Influenza Immunization

Although flu is unpleasant, healthy people usually find it clears up by itself within a week. However, flu can cause complications in certain high risk groups which include the following:

  • Those aged 65 or older
  • Women who are pregnant
  • People with an underlying health problem
  • People with weakened immune systems

A person with any of these risk factors is at an increased likelihood of developing severe complications such as pneumonia. The best way to protect against flu is to be vaccinated.

Immunization does not always prevent the flu and the degree of protection may vary between individuals, but people who do get the flu after vaccination, will probably find it is less severe and lasts for a shorter period than it otherwise would.

The flu vaccination may be given to children in the form of a nasal spray. For adults aged 65 and older, an injection is given.

When to have the jab

Ideally, people should be vaccinated by October to ensure they are immunized before the start of flu season, which is usually at a peak between December and February. The antibodies that protect against infection take about two weeks to develop after vaccination.

During this stage, individuals are still prone to infection, which is why it is better to get vaccinated early. Although this early immunization is the most effective way to protect against flu, it is not too late to receive the vaccine in January or later months.

Flu types the vaccine protects against

Every year, scientists identify the viruses that are the most likely to lead to flu, so that closely matched vaccines can be made. Most vaccines protect against the A/H1N1 strain that led to the swine flu pandemic in 2009; A/H3N2, which mainly affects the elderly and people with underlying health problems; and influenza B, which tends to affect children.

A flu vaccine is needed every season because the immune response to vaccination decreases over time. Furthermore, the viruses constantly change, meaning the vaccine formulation is reassessed annually.


Most people can receive the flu vaccination without experiencing any problems, but people who have ever suffered from a severe allergic reaction to a previous immunization should avoid it.

Other people who should avoid immunization include the following:

  • People who have ever experienced a severe allergic reaction to chicken eggs
  • Certain individuals who have previously had Guillain-Barré syndrome
  • People who have a moderate-to-severe illness should wait until they have recovered

Side effects

Various side effects are associated with flu immunization. Possible mild side effects include the following:

  • Swelling, redness and soreness at site of injection
  • Aches
  • Nausea
  • Low-grade fever
  • Fainting

More severe side effects include:

  • Breathing difficulty
  • Swelling of the lips or eyes
  • Pale complexion
  • Rapid heart rate
  • Hoarseness
  • Hives
  • Weakness
  • High-grade fever
  • Dizziness
  • Change in behavior

Any individuals who experience these more serious side effects should seek medical attention immediately. Healthcare providers log all severe reactions with the Vaccine Adverse Event Reporting System, even if an individual is not sure it was the vaccine that caused it. This system is used to aid the identification of severe reactions that may require further investigation.


  1. Immunize Australia Program, Influenza (Flu)
  2., Influenza (Flu)
  3. NHS, The Flu Jab

Further Reading

Last Updated: Feb 26, 2019

Sally Robertson

Written by

Sally Robertson

Sally has a Bachelor's Degree in Biomedical Sciences (B.Sc.). She is a specialist in reviewing and summarising the latest findings across all areas of medicine covered in major, high-impact, world-leading international medical journals, international press conferences and bulletins from governmental agencies and regulatory bodies. At News-Medical, Sally generates daily news features, life science articles and interview coverage.


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