Updated on July 30, 2021.
The Delta variant, now the dominant strain of COVID in the United States, appears to spread as easily as chickenpox (varicella-zoster), according to media reports, citing a new internal report at the U.S. Centers for Disease Control and Prevention (CDC).
Health officials warn Delta is also more likely to cause severe disease and result in “breakthrough” infections among those who have been vaccinated than other COVID strains. Although vaccinated people are unlikely to become seriously ill, there is some evidence that they could still spread the Delta variant to others, particularly those who are unvaccinated, the CDC cautions.
On July 27, the CDC reversed course on its guidance about wearing masks, advising everyone—regardless of their vaccination status—to wear masks indoors in areas where COVID-19 transmission rates are high.
Delta has shown a willingness to be an opportunist, said CDC director, Rochelle Walensky, MD, MPH in a July 26th news briefing. New data shows that Delta behaves “uniquely different from past strains of COVID,” Dr. Walensky explained, noting that some immunized people infected with Delta after vaccination may be contagious and spread the virus to others.
Delta vs chickenpox vs measles
The original COVID strain was known to spread very easily—even more efficiently than the flu but, but it wasn’t as transmissible as some of the most contagious viruses, like chickenpox or measles. The Delta variant is a different story. The CDC warns that this variant is not only more infectious than the original strain, but also more severe.
Research shows that about 90 percent of unvaccinated people will get chickenpox once exposed to someone with the varicella-zoster virus. Measles is also so contagious that if one person has it, up to 90 percent of the unvaccinated people who come into close contact with that person will also become infected.
Why this is even more concerning? The goalposts for herd immunity have shifted. Herd immunity occurs when a large portion of a population becomes immune to a disease—either through infection or immunization—making it unlikely that it will spread from person to person.
But the more contagious a disease is, the greater the number of people who need to be vaccinated to stop it from spreading. For example, it’s estimated that 94 percent of the population needs to be immune to measles to halt its spread in the community.
By comparison, as of July 29, only about 50 percent of the U.S. population is fully vaccinated against COVID-19 and about 57 percent are partially vaccinated with at least one dose. And some 30 percent of eligible U.S. adults are still not immunized against the disease, the CDC reports.
Delta on the rise
The Delta variant, a highly infectious coronavirus strain, now represents 83 percent of new COVID-19 cases in the United States, according to Centers for Disease Control and Prevention (CDC) director, Rochelle Walensky, MD, MPH.
"This is a dramatic increase, up from 50 percent for the week of July 3," Dr. Walensky said in a July 20 Senate committee hearing. "In some parts of the country the percentage is even higher, particularly in areas of low vaccination rates."
One-quarter of the global population has received at least one dose of a vaccine against COVID-19, including more than 189 million people in the United States as of July 29. In the U.S., cases have dropped dramatically from their January peak. But roughly one-third of eligible adults remain unvaccinated.
That’s worrisome because it renders them vulnerable to the Delta variant, which originated in India, hit the United Kingdom hard and has since been detected in all 50 states.
Also called B.1.617.2, Delta is poised to spread around the world, outcompeting other strains. At greatest risk: unvaccinated people.
World Health Organization officials have called Delta the “fastest and fittest,” warning that it will “pick off the more vulnerable more efficiently than previous variants.” Delta’s speedy and efficient transmission could hamper the United States’ progress in halting the pandemic.
Here’s what you need to know about Delta, including how to help protect yourself.
It’s here and highly infectious
Delta is now the most common U.S. strain of the coronavirus that causes COVID-19, the CDC reports. Missouri, Arkansas, Nevada, Colorado and Utah are among the states seeing high COVID case numbers due to Delta. Already, hospitalizations and deaths are on the rise among people who have not been vaccinated, according to Walensky.
Viruses’ genetic material mutates all the time as they replicate in an infected person. Variants that make the virus more effective are likelier to persist and spread.
Most SARS-CoV-2 mutations are either harmful to the coronavirus or have no effect either way.
But the Delta variant is at least twice as infectious as initial COVID strains, according to former CDC director, Tom Frieden, MD, MPH. That explains why highly vaccinated countries, like Israel, are still seeing a rise in COVID cases, Dr. Frieden explains.
Yale epidemiologist F. Perry Wilson has warned that Delta will speed up the pandemic, noting that while a person infected with the original coronavirus strain might infect 2.5 other people in the same room, a Delta infection could reach up to four other people.
Some evidence also suggests Delta is more dangerous than earlier strains. A study in Scotland found that people infected with Delta were more likely to be admitted to the hospital compared with Alpha, a previous variant of concern.
Vaccines offer protection
Existing coronavirus vaccines are highly effective against COVID-19. But they weren’t designed to fend off Delta specifically. Meanwhile, there is some evidence that the Delta variant might evade the neutralizing antibodies our immune systems form in response to vaccination, which could result in breakthrough infections (cases of fully vaccinated people becoming infected with COVID-19).
But just how much is unclear.
Ongoing research suggests existing vaccines effectively prevent severe Delta variant cases, leading to hospitalization or death, according to Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases (NIAID).
As Dr. Fauci told ABC News, “The bad news is that we have a very nasty variant. And the good news is that we have a vaccine that works against it.”
One study, not yet peer-reviewed, found that antibodies formed after vaccination with two doses of the Pfizer vaccine work to neutralize delta nearly ten times less effectively than they do against the original, “wild-type” Wuhan strain. Another study of 250 people suggested a still-concerning almost sixfold reduction.
But another May 2021 study, also not yet peer-reviewed, found that if a person is exposed to Delta, the effectiveness of one dose of the Pfizer vaccine has been estimated at 33.5 percent, and the effectiveness of two doses is estimated at 87.9 percent.
An Israeli study also reported that while vaccines were only 64 percent effective against Delta at preventing both infection and symptomatic disease, they were 93 percent effective at preventing hospitalizations and serious illness. Similarly, a preliminary study of 14,019 symptomatic Delta infections in England found that vaccine effectiveness against hospitalization was 94 percent after one dose and 96 percent after two doses of Pfizer.
Overall, breakthrough cases remain rare. And even though some early studies suggest the vaccines produce fewer antibodies against some newer variants, the vaccines are still highly effective thanks to other important responses by the immune system.
The body has other ways of defending itself. T-cells and B-cells that the immune system deployed to fight off the infection (or that a vaccine has taught the body to make) learn from experience and can be stored by the immune system for years, where they remain ready to do battle if the virus ever returns.
Unvaccinated people are most vulnerable
Many unknowns remain, including exactly how long immunity from the vaccines will last and whether booster shots will be needed to maintain protection.
Moderna and Pfizer are both developing COVID booster vaccines. Pfizer is reportedly meeting with U.S. health officials about potential authorization of such shots. But both the CDC and FDA say boosters are not currently needed.
As Delta causes a surge of infections among unvaccinated people, on July 14 the WHO director-general urged the global community to shift the focus from booster shots for wealthier nations to immunizations in poorer countries with low vaccination rates.
You’re much less likely to get very sick or die from Delta or other strains of coronavirus if you’re vaccinated. Some 99.5 percent of all COVID-19 deaths are among unvaccinated people, according to Fauci.
Younger people may be at higher risk. In the Scottish study, Delta was found mainly in younger groups of people. Though children aren’t inherently more susceptible to Delta than other people, they’re still more likely to catch Delta than they were to get earlier variants, simply because Delta is so much more transmissible, Fauci explains.
Fauci noted that it’s understandable that some people have chosen not to vaccinate because the FDA has not yet fully authorized the vaccines but added that he is certain full approval will come, thanks to abundant supportive data.
“Full approval, that's really only a technical issue. It's the FDA dotting the i’s and crossing the t’s,” Fauci told ABC News.
The pandemic isn’t over
Although it isn’t perfect, vaccination is a safe and effective way to protect yourself from the Delta variant. Even a partially effective vaccine is valuable and will help control the pandemic.
For example, a flu vaccine that’s only 20 percent effective can still save 21 million infections and nearly 62,000 lives if it’s administered to 43 percent of the people who should receive it, researchers have calculated.
As the virus continues to evolve in unvaccinated populations, more variants are emerging. (For instance, scientists are monitoring a new variant called Lambda, or C.37, which arose in South America in December 2020.) It’s crucial that the world avoids new variants that evade even the best current vaccines and cause widespread reinfection.
That possibility underscores the urgent need to vaccinate everyone who is eligible.
In short, now is a good time to get vaccinated if you haven’t already—not merely to protect yourself, but to reduce the chances that more dangerous variants will emerge.
In addition to vaccination, masking remains protective, especially in high-transmission areas. “Fully vaccinated people don’t need to mask in many places,” Frieden tweeted, “but it doesn’t hurt.”
