Why the Delta Variant Is So Contagious, According to Infectious Disease Doctors
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A leaked CDC document revealed the Delta coronavirus variant is just as contagious as chickenpox.
The CDC has acknowledged that both vaccinated and unvaccinated people can carry and transmit similar viral loads of the Delta variant.
Infectious disease experts explain why the Delta variant is so contagious and what this means for the future of the COVID-19 pandemic.
The Delta variant has quickly become the dominant coronavirus strain in the United States, according to data from the Centers for Disease Control and Prevention (CDC). Delta was responsible for up to 82.2% of COVID-19 cases as of July 17—and that number is likely higher now.
A leaked CDC document shows this variant is unlike others that came before it, because it’s incredibly contagious. The document, which was obtained by The Washington Post, revealed the Delta variant is more infectious than the viruses that cause MERS, SARS, Ebola, the common cold, the seasonal flu, and smallpox. In fact, the Delta variant is as contagious as chickenpox, which used to spread wildly in young children before a vaccine was developed for the virus. The document also stated that it’s important for public health officials to “acknowledge the war has changed.”
CDC director Rochelle Walensky, M.D., said last week that vaccinated people with breakthrough infections of the Delta variant—meaning they have detectable levels of the virus in their body at least 14 days after full vaccination—carry similar viral loads in their nose and throat as those who are unvaccinated, suggesting they can spread COVID-19 as easily as those who are unvaccinated, even with mild or no symptoms.
Wait, why is the Delta variant so contagious?
It’s important to remember the original SARS-CoV-2 strain “was in the range of common cold-causing viruses,” explains Amesh A. Adalja, M.D., an infectious disease expert and senior scholar at the Johns Hopkins Center for Health Security.
He says it’s “not surprising” that the Delta variant is as contagious as chickenpox, as “it is about twice as contagious as the Alpha variant.” Alpha, originally known as a B.1.1.7, first emerged in the U.K. in September 2020, per the CDC. It was associated with “more efficient and rapid transmission” and became a common variant in the U.S. after it was first detected in Colorado.
The Delta variant has several mutations on its spike protein, the crown-like piece of the virus that latches onto a person’s cells, explains John Sellick, D.O., an infectious disease expert and professor of medicine at the University at Buffalo/SUNY. Thus, Delta “binds very tightly” compared to other variants, he says, allowing it to “stay attached very well” once it gets ahold of cell receptors.
“Viruses evolve over time—that’s just part of the natural history of evolution,” says Shobha Swaminathan, M.D., associate professor in the Division of Infectious Diseases at Rutgers New Jersey Medical School. “Delta is much stickier compared to the previous variants, so it’s able to infect the host cells in a much more effective way, and with fewer viral particles.”
What does this mean for vaccinated and unvaccinated people?
If you’re unvaccinated, experts say you need to take extra precautions to stay safe. “This level of contagiousness means that if you are unvaccinated, the Delta variant will likely infect you,” Dr. Adalja says.
“You are very much at risk,” Dr. Sellick agrees, noting that the areas of the country with the highest COVID-19 cases right now also have the lowest vaccination rates.
If you’re vaccinated, there is a rare chance that you could still become infected with the virus and pass it on to others, including vulnerable people in your life who are at a higher risk for severe COVID-19 complications. That’s why the CDC now recommends that everyone, including vaccinated people, mask up indoors in areas where the spread of COVID-19 is “substantial or high.”
However, if you do happen to become infected after being fully vaccinated, you have a great chance of getting through the illness without severe symptoms or hospitalization. To put it in perspective: More than 97% of hospitalized COVID-19 patients are unvaccinated, Dr. Walensky confirmed in a recent briefing.
“In general, breakthrough infections tends to be milder and the symptoms tend to be shorter,” Dr. Swaminathan says.
How long are you contagious after you become infected with the Delta variant?
There hasn’t been any data to suggest that the amount of time that you’re contagious is any different with Delta compared to other coronavirus variants, Dr. Sellick says. As a result, you’re considered infectious for up to 10 days after you first started showing symptoms, per the CDC. If you test positive for COVID-19 without having symptoms, the CDC still recommends that you isolate for 10 days after you test positive.
What does this mean for the future? Will we ever get rid of COVID-19?
Infectious disease experts expect that COVID-19 will be sticking around. “This virus is going to become part of our ecology,” Dr. Sellick says. “Our chance of getting rid of it was a year ago. We fumbled that terribly.”
There is some hope, though: Dr. Adalja says that COVID-19 may simply become more like the common cold over time, becoming more prevalent during cold and flu season. New variants are also to be expected until a higher percentage of the country’s (and world’s) population is vaccinated, Dr. Sellick says. That’s why there may be a need for booster shots in the future.
We should be able to keep the pandemic in a place that “won’t cause the same levels of illness and death that we’ve been seeing,” Dr. Sellick says. “If we can keep people out of the hospital, that’s a good thing.”
To get to there, though, more people need to get vaccinated, even though it’s unclear if we’ll ever reach herd immunity as mutations continue to develop.
“For those of you on the fence, please get vaccinated,” Dr. Swaminathan urges. Beyond protecting yourself, you’ll protect those around you, including friends, family, and those who are most vulnerable to serious effects from the virus.
This article is accurate as of press time. However, as the COVID-19 pandemic rapidly evolves and the scientific community’s understanding of the novel coronavirus develops, some of the information may have changed since it was last updated. While we aim to keep all of our stories up to date, please visit online resources provided by the CDC, WHO, and your local public health department to stay informed on the latest news. Always talk to your doctor for professional medical advice.
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