> Seems like you would def want to compare the two groups, so what did this study actually do? You are right. Indeed, all vaccines and drugs undergo clinical t

> Seems like you would def want to compare the two groups, so what did this study actually do? You are right. Indeed, all vaccines and drugs undergo clinical trials testing for safety first, efficacy then, before being approved for sale and distribution. The study in question analyzes data that was routinely collected after the vaccine was approved, that is why they didn't do the randomization themselves. One of these vaccines is DTaP-IPV/Hib aka Pentacel, and the clinical results for it are reported here: https://www.fda.gov/files/vaccines,%20blood%20&%20biologics/published/Clinical-Safety-Review--June-13--2008---Pentacel.pdf https://www.fda.gov/files/vaccines,%20blood%20&%20biologics/... > The two controlled pivotal safety studies, overall rates of serious adverse events were similar in Pentacel and Control subjects. (Tested on about 5,000 children younger than two years old and getting two or three doses of the vaccine). Another trial for that vaccine was recently performed in Japan, also not finding significant rates of adverse events: https://pubmed.ncbi.nlm.nih.gov/32307307/ https://pubmed.ncbi.nlm.nih.gov/32307307/