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Influenza Vaccine: What Influenza is, How an Influenza Vaccine Works, and More

Reviewed by 
Brian Kurtz, DO

Article at a glance

  • Influenza symptoms are diverse and can range from mild to severe. To learn more about which symptoms are linked to the flu, read this section.

  • Influenza is a complicated virus that mutates and changes each season. Due to this, there are a plethora of strains and subtypes within our current ecosystem that are researched and targeted each year. To learn more about influenza strains and subtypes and how they affect the annual vaccine, read this section.

  • Due to the variety of strains and subtypes influenza is tied to, there are several influenza vaccine options to choose from. To learn more about each vaccine type and which option may be best for you, read this section.

  • While the CDC recommends everyone over six months to get vaccinated, there are certain patients who should avoid the influenza vaccine. To learn more about who should get vaccinated and who shouldn’t, read this section.

Influenza often referred to as the flu,” is a virus that primarily affects the nose, throat, and occasionally the lungs. Depending on which strain a patient contracts, the symptoms may range from mild to severe and can potentially lead to death. The Center for Disease Control (CDC) notes that on average, 8% of the United States population contracts the flu each year. Depending on the season, the percentage of cases can range anywhere from 3%-11%. The majority of influenza cases are seen within children and the minority is seen within the 65+ population. When broken down by age group, a CDC study found that 9.3% of cases were found in children under 18 years of age, 8.8% of cases were found in adults aged 18–64, and 3.9% were found in adults over the age of 65. After contracting the flu, it’s possible to expose a loved one to the virus because it’s contagious. Most patients are considered contagious during the first three to four days after symptoms begin. However, healthy adults might be contagious one day before symptoms show and for five to seven days after their illness begins; children and those with weak immune systems are considered contagious for an even longer period of time.

Common Influenza Symptoms

Flu symptoms are known to range from mild to severe and usually symptoms occur suddenly rather than gradually. Common influenza symptoms include:

  • Having a mild to high fever.

  • Experiencing chills or feeling feverish.

  • Dealing with a dry or wet cough.

  • Having a sore or scratchy throat.

  • Noses become runny or stuffy.

  • Experiencing consistent body or muscle aches.

  • Ongoing headaches are occurring.

  • General fatigue throughout the day.

Note: Children are also likely to experience vomiting and/​or diarrhea.

Influenza Origin

While the precise origins are still debated today, the original influenza virus was found to come from an H1N1 strain of avian descent. Decimating many country’s populations and spiking mortality rates, the flu quickly spread to become a worldwide pandemic.

Influenza Strains

There are four types of influenza strains known as strains A, B, C, and D. Strain A and Strain B are the two strains that generate the influenza epidemics every winter season. Meanwhile, strain C is known to be a milder strain that causes little to no illness, and strain D only affects cattle. These strains are then further broken down into various subtypes for strain A and two different lineages for strain B. Strain A currently has 131 classified subtypes while Strain B is split between two lineages (Victoria and Yamagata). The various strains and subtypes explain the reason behind the flu strain drift and the vaccination update each year. The CDC clarifies that antigenic drift is the main reason why people can get the flu more than one time, and it’s also a primary reason why the flu vaccine composition must be reviewed and updated each year (as needed) to keep up with evolving influenza viruses.” 

Note:Antigenic drift” is defined as the small shifts or mutations an influenza virus undergoes with time. When this occurs on a larger scale and the influenza virus changes drastically, this is called an antigenic shift” and is the main source for worldwide influenza pandemics. To learn more about influenza strains, visit this source. To learn more about the impacts of antigenic drifts and shifts, visit this source.

Flu Vaccinations

The purpose of vaccination is to boost a patient’s immunity to a specific virus or bacteria. This is done by first exposing a patient’s immune system to a weakened or killed version of a virus/​bacteria. From there, the immune system is triggered to produce antibodies that will fight off the virus/​bacteria and remember how to do so if the patient contracts it in their daily life. Specifically, with a flu vaccine, these antibodies develop approximately two weeks after a patient has been vaccinated. 

Vaccine Types

Due to the surplus of flu strains and subtypes present in our ecosystem, there is research done each year to identify which strains are predicted to be prevalent during the flu season. The vaccines themselves are broken into two categories: quadrivalent vaccines and trivalent vaccines. Quadrivalent flu vaccines are composed of a weakened influenza A (H1N1) virus, an influenza A (H3N2) virus, and two influenza B viruses to help protect a patient against four predicted flu strains. Trivalent vaccines are primarily targeted at 65 and older populations and include a weakened influenza A (H1N1) virus, an influenza A (H3N2) virus, and one influenza B virus.

Vaccine Ingredients & Options

Vaccines contain different combinations of ingredients that each have different purposes. The five components used in vaccines include preservatives, adjuvants, stabilizers, residual cell culture materials, residual inactivating ingredients, and residual antibiotics. With flu vaccines, in particular, there are a variety of vaccine options available. They are all still broken into the two categories of quadrivalent and trivalent but each contains different ingredients or is recommended for different age groups. The seven vaccine options include the standard-dose quadrivalent flu shot, the quadrivalent cell-based flu shot, the recombinant quadrivalent flu shot, the quadrivalent adjuvant-based flu shot, the high-dose quadrivalent flu shot, the live attenuated influenza vaccine (LAIV), and the adjuvant-based trivalent flu shot.

Standard-Dose Quadrivalent Shot

Afluria Quadrivalent, Fluarix Quadrivalent, FluLaval Quadrivalent, and Fluzone Quadrivalent are all considered standard-dose influenza shots. These four vaccines are approved for those over six months of age and Afluria Quadrivalent is licensed to be administered either by needle (for those over six months) or by the jet injector (for those between 18 and 64). Influenza shots in this section include the residual cell culture materials” mentioned above because the virus is grown in eggs. Growing a virus or bacteria in eggs increases the manufacturing capacity for a vaccine. 

Quadrivalent Cell-Based Shot

Flucelvax Quadrivalent is the only cell-based vaccine, meaning that its virus is grown in cell culture and is completely egg-free. This shot is licensed for those over four years of age and is primarily administered to those with egg allergies.

Recombinant Quadrivalent Shot

Flublok Quadrivalent is the only recombinant vaccine meaning that it was created from recombinant DNA technology, allowing it to be egg-free. This vaccine is licensed for those over 18 years of age and is primarily given to those with egg allergies.

Adjuvant-Based Quadrivalent Shot

An adjuvant is used in vaccines to stimulate a stronger immune response. The common adjuvant used is aluminum salts which are also found in drinking water, infant formula, antacids, aspirin, etc. An adjuvant-based influenza vaccine, such as FLUAD Quadrivalent, is licensed and approved for those 65+ years of age.

High-Dose Quadrivalent Shot

Fluzone High-Dose is the only high-dose quadrivalent shot and is licensed and approved for those 65+ years of age. The difference between Fluzone (standard-dose vaccine) and Fluzone High-Dose is that Fluzone High-Dose contains four times the antigen included in the standard Fluzone vaccine. The reason for increasing the antigen amount is to stimulate a stronger immune response in older generations, thus better protect them from the flu.

Live Attenuated Influenza Vaccine (LAIV)

LAIV is administered to patients through a nasal spray that contains a weakened version of the influenza virus. FluMist Quadrivalent is the only live attenuated influenza vaccine as of late and it is licensed and approved for those between the ages of two and 49. It’s emphasized that LAIV should not be given to those who are pregnant or immunocompromised.

Adjuvant-Based Trivalent Shot

Similar to the adjuvant-based FLUAD Quadrivalent, FLUAD Trivalent contains an adjuvant that stimulates a strong immune response. The main difference between these two FLUAD vaccines is that one fights against four flu strains (quadrivalent) while one fights against three (trivalent). Adjuvant-based trivalent vaccines are licensed and approved for those 65+ years of age.

Who Should Be Vaccinated vs. Who Shouldn’t Be

The CDC and most health officials highly encourage receiving a flu vaccination each season for varying reasons that benefit public health and safety. However, there are certain individuals that shouldn’t be vaccinated or may want to speak with their doctor prior to getting vaccinated.

Who Should Be Vaccinated?

The CDC recommends that everyone above six months of age should receive an influenza vaccine every flu season. There are many reasons to get vaccinated for the flu such as reducing the likelihood of illness, hospitalization, death, and the severity of certain chronic conditions. To learn more about why influenza vaccines are highly recommended, visit this source.

Who Shouldn’t Be Vaccinated?

Those younger than six months old are considered too young to receive a flu vaccine. Furthermore, those with severe allergies to any ingredients included in influenza vaccines should not be vaccinated. Some of those ingredients may include gelatin, antibiotics, etc. so be sure to discuss any allergies with your doctor prior to receiving any vaccine. Those with egg allergies should discuss vaccine options with their doctor prior to receiving the flu vaccine as there are egg-free alternatives. Also, those who have ever experienced Guillain-Barré Syndrome (GBS) should talk to their doctor prior to being vaccinated.

Vaccine Side Effects

Typical side effects that stem from the flu shot include:

  • Soreness

  • Redness and/​or swelling near the area where the shot was administered

  • Headache

  • Fever

  • Nausea

  • Muscle Aches

Note: Although rare, many injections, along with the flu shot, may occasionally cause fainting. If you pass out or experience abnormal side effects, be sure to consult with a doctor.

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