Why Is the New Delta COVID Variant So Contagious?

And does the vaccine protect against the Delta variant? Here, everything you need to know about the latest strain to sweep the globe, including the Delta variant symptoms and how it differs from the Delta-plus variant.

With more Americans getting vaccinated against COVID-19, you're likely eager and excited to see loved ones, travel, and enjoy a new, post-COVID "normal." While all of that signals good news for fully vaccinated folks, one potentially worrying coronavirus variant on the rise — the Delta or B.1.617.2 variant — may complicate that bright-looking future. The new Delta variant, which was first detected in India in February (and for that reason was formerly called the "Indian Variant"), has spread to more than 80 countries since originally being identified and to almost every state in the U.S. Now, health expert are noting this particularly infectious strain could be the cause of a new epidemic if global cases continue to spike.

Here's everything you need to know about the Deltavariant, including the Delta variant symptoms and answers to questions such as, "does the vaccine protect against the Delta variant?" (Read more: Why Are the New COVID-19 Strains Spreading More Quickly?)

Why Is the New Delta COVID Variant So Contagious?
Getty Images

What Is the COVID-19 Delta Variant?

The emergence of COVID-19 variants is nothing new or unexpected — all viruses change through mutation, according to the Centers for Disease Control and Prevention, which notes that sometimes variants emerge and then disappear while others persist. Scientists can then study each variant and its potential impacts on both vaccinated and unvaccinated people.

So far, the CDC is "monitoring multiple variants," with the Delta variant being among the most recent addition to the bunch. It's now the most dominant strain in the United Kingdom, accounting for more than 90 percent of coronavirus cases there, according to U.K. health secretary Matt Hancock, as reported by the BBC. In the U.S., the Delta variant now accounts for more than 20 percent of all cases, Anthony Fauci, M.D., director of the U.S. National Institute of Allergy and Infectious Diseases, said during Tuesday's White House COVID-19 Task Force Briefing. The prevalence of the Delta variant in the U.S. has doubled in the past two weeks and, according to Dr. Fauci, is now "the greatest threat" to eliminating COVID-19 in the country.

You might be wondering how and why this variant is so contagious, and if you should be worried about it if you're already two weeks out from your final shot. A new study published in The Lancetsuggests the Delta variant is linked to an 85 percent higher risk of hospitalization than other strains, and is 60 percent more transmissible than the Alpha (B.1.1.7) variant, which had first been seen in the U.K. and was the dominant strain — until Delta came along.

Scientists are still investigating why this strain, in particular, seems to be so contagious, but they believe it has to do with the mutated virus having better binding capabilities — i.e. once the virus makes its way to a person's cells, it's better able to stick to them. It also seems that infected patients have a higher viral load (aka the amount of virus in their blood), thus shedding more of the virus and making it spread more rapidly and easily. Once infected, this strain also seems to replicate more efficiently, causing more symptoms than patientsmight have otherwise had if they'd been infected with another strain. (

Another differentiating factor with the new strain is the particular symptoms associated with it. Doctors have seen an increased likelihood of hearing loss, severe stomach pains, and nausea in patients confirmed to have the Delta variant, Bhakti Hansoti, M.D., an associate professor of emergency medicine and international health at Johns Hopkins University and Bloomberg School of Public Health, told USA Today. Even scarier? Dr. Hansoti shared that in "most" cases, patients are more likely to be hospitalized, require oxygen treatments, and experience other complications.

It's important to note, however, that there is not yet enough research to confirm these Delta variant symptoms and that new strain does in fact cause more severe effects. What's more, the severity of COVID-19 — and thus the symptoms of the Delta variant or any other strain for that matter — can vary from person to person. (

Do the Vaccines Protect Against the Delta Variant?

While this all sounds worrisome, there's some good news for fully vaccinated folks. A recently released study from Public Health England showed that two doses of the mRNA vaccines (Pfizer and Moderna) were 88 percent effective against symptomatic disease from the Delta variant two weeks after the second dose. This is slightly less than the 93 percent effectiveness against the Alpha variant but still very high. One dose of a Pfizer vaccine still offered 33 percent protection, making for another strong case to return for your second dose and maintain COVID safety measures until you're safely two weeks out from either inoculation. The AstraZeneca vaccine (used widely in Europe) was found to be 60 percent effective two weeks after the second dose. Though the single-shot Johnson & Johnson wasn't tested in the study, Scott Gottlieb, M.D., the former commissioner of the Food and Drug Administration, told CBS' Face the Nation on June 13 that he believes it will offer similar protection to AstraZeneca or about 60 percent effectiveness. (

What Is the Delta-Plus Variant?

While the Delta variant's been busy becoming the most dominant strain in the U.K. — and soon enough, according to the World Health Organization, worldwide — the Delta-plus variant popped up in India.

The Delta-plus variant is believed to be a near carbon copy of its predecessor — just with a spike protein mutation known as K417N, according to Reuters. Sound familiar? That's because the K417N mutation is also present in the Beta variant (the strain first discovered in South Africa) and seems to allow the virus to spread more readily and infect cells. As of right now, it's still unknown whether the mutation will impact the Delta-plus variant in the same manner. But, according to statements from the Indian government, the Delta-plus strain is believed it be highly transmission. Keyword being "believed." Benjamin Pinsky, M.D., director of Clinical Virology Laboratory at Stanford University, told the Los Angeles Times that there's still not enough information available and more research is needed to confirm that the Delta-plus variant is worse than any other strains circulating.

Bottom Line

So even though you might be more than ready to say farewell to the pandemic, it still seems to be far from over, especially for those who aren't fully vaccinated yet and for people in countries where the vaccine isn't widely available. (

"We need masking in public areas, limited gathering sizes, and increased scrutiny in schools and public spaces where people can be symptomatic," Dr. Hansoti told USA Today. "If not, after the Delta variant, another variant will just come and surge again."

The information in this story is accurate as of press time. As updates about coronavirus COVID-19 continue to evolve, it's possible that some information and recommendations in this story have changed since initial publication. We encourage you to check in regularly with resources such as the CDC, the WHO, and your local public health department for the most up-to-date data and recommendations.

Updated by Elizabeth Bacharach
Was this page helpful?
Related Articles