(CNN)Some states are making great strides in vaccinating their residentsagainst Covid-19, but the ones that are not may soon be contending with a more transmissible variant, experts say.About 45.1% of the US population is fully vaccinated against Covid-19,CDC data showed, and in 16 states and Washington, DC, that proportion is up to half. Butsome states– such as Alabama, Arkansas, Louisiana, Mississippi, Tennessee and Wyoming — have fully vaccinated less than 35% of residents.More than 500 days and 600,000 deathssince the first person in the United States was reported to have died from Covid-19, experts have upheld vaccines as the key to reopening the country safely and containing the variants, many of which are more transmissible.

The Delta variant, which is believed to be more transmissible and cause more severe disease, could cause an upsurge in infections, but the levels will vary…