(CNN) After months of progress in the fight against Covid-19, cases are rising again as the highly contagious Delta variant spreads across the US.
States with below-average vaccination rates have almost triple the rate of new Covid-19 cases compared to states with above-average vaccination rates, according to new data from Johns Hopkins University.
As of Sunday, states with lower rates of vaccination reported an average of 6 new cases per 100,000 residents every day over the past week, according to Johns Hopkins.
States with higher vaccination rates reported an average of 2.2 new cases per 100,000 residents each day over the past week.
Arkansas, where less than 35% of residents were fully vaccinated Sunday, averaged 16 new cases per 100,000 residents every day over the past week, according to Johns Hopkins. That's about five times the nationwide rate of new cases.
And Arkansas is one of 10 states where the rate of new cases jumped more than 25% over the past week compared to the previous week. Of those 10 states, all but one -- Delaware -- had below-average vaccination rates.
On the flip side, Vermont leads the country in vaccination rates, with 66% of its residents fully vaccinated.
Vermont also has the lowest rate of new Covid-19 cases -- less than 1 per 100,000 residents each day over the past week, according to Johns Hopkins. That's a decrease of nearly 16% from the previous week.
The gap in progress between highly vaccinated states and those lagging in vaccinations keeps growing.
Parts of the South, Southwest and Midwest are starting to see surges, said Dr. Jonathan Reiner, professor of medicine and surgery at George Washington University.
Florida is getting hit particularly hard, with about 17% of all new cases in the US being reported out of the Sunshine State, he said.
"People will continue to die until we vaccinate everybody," Reiner said.
And for young people who don't think they need to get vaccinated, Reiner said his hospital has seen plenty of young adults suffering from Covid-19 or complications of long Covid months after infection.
"What I would say to young people is that Covid-19 doesn't have to kill you to wreck your life," he said.
All 50 states and Washington, DC, have reported cases of the highly contagious Delta variant.
"We learned this virus, a variant of Covid, is highly transmissible -- the most transmissible we've seen to date," US Surgeon General Dr. Vivek Murthy said last week.
"This is, again, a serious threat and we are seeing it spread among unvaccinated people."
The virus carries a cluster of mutations, including one known as L452R, that helps it infect human cells more easily.
The director-general of the World Health Organization has also said "Delta is the most transmissible of the variants identified so far."
The current vaccines protect well against all the variants so far, but that could change at any moment. That's why doctors and public health officials want more people to get vaccinated.
"The more we allow the virus to spread, the more opportunity the virus has to change," the World Health Organization advised last month.
In areas with high Covid-19 transmission and low vaccination rates, even vaccinated people may want to wear masks, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious diseases.
"When I'm in that area where there's a considerable degree of viral circulation, I might want to go the extra mile to be cautious enough to make sure that I get the extra added level of protection -- even though the vaccines themselves are highly effective," Fauci told NBC on Sunday.
But Reiner said vaccines provide strong enough protection that those who are inoculated shouldn't need to wear masks, except for those with extenuating circumstances like compromised immune systems.
Vaccination is "the ticket to get your life back," he said.
"Currently, approximately 1,000 counties in the United States have vaccination coverage of less than 30%. These communities, primarily in the Southeast and Midwest, are our most vulnerable," CDC Director Dr. Rochelle Walensky said Thursday. "As the Delta variant continues to spread across the country, we expect to see increased transmissions in these communities, unless we can vaccinate more people now."
Former US Food and Drug Administration Commissioner Dr. Scott Gottlieb said masks may become normalized for some people looking to protect themselves against respiratory pathogens -- be they coronavirus or the flu.
"I think people are going to use them on a voluntary basis," Gottlieb told CBS's "Face the Nation" Sunday. "I think going to work with the sniffles is going to be frowned upon. I think businesses are going to have access to routine testing."
Zients said the federal government will be increasing efforts this summer to get more people vaccinated so they can return to normal life safely.
"The most trusted messenger is the local doctor, the local health care provider, so increasingly we have vaccines in doctor's offices, at health care clinics, so that people can get their questions answered and roll up their sleeve and get a shot," Zients said.
If not enough people get vaccinated, health experts say, progress could be erased and Covid-19 could resurge in the winter.
Vaccine expert Dr. Paul Offit estimated in May that 80% of the population will need to become immunized through vaccines or prior infections to avoid a winter surge.
Offit is director of the Vaccine Education Center at Children's Hospital of Philadelphia and a member of the US Food and Drug Administration's vaccine advisory committee.
That 80% population immunity could be achieved through a combination of both vaccination and immunity from natural infection, he said.
"The proof will be in the pudding next winter," Offit told CNN's Jake Tapper. "If we don't get there to 80%, then I think you'll see another surge of this virus next winter."