Everything You Need to Know About the Flu Vaccine: FAQs

In this article, we will answer all of your questions about the flu vaccine and cover everything from what the flu is, when is the best time to get vaccinated, when the vaccine starts to be effective, and how the vaccine works.

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What is the Flu | Flu Vaccine 101 | Different Kinds of Flu Vaccine | How Long for Flu Vaccine to Work

The flu vaccine is an important way to protect yourself and your loved ones from the flu.

It can be confusing to figure out how the vaccine works with all of the different information available and it can be hard to know whether it is right for you.

In this article, we will answer all of your questions about the flu vaccine and cover everything from what the flu is, when is the best time to get vaccinated, when the vaccine starts to be effective, and how the vaccine works.

What is the flu?

The flu is a contagious respiratory illness caused by influenza viruses which can cause mild to severe illness and affects your nose, throat, and lungs.

There are four different types of influenza viruses: A, B, C, and D. Influenza A viruses are the most common and can cause severe illness in people of all ages.

Influenza B viruses are also common and usually only cause mild illness in children, while influenza C viruses are the least common and usually only cause mild illness in adults.

The fourth type, influenza D, only affects cattle and sometimes pigs but does not affect humans.

The flu virus is spread through respiratory droplets in the air by coughing and sneezing, and can also be spread through contact with surfaces or objects that have been contaminated with the virus and then touching your mouth, nose, or eyes.

What are the symptoms of the flu?

The most common symptoms of the flu include fever, chills, body aches, fatigue, headache, sore throat, runny nose, and cough.

These symptoms have a rapid onset of a few hours and usually appear one to four days after infection.

You may also experience vomiting and diarrhea, though this is more common in children than adults.

During the first three to four days of showing symptoms, you will likely show the most severe symptoms with the flu which is also when you are most contagious due to coughing and sneezing the most.

You can infect other people the day before you show symptoms and are usually contagious for up to a week after you start showing symptoms.

If you are immunocompromised or have a child with the flu, the symptoms may last longer due to weaker immune systems. You are no longer infectious when you do not have a fever for 24 hours without the aid of medication.

What does the flu vaccine do?

The flu vaccine, also known as the flu shot, helps protect you from the flu by injecting you with a “dead” or inactivated form of the virus.

The dead virus can not infect you but it does help your body create immunity to the live, or active, forms of the virus.

The immunity created by the vaccine usually lasts for about a year but can begin to decline after just a few months.

The flu shot is most effective in healthy adults under the age of 65. Studies have shown that there is generally a 50%-60% flu vaccine effectiveness in preventing you from getting the flu if you are a healthy adult, however, some years it may be less effective due to the changing flu viruses.

It is also important to note that the vaccine will not protect you from all types of influenza, only those that are covered by the specific vaccine you receive.

Even with a flu vaccine, you may still get the flu but your symptoms may be less severe and you have a lower risk of developing severe complications which results in a lower risk of hospitalization due to the response of your immune system from receiving the shot.

What are the different kinds of flu vaccines?

There are three different types of flu vaccines: the quadrivalent vaccine (IIV4), recombinant influenza vaccine (RIV4), or live attenuated influenza vaccine (LAIV4).

The quadrivalent vaccine protects against four different strains of the flu virus, two influenza A viruses and two influenza B viruses.

The recombinant influenza vaccine is similar to the quadrivalent vaccine but is made without using eggs and can be given to you if you have severe egg allergies.

The live attenuated influenza vaccine is a nasal spray vaccine that also protects against all four flu strains.

When is the flu vaccine available?

The flu vaccine is usually manufactured months before it is distributed and the manufacturers attempt to determine which flu viruses will be the most common during the upcoming season.

The vaccine is typically available by late summer or early fall, usually August or September, but you can get vaccinated as late as winter.

It is best to get vaccinated before the start of flu season which runs from October through May and usually peaks from December until February.

How long does it take for the flu vaccine to work?

It normally takes about two weeks after vaccination for your body to develop immunity and receive the benefits of flu vaccination.

For this reason, it is important to get vaccinated as soon as possible so you receive proper protection before the start of flu season.

Why do I need to get the flu vaccine every year?

All three types of influenza viruses that affect humans are constantly changing through mutations and each year the manufacturers create a new vaccine to protect against the most common viruses for the upcoming flu season for optimal protection.

The immunity from the vaccine can also decline over time which is why it is important to get your annual flu shot.

Who needs to get the flu vaccine?

The Centers for Disease Control and Prevention (CDC) recommends that everyone over the age of six months needs to get a flu vaccine every year with rare exceptions.

You need to avoid getting the flu vaccine if you have had severe allergies to flu vaccines from a previous dose.

If you are at a high risk of developing complications due to the flu, it is extremely important that you get vaccinated.

The people most at risk of developing flu-related complications are the following:

  • Pregnant people
  • Older adults above the age of 65
  • Young children and infants (children between the ages of six months and eight years old will receive two shots four weeks apart)
  • Immunocompromised people with weakened immune systems

If you have a chronic medical condition, you are also at risk of developing severe complications due to the flu. Examples of these chronic illnesses that can increase your risk of developing flu complications include:

  • Respiratory conditions such as asthma
  • Congestive heart failure 
  • Cancer
  • Obesity
  • Kidney disease
  • Liver disease
  • Diabetes

These are some of the most common chronic health conditions but there are other chronic medical conditions that can also compromise your immune system.

Please speak with your doctor if you have any questions or concerns about getting the flu vaccine.

Can the flu shot make me sick?

No, the influenza shot cannot cause you to get the flu as it contains dead or inactivated forms of the virus.

It is common to experience some vaccine side effects such as soreness and redness at the injection site as well as low-grade fever, body aches, and fatigue for a day or two after receiving the vaccine but these are not symptoms of the flu and are usually minor side effects.

You may also contract the flu in the two-week period after receiving the shot but before it becomes effective but this is not due to receiving the shot.

As noted above, the common strains of the flu that circulate every year may not match with the strains put in the vaccine by the manufacturers which will also result in the vaccine being less effective.

Finally, some other medical conditions such as the common cold can exhibit some symptoms of the flu and it is possible to get a cold after receiving a flu vaccine.

If you experience any severe reactions after getting the vaccine, please seek medical attention immediately or call 911.

What are other ways to prevent the flu?

In addition to getting your annual flu vaccine, there are other things you can do to prevent the spread of the flu and protect yourself from contracting the virus. Some of these things include:

  • Washing your hands regularly with soap and water for at least 20 seconds or use an alcohol-based hand sanitizer when necessary
  • Avoiding close contact with sick people
  • Practice social distancing and avoid crowds
  • Avoid touching your eyes, nose, or mouth
  • Covering your mouth and nose when you sneeze or cough with the crook of your elbow, sleeve, or tissue that you will throw away immediately
  • Cleaning high-contact surfaces that are frequently touched such as doorknobs, countertops, refrigerator door handles, or lightswitches (do this regularly and especially if someone in your household is sick)

Please remember to adhere to the list above and to get your seasonal flu vaccine every year as they are the most effective ways of preventing the spread of the flu infection.


The flu is a contagious respiratory illness caused by the influenza virus and it can lead to severe complications, hospitalization, or even death.

The best way to prevent the flu is by getting vaccinated every year as the vaccine will protect you from the most common strains of the virus circulating that season.

It takes about two weeks for the vaccine to become fully effective so it is important to get vaccinated as soon as the vaccine becomes available.

Besides the vaccine, there are things you can do to help prevent the spread of the flu such as washing your hands regularly and avoiding close contact with sick people among others.

If you have any more questions about the flu or flu vaccine, please consult with your doctor or health care provider.

References, Studies, and sources:

Mayo Clinic 



medically reviewed and fact checked
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