Final answer:
The Influenza vaccine requires a 3-week minimum between doses due to the rapid mutation of the virus, which necessitates a new formulation each year.
Step-by-step explanation:
The vaccine that has a 3-week minimum between doses, not a 4-week minimum, is the Influenza vaccine. This is because the influenza virus mutates rapidly, and therefore, a new vaccine formulation is required each year to match the circulating strains. The other vaccines listed, such as HPV (Human papillomavirus), MMR (Measles, mumps, rubella), and Pneumococcal, have different administration schedules.