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[Pharmacology](https://durgical.com/en/pharmacology/) · [Viral vaccines](https://durgical.com/en/pharmacology/classes/viral-vaccines/)

# Influenza vaccine

Seasonal vaccine against influenza A and B. Its composition is updated every year because the virus's surface proteins keep changing.

Updated 2026-11-01

- Class: [Viral vaccines](https://durgical.com/en/pharmacology/classes/viral-vaccines/)
- Target: Influenza haemagglutinin (HA)
- ATC code: J07BB
- WHO Model List of Essential Medicines

Also called: Inactivated influenza vaccine (IIV), Recombinant influenza vaccine (RIV), Live attenuated influenza vaccine (LAIV, intranasal)

◆**Influenza haemagglutinin (HA)**Influenza vaccine

ⓘ The figure shows haemagglutinin from the 2009 pandemic H1N1 virus; vaccine antibodies are mainly directed against this protein.

**Target in 3D:** Influenza haemagglutinin (HA). PDB structure 3LZG (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/3LZG) · Drag to rotate · pinch to zoom

## Mechanism of action

Haemagglutinin binds the virus to sialic acid on respiratory cells, and neuraminidase releases new virions. The vaccine induces antibodies mainly against haemagglutinin. Small point mutations in these proteins (antigenic drift) produce new strains each season, so composition is reviewed and vaccination repeated every year; a major exchange of genes between influenza A viruses (antigenic shift) can cause a pandemic.

- The vaccine presents the season's haemagglutinin and neuraminidase
- Antibodies against haemagglutinin block attachment to respiratory cells
- Antigenic drift makes last season's antibodies less useful
- Composition is updated and the vaccine is repeated every season

## Pharmacokinetics

**Onset and duration**
Immunity usually lasts less than one season, because antibodies wane and the virus drifts

## Indications

- Seasonal prevention of influenza, especially in groups at risk of complications
- Pregnant women, to protect mother and newborn; MinSalud promotes it during pregnancy
- Indirect reduction of influenza-related pneumonia [Community-acquired pneumonia →](https://durgical.com/en/conditions/community-acquired-pneumonia/)

## Adverse effects

### Common

- Pain and redness at the injection site
- Low-grade fever, myalgia, malaise

### Serious (rare, important)

- Anaphylaxis (rare)
- Guillain-Barré syndrome: any risk is very small

## Contraindications

- Severe allergic reaction to a previous dose or a component
- Live intranasal vaccine: pregnancy and immunosuppression (the inactivated vaccine is used instead)

## Key interactions

**Immunosuppressive therapy**
The live intranasal vaccine is avoided; inactivated or recombinant vaccine is used

**Acetylsalicylic acid in children**
Avoided in children and adolescents with influenza because of Reye syndrome

## What is monitored

- History of Guillain-Barré syndrome within six weeks of a previous influenza vaccine (a precaution)
- Reaction after the dose

## Administration and safety

- Inactivated and recombinant vaccines by intramuscular route; the live vaccine by intranasal spray
- Check the season's formulation and the product's age indication
- Some egg-based products contain residual egg protein, and some multidose vials contain thimerosal: read the product label
- Record the batch and product

## Nursing care

- Ask about previous reactions and Guillain-Barré syndrome after an influenza vaccine
- Check pregnancy and immune status before choosing the live intranasal vaccine
- Explain that the vaccine does not cause influenza and must be repeated every season
- Remind families not to give acetylsalicylic acid to children with influenza

## Pregnancy and breastfeeding

Inactivated or recombinant influenza vaccine is recommended in pregnancy and can be given in any trimester; the live intranasal vaccine is contraindicated.

## Memory aid

"Drift = drizzle, shift = storm": small yearly drift means a new vaccine each year; a big shift brings a pandemic. H for Hook (attaches), N for Nip (releases).

## Test yourself

Think of the answer, then tap the question to see it.

## Why is the influenza vaccine given every year?

Antigenic drift changes haemagglutinin and neuraminidase each season, and immunity wanes in less than a year.

## What is the difference between antigenic drift and antigenic shift?

Drift is gradual point mutation (seasonal epidemics); shift is an abrupt change from gene exchange between influenza A viruses (pandemics).

## Which influenza vaccine is avoided in pregnancy?

The live attenuated intranasal vaccine; inactivated or recombinant vaccines are used.

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## Related drugs

- [Respiratory syncytial virus (RSV) vaccine](https://durgical.com/en/pharmacology/respiratory-syncytial-virus-vaccine/)

## Related conditions

- [Community-acquired pneumonia](https://durgical.com/en/conditions/community-acquired-pneumonia/)

[← Back to the class: Viral vaccines](https://durgical.com/en/pharmacology/classes/viral-vaccines/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Chapter 12: Influenza | Pink Book | CDC](https://www.cdc.gov/pinkbook/hcp/table-of-contents/chapter-12-influenza.html) (Centers for Disease Control and Prevention, 2021); [Lineamiento Técnico y Operativo de la Primera Jornada Nacional de Vacunación](https://www.minsalud.gov.co/sites/rid/Lists/BibliotecaDigital/RIDE/VS/PP/PAI/lineamientos-t%C3%A9cnicos-primera-jornada-nacional-vacunaci%C3%B3n-2026.pdf) (Ministerio de Salud y Protección Social (Colombia), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=J07B&showdescription=no) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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