Influenza vaccine is generally well tolerated and most people will experience only minor unwanted effects. The frequency and severity of these effects depends mainly on the individual's susceptibility. Possible unwanted effects include:

• soreness at the injection site
• fever
• feeling unwell
• shivering
• tiredness
• headache
• sweating
• muscle joint pain
• warmth

Rare:

• inflammation of the nerves causing pain, weakness and paralysis (a temporary condition called Guillain-Barré syndrome)
• sore red itchy eyes and difficulty breathing or other lung problems (e.g., coughing, wheezing), a condition called oculorespiratory syndrome
• allergic reactions

The influenza vaccine causes the body to produce antibodies against the virus. This means that when your body is exposed to the influenza virus, your body is able to defend itself. The antibodies stop the attacking virus. Influenza viruses change all the time, so different vaccines are made every year.

Influenza vaccine should not be given to:

• people who have had an anaphylactic reaction (e.g., throat constriction, difficulty swallowing) to the influenza vaccine
• people who have had an anaphylactic reaction to any of the vaccine components, with the exception of egg 
• people who have developed Guillain-Barré Syndrome (GBS) within six weeks of influenza vaccination

Influenza vaccination should be postponed:

• in serious acute illness - until symptoms have abated. Immunization should not be delayed because of minor acute illness, with or without fever.