The flu vaccine is a vaccine that protects people against influenza viruses that cause the flu.

The flu vaccine is reformulated each year to provide protection (immunization) against the particular flu viruses that are most common for that year. Getting the flu shot one year will not provide sufficient protection against the influenza virus in future years. As a result, individuals eligible for the flu shot should get the flu shot every year.

The flu vaccine may be given as a "flu shot" or nasal spray vaccine.

When should one get the flu shot?

Flu vaccines are generally given starting in September, or as soon as the vaccine is available. The Centers of Disease Control and Prevention (CDC) recommends that people get their seasonal flu vaccine as soon as the vaccine becomes available in their community.

The flu season can begin as early as October and last as late as May. Vaccination before December is ideal because this gives the body several weeks to develop protective antibodies before an outbreak occurs.

Over the course of the flu season, many different influenza viruses can circulate at different times and in different places. As long as flu viruses are still spreading in the community, vaccination can provide protective benefit.

Who Should Get Vaccinated Against Seasonal Flu?

Talk to your health-care provider if you have questions about whether you should get a flu vaccine.

On February 24, 2010 vaccine experts voted that everyone 6 months and older should get a flu vaccine each year.. CDC’s Advisory Committee on Immunization Practices (ACIP) voted for “universal” flu vaccination in the U.S. to expand protection against the flu to more people.

Certain people should be sure to get vaccinated each year either because they are at high risk of having serious flu-related complications, such as those with asthma, diabetes, and heart disease, or because they live with or care for high-risk persons.

During flu seasons when vaccine supplies are limited or delayed, the Advisory Committee on Immunization Practices (ACIP) makes recommendations regarding priority groups for vaccination.

Who Should Not Get Vaccinated Against Seasonal Flu?

Some people should not be vaccinated without first consulting a physician. They include:

  • People who have a severe allergy to chicken eggs.
  • People who have had a severe reaction to an influenza vaccination in the past.
  • People who developed Guillain-Barre Syndrome (GBS) within 6 weeks of getting an influenza vaccine previously.
  • Children less than 6 months of age (influenza vaccine is not approved for use in this age group).
  • People who have a moderate or severe illness with a fever should wait to get vaccinated until their symptoms lessen.

Why Does The Vaccine Need To Be Given Every Year?

There are two reasons for getting a yearly flu vaccine:

  • Flu viruses are constantly changing. The flu vaccines are updated from one season to the next to protect against the most recent and most commonly circulating flu viruses.
  • Yearly vaccination is recommended because a person’s immune protection from vaccination declines over time and annual vaccination is needed for optimal protection. The decline in protection against the flu is influenced by several factors, including a person’s age.

Types Of Flu Vaccines

There are two types of flu vaccines:

  1. The flu shot contains killed (inactive) viruses, so it is not possible to get the flu from this type of vaccine. However, some people do get a low-grade fever for a day or two after the shot as their immune systems gear up to recognize the virus. The flu shot is approved for use in people older than 6 months, including healthy people and people with chronic medical conditions.
  2. A nasal spray-type flu vaccine called FluMist uses a live, weakened virus instead of a dead one like the flu shot. The vaccine helps the lining of the nose fight off actual viral infections. Vaccination with the nasal-spray flu vaccine is an option for healthy people 2-49 years of age who are not pregnant, even healthy persons who live with or care for those in a high risk group. The one exception is healthy persons who care for persons with severely weakened immune systems who require a protected environment; these healthy persons should get the inactivated flu vaccine. The nasal-spray-type vaccine should not be used in those who have asthma or children under age 5 who have repeated wheezing episodes.

Will Someone Still Get Flu-Like Symptoms After Getting The Flu Vaccine?

There are several reasons why someone might get flu-like symptoms even after they have been vaccinated against the flu.

  • People may be exposed to an influenza virus shortly before getting vaccinated or during the two-week period that it takes the body to gain protection after getting vaccinated. This exposure may result in a person becoming ill with flu before the vaccine begins to protect them.
  • People may become ill from other (non-flu) viruses that circulate during the flu season, which can also cause flu-like symptoms.
  • A person may be exposed to an influenza virus that is not included in the seasonal flu vaccine. There are many different influenza viruses that circulate every year. The flu shot protects against the 3 viruses that research suggests will be most common. Unfortunately, some people can remain unprotected from flu despite getting the vaccine. This is more likely to occur among people that have weakened immune systems. However, even among people with weakened immune systems, the flu vaccine can still help prevent influenza complications.

Seasonal influenza vaccine provides the best protection available from seasonal flu — even when the vaccine does not closely match circulating flu strains, and even when the person getting the vaccine has a weakened immune system. Vaccination can lessen illness severity and is particularly important for people at high risk for serious flu-related complications and for close contacts of high-risk people.

Does Flu Vaccine Work Right Away?

No. It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection. In the meantime, you are still at risk for getting the flu. That’s why it’s better to get vaccinated early in the fall, before the flu season really gets under way.

Reference: Centers for Disease Control and Prevention

Last updated May 8, 2015.

This information is for general educational uses only. It may not apply to you and your personal medical needs. This information should not be used in place of a visit, call, consultation with or the advice of your physician or health care professional.

Communicate promptly with your physician or other health care professional with any health-related questions or concerns.

Be sure to follow specific instructions given to you by your physician or health care professional.

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