A growing number of people -- now more than one in 10 -- have missed their second dose of Covid-19 vaccine in the United States, a cause for concern as the more transmissible Delta variant gains a foothold. As of June 16, about 88% of those who received one dose of vaccine and were eligible for their second -- recommended 21 days after the first Pfizer/BioNTech shot or 28 days after the first Moderna shot -- had completed their two dose series, according to data shared with CNN by the CDC. That's down from a 92% completion rate earlier in the year.