I got to thinking about how vaccine dosage goes for delayed kids who are much larger than the recommended age for a particular vaccine OR for vaccines that are given at intervals throughout development.
Who knows about how this is determined?
It depends. E.g. HiB and Prevnar, if given after a certain time (15 and 18 months I think) only need one dose. Measles induced sufficient titers in 95% of all vaccinated people. So I would run a titer test instead of revaccinating.
Others technically require a basic 3 injection round to establish some immunity (DTaP, IPV, HepB) - but if you are like me and only interested in the T portion, if started after age 2, 2 shots usually induce high tetanus titers. After 2 years of age the child's immune system is quite mature and doesn't need a gazillion doses of repeated vaccines. I'm a big fan of titer tests - drawing blood hurts for a split second, lab personnel is quite nice and it's quick. No harm done in taking a little blood!
As to same dosage: it doesn't make sense to me. A child receives a lot more antigens/weight than an adult would. I know it is often justified with kids need more antigen in a vaccine to evoke sufficient antibody response. I never do flu shots but I think I read, and correct me if I'm wrong, if given at 6 and 7 months, they only give them half a vial each time.
I decided I wanted to see a bit more action on this subject so I posed another thread in D&D. http://www.mothering.com/community/t/1374518/help-me-understand-vaccine-dosage