Hey! I'm quite uninformed about medicine and vaccine science in general, so I apologize if this is an obvious question, but I couldn't figure it out myself.

Why are vaccines not always placebo-tested against saline or similar in order to evaluate the true background rates of adverse events/serious adverse events? I understand the ethical argument of withholding potentially life saving medicine (so we compare against similar vaccines of previous generations), but I don't understand why some lines of vaccines seem to never have been tested against placebos.

For example, Prevnar-13 is the current de-facto pneumococcal vaccine for children <2yo, and it was tested in randomized clinical trials against Prevnar-7 (previous generation vaccine) to compare rates of adverse events. However, Prevnar-7 itself seemed to never have been tested against a true placebo, just an "investigative meningococcal vaccine", even though it seems to be the first conjugate vaccine of its kind.

Similarly, the current infanrix DTAP vaccine was tested for adverse events against the whole-cell pertussis DTP vaccine, but I can't seem to find any randomized clinical trials testing the whole-cell pertussis DTP vaccine itself against a true placebo.

I feel like there's something I'm missing here, would appreciate if someone with better understanding could help me. Thanks!