You have reversed the logical sequence. You can't know how efficient a vaccine is without a trial. You need an authorization first to do that. But you wouldn't

You have reversed the logical sequence. You can't know how efficient a vaccine is without a trial. You need an authorization first to do that. But you wouldn't get an authorization if something exists, even "only" 70% better, because that 70% would make Covid less lethal than the yearly flu. Hence 70% is enough to block emergency vaccines trial authorization.