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What is the flu vaccine? And more commonly asked questions

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, 7 min read

What is the flu vaccine? And more commonly asked questions

The flu vaccine is a vaccine that protects you from catching the flu. It is usually given as an injection in the arm but in some cases it can also be given as a nasal spray. It protects against the three or four flu viruses that researchers think may be most common during the upcoming season.

Flu vaccines work by causing antibodies to develop in the body about two weeks after vaccination takes place. These antibodies provide protection against infection by the flu, meaning that vaccinated people are far less likely to catch the flu in the upcoming flu season than those who haven’t had the shot.

Why should people be vaccinated against the flu?

Unlike colds, flu can cause serious complications, particularly in young children, older adults and people with certain medical conditions. In some people, particularly those with weakened immune systems, the flu can be a potentially serious disease that can lead to hospitalization and sometimes even death. In fact, thousands of people die from flu-related causes every year. A seasonal flu vaccine is the best way to help protect against flu.

Flu vaccine effectiveness

Getting an influenza vaccine — though not 100% effective — is the best way to prevent contracting the flu and having any complications from infection. The Centers for Disease Control and Prevention (CDC) recommends annual flu vaccinations for everyone age 6 months or older.

How does the flu vaccine work?

The flu vaccine works by building up your body’s own immunity to flu viruses (i.e. its ability to destroy or neutralize these viruses).

When they are exposed to foreign substances (like viruses), your white blood cells (in your immune system) respond by producing antibodies. These are special proteins that latch onto foreign substances. Antibodies can prevent a virus from being able to enter and infect a cell, or may tell other white cells to attack the invader.

The goal of the flu vaccine is to expose your immune system to specific, non-dangerous parts of the virus before you are infected by the live virus. In this way, your body will have antibodies ready-made when you are exposed to the flu in your community. As a result, you may not get sick at all or you may have a milder case of the flu.

What are the different types of flu vaccine?

There are a number of different flu vaccines available. They differ according to four main factors.

  • How they are administered.
  • What type of viral material they contain.
  • How many flu strains they protect against.
  • Whether they are “adjuvanted” or not. That means created with an ingredient that helps create a stronger immune response in the vaccinated person.

Note that a possible fifth category is how the vaccines are produced, i.e. using egg proteins, cells, or in another way.

Method of administration

Flu vaccines can either be administered as an injection (usually in the upper arm), or given as a nasal spray.

Viral material used

There are three different types of viral material that can be included in flu vaccines. The different types include: inactivated influenza vaccines [IIV], recombinant influenza vaccines [RIV], or live attenuated influenza vaccine (LAIV). As their name indicates, inactivated flu vaccines contain inactivated viral particles, so they do not cause flu. Recombinant vaccines use synthetically created virus particles. Live attenuated vaccines are made from weakened viruses. The CDC does not favor the use of one vaccine type over another.

Protection against flu strains

The trivalent vaccines protect against three different flu viruses: the two most common A-type flu strains and one B strain. A quadrivalent influenza vaccine on the other hand is designed to protect against four different flu viruses, including two influenza A viruses and two influenza B viruses.

Adjuvanted vs. non-adjuvanted vaccines

Adjuvanted vaccines work to produce a stronger immune response in vaccinated people. They are usually recommended for seniors aged 65+ who may not show enough of an immune response to the traditional (non-adjuvanted) vaccine.


Q. Why do I need a new flu vaccine every year?

There are two reasons that it’s necessary to have a new vaccination each year. Firstly, a person’s immune protection from vaccination declines over time. This means that getting vaccinated annually helps provide optimal protection. The second reason is because flu viruses are constantly changing (i.e. mutating). That means that prior years’ vaccines may not be effective against the current year’s flu strains. Flu vaccines are updated from one season to the next to protect against the viruses that research suggests may be most common during the upcoming flu season.

Q. What are common side effects of the flu shot?

Most of the side effects of the flu shot are not serious. They are also temporary and will fade on their own. Common side effects from a flu shot include soreness, redness, or swelling where the shot was given, mild headache, fever, nausea, muscle aches and tiredness. The flu shot, like other injections, can occasionally cause fainting.

Q. Can the flu shot cause serious side-effects?

Life-threatening reactions to flu shots are very rare. But signs of serious allergic reaction can include breathing problems, hoarseness or wheezing, skin rash, paleness, weakness, a fast heartbeat, or dizziness. If they do occur, it is usually within a few minutes to a few hours after receiving the shot. These reactions can occur among persons who are allergic to something that is in the vaccine, such as egg protein or other ingredients. While severe reactions are uncommon, you should let your doctor or nurse know if you have a history of allergy or severe reaction to the influenza vaccine or any part of flu vaccines.

Q. Can the flu vaccine give you the flu?

No. Flu vaccines cannot cause flu illness. Injectable flu vaccines are made with inactivated viruses, or with only a single protein from the flu virus. The nasal spray vaccine contains live viruses that are attenuated (weakened) so that they will not cause illness. However, the vaccine can trigger an immune response from your body, so you may have a few mild symptoms, like achy muscles or a low-grade fever.

Q. How long does the flu vaccine take to work?

It takes about two weeks for your body to create the maximum number of antibodies against the flu after you are vaccinated. That is why many medical professionals recommend getting vaccinated in the fall, before the flu season begins. This gives your body time to build immunity.

Q. Who should and who should not get a flu vaccine?

The CDC recommends that everyone older than six months should get a flu vaccine every flu season with only very rare exceptions, as long as the correct form of the vaccine is given. For example, the recombinant influenza vaccine (RIV) is approved for people aged 18 years and older, while adjuvanted and high-dose inactivated vaccines are approved for people aged 65 years and older.

Pregnant women and people with certain chronic health conditions can get a flu shot. Most people with egg allergies can get a flu shot. Children younger than 6 months of age are too young to get a flu shot.

On the other hand, people with severe, life-threatening allergies to flu vaccines or any ingredient in the vaccine should not get the shot. These allergies might include gelatin, antibiotics, or other ingredients. People with any serious underlying health conditions should also talk to their doctors about getting the flu shot and which type of vaccine would be right for them.

Q. Can I get the nasal spray instead of a shot?

The nasal spray flu vaccine is approved for use in healthy non-pregnant individuals, 2 years through 49 years old. Pregnant women, people with certain medical conditions, and people taking certain medications should not receive the nasal spray flu vaccine. Talk to your doctor or pharmacist to see if you are a candidate for the nasal spray flu vaccine.

Q. Is the flu vaccine effective?

In general, flu vaccines are effective, however, effectiveness can vary. The protection provided by a flu vaccine depends firstly on the age and health status of the person getting it. It also depends on the similarity or “match” between the viruses in the vaccine and those in circulation in a given community in a given year. If a person is vaccinated against a certain strain but then contracts another strain, they may still get sick. Every year, the CDC does its best to anticipate which flu strains will be most active, to ensure vaccines provide the greatest possible protection.

Q. Why does my child need a second flu shot?

Special vaccination instructions for children aged 6 months through 8 years old

  • Some children 6 months to 8 years old require two doses of flu vaccine for adequate protection from flu. Children in this age group getting vaccinated for the first time, and those who have only previously gotten one dose of vaccine, should get two doses of vaccine this season—spaced at least 4 weeks apart. Your child’s health care provider can tell you if your child needs two doses.
  • If your child needs the two doses, begin the process early. This will ensure that your child is protected before flu starts circulating in your community.
  • Be sure to get your child a second dose if they need one. It usually takes about two weeks after the second dose for protection against flu to begin.

The information provided is for educational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Seek the advice of a doctor with any questions you may have regarding a medical condition. Never delay seeking or disregard professional medical advice because of something you have read here.

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