The two possible reasons for these setting choices are that it takes to the body around 14 days to develop immunity and that vaccinating a lot of people is logistically demanding.
Vaccines are biological substances that help the body develop immunity against a virus, bacteria, etc. This is possible because vaccines contained a weakened or modified version of the pathogen that would help the body secrete specific antibodies for the real pathogen.
Immunity does not occur immediately, indeed, it takes the body up to 14 days to build up immunity. This includes the modern vaccines.
On the other hand, giving vaccines to large populations is quite demanding and challenging. Due to this programs with multiple weeks including four-week programs are ideal.
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