How — and When — We’ll Know If a Covid-19 Vaccine Is Working

Phase 3 trials explained

Photo: ANDREW CABALLERO-REYNOLDS / Getty Images

There are more than 150 experimental Covid-19 vaccines in development right now, and a handful of those have recently entered the final stages of human testing.

Initial findings show a glimmer of promise: In early trials, these vaccines appear safe and seem to spur an immune response in the…