They compare similar populations, mathematically control the variables, and see if there's a difference. It's all explained in detail in the study. It's also excruciatingly reviewed by other scientists for errors in a peer review before it's published and can be referenced in other studies/experiments
RocknrollClown09 t1_j5xnlaw wrote
Reply to comment by lintinmypocket in The bivalent mRNA boosters from Pfizer-BioNTech and Moderna were 48% effective against symptomatic infection from the predominant omicron subvariant (XBB/XBB.1.5) in persons aged 18-49 years according to early data published by the CDC by shiruken
They compare similar populations, mathematically control the variables, and see if there's a difference. It's all explained in detail in the study. It's also excruciatingly reviewed by other scientists for errors in a peer review before it's published and can be referenced in other studies/experiments