Prevnar 7 was licensed for infants at two, four, and six months of age with no placebo-controlled trial. There was no ethical reason to skip one. Instead, the control used in the clinical trial was another experimental vaccine, an investigational meningococcal conjugate vaccine that had never been licensed either. A JAMA article authored by FDA scientists acknowledged this approach can mask serious adverse events. Prevnar 7 then became the control used to license Prevnar 13. In that trial, serious adverse events including death, hospitalization, and permanent disability were reported in 8.2% of Prevnar 13 recipients and 7.2% of Prevnar 7 recipients. Prevnar 13 then became the control used to license Prevnar 15. Serious adverse events were reported in 9.6% of Prevnar 15 recipients and 8.9% of Prevnar 13 recipients. The FDA called it safe. Each generation of this vaccine was tested against the last one, and the last one was never tested against a true placebo. @AaronSiriSG calls it a pyramid scheme of safety, with a baseline that has never been shown to be safe at all.