What's in a vaccine

So what is a vaccine? It's a substance that makes your immune system create antibodies. Antibodies help you combat infections.

All vaccines contain an ingredient called an antigen: weakened forms of a real germ ("live" vaccines) or pieces of a real germ ("subunit" vaccines). Antigens trigger a response from your immune system when they're injected.

Non-live vaccines usually need an adjuvant like aluminium salt to trigger a response. The adjuvant may contribute to reactions at the site of injection.

Other ingredients may include:

• Preservatives: sometimes vaccines need preservatives to stop contamination during storage. Phenoxyethanol is the most common preservative in vaccines. It's also used in cosmetics, baby care products and eye drops.
  
  
• Stabilisers: These prevent unwanted chemical reactions and include sugars like lactose, amino acids, and albumin (a protein often extracted from bovine blood).
  
  
• Buffers: Buffers stop changes in the vaccine's acidity level. Salt's the most common buffer.
  
  
• Diluents: Saline solution and sterile water are common diluents. Diluents make up most of the vaccine's volume.
  
  
• Emulsifiers: An emulsifier holds two liquids together. "Tween 80" is common in vaccines - it's also common in ice cream (where it holds milk protein and fats together).

Trace residuals may be present in some vaccines. These are leftovers from the manufacturing process: cellular culture fluids, egg proteins, yeast, antibiotics and formaldehyde. These are found at extremely low levels and often measure parts per million or billion.

Common side effects: There are minor side-effects associated with each vaccine. For example, the Hepatitis B jab sometimes causes a low-grade fever and swelling at the injection site. In addition to a low-grade fever, the MMR jab can also make children tired or drowsy. Minor side-effects can usually be fixed with rest, extra fluids and paracetamol.

The Immunisation Advisory Centre's Helen Petousis-Harris says severe allergic reactions (anaphylaxis) occur at a rate of around one per million doses of a vaccine.

Vaccine efficiency

No vaccine is 100 percent effective. Here's a glance at the efficiency of two of the jabs on the National Immunisation Schedule:

• Tetanus: 96 percent of people who receive all five tetanus jabs will be protected against the infection for 14 years; 72 percent will be protected for 25 years or more.
• Pertussis: The whooping cough vaccine only provides immunity for four to six years. Whooping cough is most serious in infants, so it's important the shots are given on time.

More information

• Health Outcomes Associates: www.hoa.co.nz
• Immunisation Advisory Centre: www.immune.org.nz
• Immunisation Awareness Society: www.ias.org.nz
• Ministry of Health: www.moh.govt.nz

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Report by Luke Harrison