Why Is the Delta Variant More Transmissible?

The Delta variant has taken hold in the United States, driving a worrisome spike in COVID-19 infections, especially among unvaccinated populations.  

Delta is the most transmissible COVID-19 variant yet. It could be as contagious as chickenpox—which spreads more easily than the common cold, Ebola, and the 1918 flu—says an internal presentation from the Centers for Disease Control and Prevention (CDC).

Chinese researchers found that people infected with Delta had about 1,000 times the viral load in their respiratory tract compared to those infected with the original strain. This variant also make people contagious in about four days, rather than six, as with other variants.

COVID-19 vaccines still offer strong protection against the Delta variant. But its high transmissibility means it's able to wreak havoc, raising hospitalizations and death rates among unvaccinated people.

Scientists are seeking to understand what makes this variant so contagious—here’s what we know so far.

Delta: A More Contagious Variant

The CDC estimates that the Delta variant is responsible for 82% of COVID-19 cases in the U.S. in mid-July, just two weeks after it became the country’s dominant variant.

Scientists believed vaccines could prevent the Alpha variant spread, but Delta may be transmitted by infected vaccinated people, CDC director Rochelle Walensky said in a news briefing last week.

"The Delta variant is showing every day its willingness to outsmart us," Walensky said. "When we examine the rare breakthrough infections and we look at the amount of virus in those people, it is pretty similar to the amount of virus in unvaccinated people."

A COVID-19 outbreak in Massachusetts this month appears to be a case study of this. During the outbreak, which was fueled by the Delta variant, three-quarters of those infected were fully vaccinated, according to a CDC report.

Infection with Delta seems to carry an increased risk of hospitalization, too. An early study from Scotland showed that unvaccinated people infected with Delta were twice as likely to be hospitalized than unvaccinated people infected with Alpha.

Fortunately, Marina Van Kerkhove, technical lead for COVID-19 at the World Health Organization, said in a briefing that the organization hasn’t seen a substantial increase in severe sickness or death, meaning people infected with the Delta variant don’t seem to die more readily than those infected with other variants.

Looking Beyond the Spike

To understand why Delta is so infectious, researchers are looking to the ways it differs from other variants of concern.

For the most part, scientists have sought to understand the signature crown-like spike protein on the outside of the COVID-19 virus. The spikes of this protein act like keys, which bind to receptors that can unlock the path to human cells. Most of the COVID-19 vaccines target this spike protein to block the virus from entering cells.

Various mutations in the spike protein, such as the receptor-binding domain, make it especially effective at tricking first wave antibodies. Delta carries a mutation in the spike protein called D614G, sometimes known as "Doug," which may increase the density of spike protein on the surface of viral particles.

In addition to these worrisome changes, Nevan Krogan, PhD, professor and director at the Quantitative Biosciences Institute at the University of California San Francisco, says it’s worth paying attention to mutations in the variant’s genome that are unrelated to the spike protein.

“Spike is a key one,” Krogan tells Verywell. “We have a lot of reason to believe that mutations in these other non-spike proteins are also playing a role in making some of these variants more transmissible and ultimately more deadly.”

Scientists are still trying to understand Delta’s unique combination of genetic mutations. For instance, it lacks two key mutations that made the Alpha, Beta, and Gamma variants particularly invasive, Kaiser Health News reported. And it contains a unique mutation in an area of the genome that allows the virus to fuse with human cells.

Krogan thinks that Delta is able to suppress the immune system once it's inside human cells. In a preprint study, his team found that non-spike mutations in the Alpha variant allowed the variant to better evade immune responses. Krogan says Delta may operate in a similar way—by infecting cells without triggering the body’s alarm system, giving it opportunity to create more viral copies before the immune response stops it.

“If we can't mount a response as effectively, the virus will grow more and then it'll spread more,” Krogan says. “That will ultimately contribute to, at least indirectly, increased mortality.”

Studying the purpose of the virus’ various proteins and the mutations found in each variant will help scientists better respond to future variants, he says.

Vaccines Remain the Best Protective Tool

Existing COVID-19 vaccines largely remain effective at preventing serious sickness and death from Delta. Two doses of the Pfizer vaccine protect 88% of people from any symptomatic infection by Delta, compared to 94% efficacy against Alpha, for instance.

Vaccinated people can transmit the virus to others.  But such breakthrough cases are rare and the CDC and other health experts still say that vaccination is the best tool to protect oneself and others from severe illness and death from COVID-19. Masks are also a key tool to keep cases down.

In the U.S., fewer than half of adults are fully vaccinated. Krogan says he worries that low vaccination rates will allow the virus to evolve into new, potentially more dangerous forms.

“Not only can you help yourself, but you can help the species,” Krogan says. “The more people this virus goes through, the more chance it has to mutate around some of our preventative measures.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.