What to know about the annual flu shot

Influenza viruses change over time, and immune protection can fade, which is why vaccination is offered for each flu season. The injection cannot cause influenza when it contains inactivated virus components.

The vaccine is updated to target influenza strains expected to circulate during the coming season. Receiving the current formulation gives the immune system a more relevant reminder than relying only on an earlier vaccination or a previous flu infection.

Vaccination does not treat an infection that has already started, and it does not cover every virus that causes cough, fever or a sore throat. A person can therefore become unwell after vaccination for reasons that include another respiratory virus, exposure before immune protection developed, or influenza not fully prevented by the vaccine.

Tell the vaccinator about previous severe reactions to vaccines, current illness, immune-system conditions, pregnancy, regular medicines and any allergy concerns. A mild everyday illness may not prevent vaccination, but the clinician can decide whether your current symptoms warrant waiting or assessment.

After the injection, soreness at the site, tiredness, headache, muscle aches or a mild fever can occur as the immune system responds. Gentle arm movement, rest and fluids may help. For medicine to ease discomfort, check which option and dose suit your age, health conditions and other medicines.

Call emergency services for trouble breathing, swelling of the face or throat, widespread hives with faintness, collapse, or rapidly worsening symptoms after vaccination. These features may indicate a severe allergic reaction and require immediate treatment.

Get medical advice if side effects are intense, continue to worsen, do not begin to settle, or include a symptom you did not expect. Symptoms of influenza can still need assessment after a flu shot, particularly when breathing is difficult, chest pain develops, confusion appears, or the person cannot keep fluids down.

Ask whether the available formulation is appropriate for your age and health, how it fits with other planned vaccines, and what to do if you previously reacted to a flu shot. If you take medicines that affect immunity or have recently been acutely unwell, ask whether the timing should be adjusted.