Covid-19 Delta Variant Emerges as Primary Threat Around the World

A surge of cases in the United Kingdom suggests that Delta is the most transmissible variant yet identified

An Indonesian doctor injects a dose of Sinovac COVID-19 vaccine to a man's upper arm.
Officials at the U.S. Centers for Disease Control and Prevention anticipate that Delta will become the dominant variant in the United States within the next few months. Photo by Afriadi Hikmal/NurPhoto via Getty Images

Surges in Covid-19 cases around the world have been linked to the highly transmissible Delta variant of the pandemic coronavirus.

The Delta variant has contributed to rising cases in India, Nepal, the United Kingdom, Portugal and Australia, reports Ewen Callaway for Nature News. In the United States, the variant makes up about 20 percent of genetically analyzed coronavirus cases, a rate that has doubled in the last two weeks. Studies have shown that vaccines are effective at preventing Covid-19 symptoms from the Delta variant, but in places where vaccines are not yet widely available, or in communities where people have not gotten vaccinated despite availability, the variant could spread rapidly.

At a White House briefing on Tuesday, National Institute of Allergy and Infectious Diseases (NIAID) director Anthony Fauci said the Delta variant presents the “greatest threat in the U.S. to our attempt to eliminate Covid-19,” Emily Anthes reports for the New York Times.

Computer models that predict how the coronavirus could spread suggest that a variant like Delta could cause Covid-19 cases to rise in the U.S. later this year.

Because Delta is more transmissible, "it looks like we do see a resurgence late in the summer, or in the early fall," says Johns Hopkins Bloomberg School of Public Health epidemiologist Justin Lessler to CNN’s Michael Nedelman and Nadia Kounang.

The Delta variant was first identified in India when the country faced a wave of Covid-19 cases in the spring, but it was difficult for researchers to study the characteristics of the variant amid other driving factors of the rising cases, like large gatherings, reports Nature News.

Analysis of the variant’s rise in the United Kingdom, however, shows that Delta is 60 percent more transmissible than the Alpha variant, the country’s previous dominant type of the coronavirus. The Alpha variant was itself about 50 percent more transmissible than the initial pandemic coronavirus, per Beth Mole at Ars Technica.

The Delta variant was first identified in the U.K. in late April, and has been separately introduced to the country about 500 times, per BBC News’ Rachel Schraer. Now it accounts for 99 percent of sequenced coronavirus cases in the nation, reports NPR’s Jaclyn Diaz.

“The data coming out of the UK is so good, that we have a really good idea about how the Delta variant is behaving,” says Aalborg University bioinformatician Mads Albertsen to Nature News. “That’s been an eye-opener.”

Officials at the U.S. Centers for Disease Control and Prevention anticipate that Delta will become the dominant variant in the United States within the next few months, report Madeline Holcombe and Jay Croft at CNN.

A study published in May from Public Health England shows that Pfizer/BioNTech’s Covid-19 vaccine is 88 percent effective at preventing symptomatic illness associated with the Delta variant after a person receives both doses. A single dose of the vaccine is only 33 percent effective on its own.

“Fully immunized individuals should do well with this new phase of the epidemic,” says Baylor College of Medicine pediatrician and vaccine researcher Peter Hotez to the New York Times. “However, the protection offered by a single dose appears low, and of course if you are not at all vaccinated, consider yourself at high risk.”

In the U.S., about 53 percent of eligible people (those 12 years old and older) have been fully vaccinated against Covid-19, according to the CDC. But vaccination rates now vary by region—for example, in Missouri, just 38 percent of people are fully vaccinated, per CNN.

Countries with limited vaccine availability will face the greatest risk from the Delta variant, reports Nature News. Many nations in Africa have vaccinated less than five percent of their populations, and the Delta variant has been detected in the Democratic Republic of the Congo, Malawi, Uganda and South Africa.

“The vaccines will never come in time,” says Catholic University of Leuven evolutionary biologist Tom Wenseleers to Nature News. “If these kinds of new variant arrive, it can be very devastating.”