Delta Variant Becomes Dominant in the U.S., CDC Estimates

Conceptual paper illustration of human hands and coronavirus in a lab.

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Key Takeaways

  • The Delta variant, which originated in India, now accounts for the most U.S. COVID-19 cases.
  • Studies estimate Delta is 30-60% more transmissible than Alpha, the previously dominant variant.
  • Vaccination remains the best tool to combat the spread and evolution of viral variants.


In June, the Delta variant accounted for a mere 10% of infections in the United States. Now, only a month later, it is responsible for 51.7 % of infections.

The new variant, which was first detected in India, may be 40 to 60% more contagious as Alpha, the variant originally detected in Britain. Alpha, which became dominant in the U.S. this spring, now accounts for 28.7% of infections.

Delta’s rapid spread leaves some health experts worried about surging cases in some pockets of the U.S. Around 1,000 counties currently have vaccination rates below 30%, Rochelle Walensky, director for the CDC said last week. Experts say that the easily transmissible variant makes unvaccinated groups increasingly vulnerable to COVID-19 infection.

“The fact that we're here on July 7, and more than 50% of the isolates are Delta, it is eye-opening and it underscores just how much more transmissible this is than the previously dominant variant,” F. Perry Wilson, MD, MSCE, Associate Professor of Medicine at Yale School of Medicine, tells Verywell. 

As the virus continues to evolve and spread, Wilson says the risk to unvaccinated “is increasing day by day.”

A More Transmissible Variant

As SARS-CoV-2, the virus responsible for COVID-19, spreads, it mutates and creates new variants. Such genetic changes allow the Delta variant to better infect people.

“You see how quickly a virus, like the Delta strain of the virus, can be found in India. And then it's knocking on our doorstep within a few weeks to a month,” Mehul Suthar, PhD, assistant professor at the Emory Vaccine Center, tells Verywell. “This is just truly remarkable.”

Compared to the Alpha variant, scientists estimate that the Delta variant is up to 60% more transmissible. Studies also show that it may better take hold of the immune system—one dose of the vaccine appears to offer only weak protection against the variant.

More Symptomatic Cases 

For the most part, the vaccines appear to remain effective against the Delta variant. The Pfizer vaccine appears to be 88% effective at protecting against symptomatic disease caused by Delta, according to a pre-print study from May.

Most vaccinated people who experience symptomatic COVID-19 likely have mild symptoms, like a runny nose and a low-grade fever. However, these people can carry more of the virus than those with asymptomatic infection, making it more likely they can spread it to others.

In the U.K., Delta accounts for nearly all infections. But while case rates have increased among all demographic groups, Public Health England reports that there is no surge in the rate of hospitalizations.

“It's easier to get, but once you've got it, there’s maybe a slight increase in the risk of being hospitalized, but probably not a dramatic increase in risk of death,” Wilson says.

Surges May Happen in Pockets

As of Thursday, 67.3% of American adults have received at least one vaccination and 47.7% are fully immunized, says the CDC.

But herd immunity, Wilson says, is a local phenomenon. People tend to interact more with others in their community than with people from different counties or states. So, in counties where only a fifth of residents are vaccinated, the virus is more likely to spread than in those with higher immunization rates. 

“Those counties and local areas with relatively low vaccination rates are essentially like tinder sitting on the forest floor with sparks flying around,” Wilson says.

In these areas, where the majority of the population remains vulnerable to infection, Suthar foresees a strain on local health services.

“If you think about where these pockets are—in rural areas in various states—they may not have large hospitals,” Suthar says. “This virus may certainly inundate these hospitals.”

Should You Be Concerned?

The vaccines authorized in the U.S. appear to all be effective at protecting fully vaccinated people against serious illness and death. While the Delta variant appears to cause more cases of symptomatic infection in vaccinated people compared to most previous variants, there does not seem to be a greater risk of hospitalization or death. 

Suthar says it may be premature to reinstate interventions like mask mandates or lockdowns. But it’s also important to be situationally aware. If you are entering a crowded space or live in an area with low vaccination rates, he recommends wearing a mask and being careful about your interactions with others.

For unvaccinated people, the risk is greater. With greater transmissibility comes greater chances of getting sick if you are exposed. Experts say the best tool to protect yourself and others from COVID-19 is to get fully vaccinated.   

Plus, until transmission is further reduced, the virus will still have opportunities to evolve into new, potentially more dangerous variants.

 “It's a race against time,” Wilson says. “The way you stop variants getting produced is you stop the chain of infection, and the best way we have to stop the chain of infection is vaccination.”

What This Means For You

Experts say vaccination is the best way to protect yourself and others from the spread of all COVID-19. Studies show that the approved vaccines appear to be effective at preventing most serious disease and death against the COVID-19 variants circulating in the U.S. Visit for information on how to schedule a vaccination appointment near you.  

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.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. COVID Data Tracker.

  2. European Centre for Disease Prevention and Control. Implications for the EU/EEA on the spread of the SARSCoV-2 Delta (B.1.617.2) variant of concern - 23 June 2021. ECDC: Stockholm.

  3. Bernal JL, Andrews N, Gower C, et al. Effectiveness of COVID-19 vaccines against the B.1.617.2 variant. medRxiv. doi:10.1101/2021.05.22.21257658

  4. Public Health England. Weekly national Influenza and COVID-19 surveillance report.

By Claire Bugos
Claire Bugos is a health and science reporter and writer and a 2020 National Association of Science Writers travel fellow.