For the standard flu injection (flu jab), the risk of getting an infection from the vaccine itself is extremely low because the vaccine does not contain infectious flu virus. The viruses in flu shots are inactivated (killed) or made from only part of the virus, so they cannot cause influenza.

Common side effects that are sometimes mistaken for infection

After vaccination, some people experience:

• Soreness, redness, or swelling at the injection site
• Mild fever
• Headache
• Muscle aches
• Tiredness

These symptoms are usually mild and last 1 to 2 days. They reflect the immune system responding to the vaccine, not an infection

Are there any infection risks?

The main theoretical infection risk is from the injection procedure itself (for example, a skin infection at the injection site), but this is very rare when standard vaccination practices are followed. Serious adverse events are uncommon.

Rare serious reactions

Rarely, people may experience:

• Severe allergic reactions (usually within minutes to hours)
• Guillain-Barré syndrome (estimated at fewer than 1 to 2 cases per million vaccinations, and even this association is uncertain).

Potential Complications if not immunised

Potential Complications of Influenza

• Pneumonia (viral or secondary bacterial pneumonia), which is one of the most common serious complications and may require hospital admission.
• Worsening of existing medical conditions, such as:
  • Asthma
  • Chronic obstructive pulmonary disease (COPD)
  • Heart disease
  • Diabetes
  • Neurological conditions
• Bronchitis and other lower respiratory tract infections.
• Hospitalisation due to severe respiratory illness, dehydration, or complications from underlying health conditions.
• Heart complications, including inflammation of the heart muscle (myocarditis) and increased risk of cardiac events in susceptible individuals.
• Neurological complications, such as encephalitis (brain inflammation) and, rarely, other nervous system disorders.
• Sepsis, a severe body-wide response to infection that can lead to organ failure.
• Death, particularly among older adults, pregnant women, young children, immunocompromised individuals, and those with chronic medical conditions.

Higher-Risk Groups

The consequences of influenza can be particularly serious for:

• Adults aged 65 years and over
• People with chronic respiratory, cardiac, kidney, liver, or neurological conditions
• People with weakened immune systems
• Pregnant women
• Residents of care homes

Summary

Influenza is more than a heavy cold. Without immunisation, infection can lead to severe respiratory disease, worsening of existing health conditions, hospitalisation, and in some cases death. Annual vaccination helps reduce the risk of infection and significantly lowers the likelihood of serious complications