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Health officials warn delta Covid variant could become dominant in U.S. as vaccination rate drops 

Half of U.S. adults are fully vaccinated against Covid-19, but health officials are warning that the delta variant, first detected in India, could become dominant across the country unless more Americans get the shot. NBC News medical contributor, Dr. Natalie Azar, breaks down whether the delta variant is more contagious than the original strain and why younger people are testing positive for Covid-19 at a higher rate.