What makes an mRNA vaccine so effective against severe COVID-19?

The first two vaccines created with mRNA vaccine technology — the Pfizer/BioNTech and Moderna COVID-19 vaccines — are arguably two of the most effective COVID vaccines developed to date. In clinical trials, both were more than 90% effective at preventing symptomatic infection, easily surpassing the 50% threshold the Food and Drug Administration had set for COVID-19 vaccines to be considered for emergency use authorization.