There’s good evidence that the shots made by Pfizer Inc.-BioNTech SE and Moderna Inc. are very effective — as much as 95% — at preventing recipients from developing Covid-19 itself. However, data haven’t been released on their ability to prevent people from developing asymptomatic infections or transmitting the virus to others. The gold standard in vaccinology is to stop infection as well as disease, providing so-called sterilizing immunity.