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Delta variant makes up 20 percent of new U.S. Covid cases

03:18

The White House is expected to fall short of its goal of having 70 percent of Americans partially vaccinated against Covid-19 by July Fourth as health officials warn that younger adults are at risk of catching the delta variant. NBC News medical contributor, Dr. Uche Blackstock, explains the risk the delta variant poses to younger people and whether the new strain could mutate further.