11.2.6—Vaccination programmes

A vaccination programme protects a population when a sufficiently large proportion of people become immune. This population-level protection is herd immunity: fewer susceptible hosts make it harder for a pathogen to spread.

• Coverage: A coordinated immunisation schedule makes vaccination available across the relevant population. Coverage is not just an individual decision: delivery, access, resources and acceptance affect how many people become immune.
• Fewer susceptible hosts: Each immune person is less likely to contract and pass on the disease, so the pathogen encounters fewer suitable transmission opportunities.
• Transmission falls: When enough people are immune, chains of transmission become harder to maintain and disease levels stay low within the population.
• Indirect protection: People who are not immunised can be less likely to encounter the pathogen because spread is reduced around them. This matters especially for people who cannot rely on vaccination or have higher risk from infection.
• Limits: Herd immunity is not an absolute barrier. If coverage is too low, uneven, inaccessible or poorly maintained, enough susceptible people remain for transmission to continue; protection also depends on the vaccine and pathogen combination rather than on a universal guarantee.

This card is about population coverage and transmission, not the individual vaccine mechanism in 4637 or the active/passive classification in 4636. Herd immunity protects through reduced spread; it does not mean every person is directly vaccinated or that infection is impossible.