Influenza Vaccines

Intro

Influenza (the flu) is a viral respiratory illness that causes seasonal epidemics and can lead to severe illness, hospitalization, and death — particularly in young children, older adults, and people with chronic conditions. The influenza vaccine is updated each year to match circulating strains, making it one of the most widely administered vaccines globally.

Key Points

Annual vaccination is needed because influenza viruses constantly evolve (antigenic drift).
Effectiveness varies by year, depending on how well the vaccine matches circulating strains.
High-risk groups benefit most, including older adults, children, pregnant women, and people with chronic illnesses.
Safety record is strong, with mostly mild side effects (soreness, mild fever).

Background

History: The first flu vaccines were developed in the 1940s. Annual programs expanded globally in the post-war decades.
Types of vaccines:
- Inactivated influenza vaccine (IIV) – injected.
- Live attenuated influenza vaccine (LAIV) – nasal spray (used in children in some countries).
- High-dose and adjuvanted vaccines – designed for older adults to improve immune response.
Strain selection: Twice a year, WHO convenes experts to recommend strains for the Northern and Southern Hemisphere vaccines.

Evidence

Effectiveness: Typically 40–60% in preventing symptomatic flu when well-matched; effectiveness against severe outcomes (hospitalization, ICU admission, death) is usually higher.
Population impact: Even with modest effectiveness, widespread vaccination reduces transmission and protects vulnerable populations.
Safety: Decades of global surveillance (VAERS, AusVaxSafety, EudraVigilance) confirm influenza vaccines are safe.

Risks / Benefits

Benefits: Reduces flu illness, hospitalizations, and deaths. Especially critical for healthcare workers and those caring for vulnerable groups.
Risks: Most adverse effects are mild (soreness, mild fever, aches).
Rare events:
- Severe allergic reactions are extremely rare.
- A small increased risk of Guillain–Barré syndrome has been observed in some years (~1–2 per million doses), but the risk of Guillain–Barré after influenza infection is much higher.

Global Recommendations

WHO: Annual vaccination recommended, prioritizing high-risk groups (children, elderly, pregnant women, healthcare workers).
CDC (US): Everyone 6 months and older should receive annual flu vaccination, unless contraindicated.
ATAGI (Australia): Annual vaccination recommended for all people ≥6 months; provided free under the National Immunisation Program for high-risk groups (e.g., >65s, Indigenous Australians, pregnant women, children <5).

FAQ

Q: Why do I need the flu vaccine every year?
A: Because influenza viruses mutate quickly, and immunity wanes over time. Annual vaccination ensures protection against current strains.

Q: Can the flu vaccine give me the flu?
A: No. Inactivated vaccines cannot cause infection. The live attenuated nasal spray is weakened and does not cause full influenza in healthy people.

Q: Is flu really dangerous?
A: Yes. While many cases are mild, flu causes hundreds of thousands of hospitalizations and tens of thousands of deaths globally each year.

Q: What if the vaccine is a poor match?
A: Even in mismatch years, vaccination reduces severity, hospitalization, and complications.

Disclaimer

This guide is for educational purposes only and is not a substitute for professional medical advice.
Always consult a qualified healthcare provider for diagnosis, treatment, and personal medical advice.