Health,
You’re not really answering the question. Hib is recommended for children under 5 years old, so your answer regarding 15 months of age makes no sense. Hib vaccines yield a weaker and shorter lasting immune response in infants than they do older children. That is the reason that multiple doses are needed in infancy and a final booster at ~15 months is given. And that is also why no further subsequent doses are needed after 15 months, even if the child didn’t receive the infant series. The immune response is more robust.