A standard NSW vaccination schedule for diphtheria/pertussis/tetanus (DPT) is shown - HSC - SSCE Biology - Question 28 - 2003 - Paper 1

Immaturity of the Immune System: Infants have an immature immune system that is not fully developed, making them more susceptible to infections. Frequent vaccinations help to provide the necessary immunological defense against these diseases at a vulnerable stage.

Primary Immune Response: The effectiveness of a single dose may not provide long-lasting immunity. Repeated doses help to ensure a strong primary immune response, enabling the body to create a robust defense against diphtheria, pertussis, and tetanus.

Booster Effect: Multiple vaccinations during early childhood can trigger booster effects, further enhancing the immune response. This process helps in developing long-term immunity, thus reducing the risk of outbreaks.

Vulnerability to Disease: The diseases targeted by the DPT vaccine can have serious complications in young children. Early and frequent vaccinations minimize the risk of severe illness and associated mortality.

Public Health Strategy: Frequent vaccinations in early childhood also serve as a public health strategy to control the spread of these communicable diseases within the community, protecting not only the vaccinated individuals but also those who are unvaccinated.