ok. 1. sample sizes are still pretty small. 2. participants are screened to be those with best possible health, this does not reflect real population of real kids with possible health issues, family histories or allergies and reactions, autoimmune disease, etc. --- also this is an issue I find really important about preemies, 3. no true placebos, 4. you are saying years, but as far as I have seen it is much shorter than that, except where testing for remaining immunity.
Again with the sample sizes. It is a common statistical misconception that it takes a very large sample size to get good data. However, the final phase of testing for vaccines includes thousands of people. Vaccines go through at least three stages of testing, each of which lasts over a year, in some cases several years. Individuals selected for testing are selected to try and match the age and health of the intended recipients of the vaccine, not cherry picked for just very healthy people.
This link does not have info about the time frame, I will try to find you something about that, though.