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This is a group with lower overall efficacy than we have seen with the mrna vaccines, so there is some urgency here. >> reporter: the advisers also launching into a spirited debate over whether j&j should have been a two-dose vaccine to begin with. the company defending the need for a single-shot option in a pandemic. and today, a key question on mixing and matching vaccines, specifically whether its safe and effective to boost johnson & johnson with an mrna shot. a small study shows that a john...