Delta Variant May Be 4 Times More Infectious Than Original COVID Virus

People infected with the Delta variant of COVID would go on to infect between five and eight other people on average without any measures in place, a researcher has told Newsweek.

Dr. Sonja Rasmussen is a clinical geneticist in the Department of Pediatrics and Epidemiology at the University of Florida who has studied COVID-19 throughout the pandemic.

Speaking about the Delta variant, she said its increased transmissibility is part of the reason the COVID mutant is causing so much concern around the world.

One way of measuring just how easily Delta can spread between people is by looking at its R0 number, or reproductive number. This number refers to how many people an infected person can expect to spread the virus to if there were no measures in place to prevent this, and it differs between variants.

According to The Lancet medical journal, the reproductive number of the original strain of COVID was around 2.5. The Alpha variant was then around 60 percent more transmissible than this.

Delta is thought to be about 60 percent more transmissible again, resulting in a reproductive number that could be as high as R8, said Rasmussen.

"The R0 of the Delta variant is estimated to be in the range of five to eight. So on average, one person spreads the virus to five to eight others," she said. "If you follow that a few generations forward, it shows how quickly you can end up with large numbers of people infected in a short amount of time."

Dr. Francis Perry Wilson, associate professor of medicine at Yale University School of Medicine, echoed the point. He told Newsweek that individual studies vary, but the best estimate is that the Delta variant could be "somewhere between two and four times more contagious than the original Wuhan variant" of COVID.

Delta is more transmissible because of the way it has evolved over time. Viruses always evolve, generating new mutations as they go. Sometimes these mutations are unhelpful or even harmful to the virus, and that particular line of evolution dies off. But sometimes, these mutations give the virus dangerous advantages.

"The Delta variant has some mutations—genetic changes—in its spike protein that make it better at attaching to and entering human cells," Rasmussen said.

"In addition, some studies suggest that the viral load—the amount of virus in people's throats—is 1,000 times higher with Delta.

"These lead to the higher transmissibility. Because of its high transmissibility, the Delta variant has outcompeted the other variants in the United States."

Indeed, Centers for Disease Control and Prevention (CDC) data from August 28 shows that Delta accounted for more than 99 percent of sequenced COVID samples found in the U.S. in the week prior.

But Wilson points out that there is a crucial caveat to the reproductive number—it refers to the rate of spread without any mitigation. That is, in a world without vaccines, distancing, and masks.

"It helps as a standard, but in the real world the typical person infected with Delta will not pass it to five people because a bunch of people are vaccinated, and we are engaging in social practices to limit the spread of disease," he told Newsweek.

"Probably, the 'effective reproduction number' often called Rt is closer to 1.5. Anything greater than 1 will lead to increasing case loads, but it isn't as catastrophic as it would be in a world without vaccination or transmission mitigation."

Man coughing on plane
A stock image shows a man coughing on an airplane. The Delta variant spreads more easily between people than earlier variants, experts say. Jun/Getty