Lifestyle Health The Delta Variant Is Expected to Cause 'Very Dense Outbreaks' in These Undervaccinated States With part of the U.S. struggling to get residents vaccinated against COVID-19, cases of the Delta variant will likely be "hyper-regionalized," said the former FDA commissioner By Julie Mazziotta Julie Mazziotta Twitter Julie Mazziotta is the Sports Editor at PEOPLE, covering everything from the NFL to tennis to Simone Biles and Tom Brady. She was previously an Associate Editor for the Health vertical for six years, and prior to joining PEOPLE worked at Health Magazine. When not covering professional athletes, Julie spends her time as a (very) amateur athlete, training for marathons, long bike trips and hikes. People Editorial Guidelines Published on June 28, 2021 04:05 PM Share Tweet Pin Email Person refusing vaccine. Photo: Getty Images With the Delta variant now making up 20.6% of all COVID-19 cases in the U.S., health experts are concerned about how the highly contagious strain will continue to spread, particularly in areas with fewer people vaccinated against the virus. The Delta variant will likely be more of an issue in states with the lowest vaccination rates, Dr. Scott Gottlieb, the former commissioner of the Food and Drug Administration, said Sunday on CNN. "It's not going to be as pervasive. It's going to hyper-regionalized," he said. "There's certain pockets of the country where you're going to have very dense outbreaks." Fauci Says the Delta Variant Is the 'Greatest Threat' to Ending the COVID Pandemic in the U.S. Gottlieb said those areas are going to be in the rural and southern states where fewer people are vaccinated, such as Alabama, Arkansas, Louisiana, Mississippi and Wyoming. Those five states have the lowest vaccination rates in the country, with under 35% of their populations fully vaccinated. "The Delta variant is currently the greatest threat in the U.S. to our attempt to eliminate COVID-19," Dr. Anthony Fauci, the nation's top infectious diseases expert, said during a White House briefing last Tuesday. RELATED VIDEO: Nurse Whose Husband Died of COVID Is on a Door-to-Door Vaccine Crusade: 'I Can Help Save Others' The Delta variant, which was first identified in India, is "the most transmissible of the variants identified so far," the head of the World Health Organization, Tedros Adhanom Ghebreyesus, said Friday, according to NBC News. It's now spreading in 85 different countries, and moving faster due to the lack of vaccines in poorer countries, Ghebreyesus said. 153 Houston Hospital Workers Were Fired or Quit After Refusing to Get the COVID Vaccine Getting vaccinated is the best defense against the Delta variant, Fauci said. Testing has shown that both Pfizer and Moderna's COVID-19 vaccines are extremely effective against the variant, and prevent illness 90% of the time, and prevent hospitalization or severe illness 94% of the time. As of June 28, more than half of the U.S. population, or 54% of Americans, have received at least one dose of a COVID-19 vaccine and 46.1% are fully vaccinated, according to the Centers for Disease Control. Of the vaccine-eligible Americans, meaning those aged 12 and up, 63.1% have received at least one dose and 53.9% are fully vaccinated. As information about the coronavirus pandemic rapidly changes, PEOPLE is committed to providing the most recent data in our coverage. Some of the information in this story may have changed after publication. For the latest on COVID-19, readers are encouraged to use online resources from the CDC, WHO and local public health departments. PEOPLE has partnered with GoFundMe to raise money for the COVID-19 Relief Fund, a GoFundMe.org fundraiser to support everything from frontline responders to families in need, as well as organizations helping communities. For more information or to donate, click here.