Childhood immunisation follows a timetable in which each dose has a specified age, and the intervals differ between vaccines. The dates are determined by immunology rather than by administrative convenience.
Maternal antibodies set an early limit
Infants are born carrying antibodies transferred from the mother, which provide protection during the first months of life.
Those same antibodies can interfere with a vaccine, binding the antigen before the infant's own immune system responds to it fully.
Schedules therefore place certain vaccines after maternal protection has waned sufficiently, which is why some are given in infancy and others deferred until later in the first or second year.
Disease risk pulls in the opposite direction
Some infections are most dangerous in the earliest months, so waiting for an ideal immune response would leave infants exposed during the period of greatest danger.
Schedules balance these two pressures, giving protection as early as an adequate response can be generated rather than as early as physically possible.
Where local disease patterns differ, national schedules differ accordingly, which is one reason timetables are not identical between countries.
Multiple doses build durable memory
A first dose generates an initial response, and subsequent doses produce a stronger and longer-lasting one because the immune system has already encountered the antigen.
The interval between doses matters, since spacing them too closely limits the benefit of the second while the first response is still active.
Boosters given years later address the gradual decline in protection that follows the primary course for certain diseases.
Combination vaccines shape the calendar
Several antigens are frequently delivered in a single injection, which reduces the number of visits and improves the proportion of children who complete the course.
Combining vaccines requires evidence that each component still works properly alongside the others, and that constrains which can be grouped.
The resulting groupings largely determine the visit structure, since the calendar is built around the combinations available.
Schedules are reviewed continuously
National advisory committees assess new evidence, disease surveillance data and vaccine supply, and revise recommendations accordingly.
Changes occur when a disease becomes more or less prevalent, when a new product becomes available, or when data on protection duration accumulates.
Because schedules differ by country and are updated over time, the applicable timetable for any individual child is a matter for their own health service and clinician.