Science Explained: Why is the Delta variant so contagious?

Strain replicates better than previous ones; vaccination remains best preventative measure



People infected with the delta variant are more likely to spread the virus because there are more viral particles present in their nose and mouth. The variant is a mutation of previous strains.

EMMA LEDBETTER, Evergreen news editor

The Delta variant has been dominating headlines recently. We keep hearing that it is “more contagious” than previous strains — but what exactly does that mean?

The answer lies in the viral strain’s ability to replicate itself, said Guy Palmer, WSU Global Health senior director and regents professor of pathology and infectious diseases. 

“This virus replicates much better, meaning it makes more copies of itself much faster and to a higher level than did the previous viruses,” Palmer said. “If you compare it to what was called ‘Alpha,’ it’s about twice as contagious.”

There are more viral particles in the nose and mouth of a person infected with Delta because the virus has mutations that make it more efficient at replicating, he said. 

People who are infected with Delta are more likely to spread the virus simply because there are more viral particles present, Palmer said.

“If you give enough virus, you can overwhelm any level of immunity,” he said. “It’s just more than the immune system can handle.”

Vaccinated people produce the same high level of viral particles as unvaccinated people. However, because they are vaccinated, they get rid of the virus faster — they are contagious for a shorter period of time. Shortening the window of infection means vaccinated people are much less likely to spread the virus to others while they are infected, Palmer said. 

“Some people have called the Delta variant an epidemic — or a pandemic — of the unvaccinated because that’s where it’s having its big impact,” he said.

The R-naught, or reproduction number, is a value disease scientists use to quantify how contagious a disease is. R-naught represents how many people — on average — a contagious person can infect during the period in which they are contagious. Palmer said the Delta variant has an R-naught about twice that of previous strains, meaning someone infected with the Delta variant could infect twice as many people while they are sick.

Based on R-naught values, the Delta variant is about as contagious as chickenpox, a disease most young people today have never experienced because of vaccines, he said.

Available vaccines are about 90 percent effective at preventing infection, which is very high, Palmer said. Vaccines are also about 96 percent effective at preventing hospitalization, which is what scientists are most worried about for infected people.

The Delta variant has likely been the dominant strain in Washington state and Whitman County since late July or early August, said Chris Skidmore, director of Whitman County Public Health.

Since Delta became the dominant strain in Whitman County, about 20-25 percent of cases are considered “breakthrough” cases, meaning they occur in people who are fully vaccinated, Skidmore said. 

The most effective steps to prevent infection are getting vaccinated, wearing a mask, and social distancing — in that order, he said. People who want to get vaccinated or tested can visit Whitman County Public Health’s website for a list of vaccine clinics and testing options.

Science Explained is an article series that aims to describe popular science topics without technical jargon that can be confusing for non-science audiences. Each article will cite professors, researchers or experts at WSU, as well as available online research to explain science in a way that is understandable and interesting.

Emma Ledbetter is the author of Science Explained. She is a microbiology major and is going into her fourth year working at The Daily Evergreen.