post-Master’s in emergency medicine. I’m a member of Team Rubicon, an NGO that provides both domestic disaster relief & international medical/humanitarian aid. I also serve as a medical provider for several sporting events, including Texas IRONMAN, Houston Rodeo, and Wings Over Houston. Additionally, I’ve volunteered as a mentor with the Big Brother Big Sister program, as a catechist and youth leader in the Catholic Church, and am an honorary lifetime member of the American Legion

Recommended Posts