There is growing concern that the Food and Drug Administration, under political pressure, could approve a Covid-19 vaccine before it has robust safety and efficacy data. The consequences of such a decision could be significant, particularly if the vaccine is ultimately shown to be less effective than early data suggest. But an approval before the completion of large, Phase 3 trials does not have to be problematic. Experts aren’t ruling out the possibility that a vaccine could be cleared this fall if it is very effective.