director is calling it an opportunistic violence. >> about two weeks ago, it was 10% being delta and now more recently about 20% of the straensstraen strains here in the u.s. are the delta. >> reporter: a rapid increase in the potentially more dangerous strain of coronavirus. and a warning. >> it will be the dominant strain among those regions where the vaccination rate is lower than we would like. >> reporter: these four states have the lowest vaccination rates for adults in the country. among those 18 and older, less than half have at least one shot. compare that to vermont where nearly 85% have at least one dose and 75% are full will he vaccinated. yet even in areas doing well, the push continues. >> we are going to keeinnovatin new ways to get people the vaccine, new ways to get it to work for them.