At a Thursday press briefing by the White House COVID-19 response team and public health officials, CDC director Dr. Rochelle Walensky said the disease was one of the most contagious ever seen by scientists. Walensky said the Delta variant of COVID-19 is much more transmissible than other previously circulating strains.

“It is one of the most infectious respiratory viruses we know of, and that I have seen in my 20-year career,” Walensky said in the briefing.

The viral loads in Delta infections were around 1,000 times higher than those in earlier strain infections from when the pandemic first began, according to recent research from Guangdong CDC in China.

The Delta variant now represents 83% of the virus circulating in the U.S., Walensky said, and it is important that the more than 30% of Americans who haven’t received a single dose yet get vaccinated.

“The good news is that current scientific evidence shows that our current vaccines are working as they did in clinical trials, even against the Delta variant,” Walensky said.

Walensky also touched on the problem of states, which are especially in the South and Midwest, with low vaccination rates and high prevalence of COVID-19. Those that are unvaccinated have much higher chance of being significantly affected by the Delta variant.

The region made up of Montana, North Dakota, South Dakota, Colorado, Wyoming, and Utah had a rate of 93.4% of Delta variant cases among new COVID cases, according to Fortune’s vaccine tracker.

Among this group are some of the states with the lowest vaccination rates, including Wyoming (where 55.2% have had at least one dose) and North Dakota (56.3%). In the South, states such as Louisiana, Alabama, and Mississippi have vaccination rates near 50% while the Delta variant increases in prevalence.

At the same press briefing, Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, said it is normal for some to contract COVID after being vaccinated. “No vaccine is 100% effective,” Fauci said.

Despite the vaccine not protecting completely against COVID, it still protects against serious disease, he said. Available data shows vaccines are 96% effective in preventing COVID hospitalizations and deaths from Delta variant infections.