Immunisation schedules look arbitrary because the ages are specific and vary between countries. The dates are derived from three biological constraints and one local one.
Maternal antibodies fade on a timetable
Babies are born with antibodies transferred across the placenta in late pregnancy, which give partial protection against some infections in early life.
Those same antibodies can bind a vaccine component and blunt the infant's own response. Giving certain vaccines too early therefore produces weaker immunity than giving them slightly later.
The waning of maternal antibody is gradual and differs by disease, which is one reason different vaccines on the same schedule are given at different ages rather than together.
The infant immune system matures unevenly
Young infants respond well to some types of antigen and poorly to others. Responses to plain bacterial sugar coatings are particularly weak in the first months of life.
Conjugate vaccines were developed to work around this by linking the sugar to a protein, which recruits a different arm of the immune response that is functional earlier.
Where no such workaround exists, the schedule waits. That is why a few vaccines appear only in the second year rather than alongside the early infant doses.
Multiple doses build durable memory
A first dose primes the immune system, and later doses drive the maturation of antibody quality and the expansion of memory cells. The interval between doses affects the result.
Intervals that are too short produce a weaker response because the priming process is incomplete. Longer intervals often produce a stronger one but leave a child unprotected for longer.
Schedules resolve that trade-off in favour of earlier protection where the disease strikes young, and in favour of stronger response where it does not. Booster timing follows the same logic.
Local epidemiology moves the dates
The age at which a disease is most dangerous, and how common it is locally, both shift the schedule. A vaccine given routinely in one country may be targeted or absent in another.
Countries with high early exposure to a particular infection often start that vaccine earlier, accepting a somewhat weaker response in exchange for closing the window of vulnerability.
This is the main reason schedules are not identical worldwide, and why a family moving between countries may be advised to adjust rather than simply continue.
Schedules are reviewed, not fixed
National advisory committees review schedules as new vaccines are licensed, as disease patterns change and as long-term data accumulate. Changes are usually incremental.
Because of that, the schedule a child follows may differ from the one an older sibling followed, and older printed information can be out of date.
Which vaccines apply to an individual child, and when, is a matter for a paediatrician or immunisation service, particularly where a child has been unwell, was born early, or has moved countries.