How vaccines work

A vaccine trains your immune system to recognise a threat without you having to survive the full infection first.

The immune system has memory

When a pathogen enters the body, immune cells identify markers on its surface called antigens. Some cells attack directly; others produce antibodies that stick to the pathogen and help destroy it. After the infection, a small population of memory cells remains.

If the same pathogen returns, those memory cells respond faster and more strongly. That is why many childhood illnesses do not come back.

Vaccines exploit this memory. They expose the immune system to a harmless version of the antigen — a weakened or inactivated pathogen, a protein fragment, or genetic instructions to make a fragment — without causing the disease.

Main types

• Inactivated vaccines use a killed pathogen. Examples include some flu and polio vaccines.
• Live attenuated vaccines use a weakened pathogen. Examples include MMR and some chickenpox vaccines.
• Subunit and conjugate vaccines use a piece of the pathogen, often with an adjuvant to strengthen the response. Examples include some hepatitis B and HPV vaccines.
• mRNA vaccines deliver instructions for cells to make a spike protein temporarily, which the immune system learns to recognise.
• Viral vector vaccines use a harmless virus to deliver similar instructions.

Each approach has trade-offs in storage, cost, immune response and suitability for different groups.

Herd immunity

When enough people are immune, a pathogen struggles to find new hosts. This protects people who cannot be vaccinated — newborns, some immunocompromised people and those with certain allergies.

The threshold depends on how contagious a disease is. Measles is highly contagious, so a very high proportion of the population needs immunity to interrupt spread. That is why outbreaks can return when vaccination rates fall.

Side effects

Temporary soreness, fatigue, headache or fever are common. They usually reflect the immune system responding, not an infection. Serious adverse events are rare but possible, which is why vaccines are monitored after approval.

What vaccines do not do

No vaccine is 100% effective, and protection can fade. Some vaccines reduce severe disease more than they reduce infection. Vaccination advice depends on age, pregnancy, immune status, allergies and local guidance.

This article is general information, not medical advice. For decisions about you or your family, speak to a qualified healthcare professional and use official public health guidance.