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3 Things to Know About the Delta Variant, According to an Infectious Disease Expert

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Robert L. Quigley, MD, DPhil, is the Senior Vice President and Global Medical Director, Corporate Health Solutions at International SOS & MedAire. After 25 years working in surgery, critical care, and immunology, he's using his expertise to advise on crisis management, infectious disease, and health care. Here, he shares what people in the U.S. should know about the Delta variant.

The Delta coronavirus variant is making headlines daily, posing threats to unvaccinated communities and fully-vaccinated individuals alike. But it's not exactly a surprise to the scientific community. Delta is behaving in the same way that all viruses do: by replicating and mutating within host cells. 

The way a virus infects someone is by invading a cell in their body, which provides the virus with "machinery" to survive and replicate. Each time the virus replicates, there is a risk of mutation. Some of these mutations are insignificant and cause no additional harm, but other mutations may enhance the virus in ways that make it more infectious. For instance, It may become better at attaching to cells or start replicating faster. These more dangerous mutations can result in a person becoming ill more quickly and severely, as is the case with Delta.

The ultra-transmissible variant has spread to at least 90 countries and is estimated to make up over half of U.S. cases. In mid-May, the Delta variant accounted for only 2.5% of U.S. cases, but that number rose to 31% by mid-June. Because Delta accounted for 90% of COVID-19 cases in the U.K. in early June, experts anticipated the spike would make its way stateside, too.

As scientists learn more about how the COVID-19 virus mutates and spreads, here are three things you should know about the Delta variant. 

Delta Seems More Easily Transmissible and Harder to Combat

The way the Delta variant has mutated has caused the surface of the virus to change, which can affect how the virus attaches to other cells. This means that it may be more difficult for existing antibodies, whether from previous COVID-19 infection or vaccination, to tightly bind to the virus and effectively neutralize the new infection.

Similarly, drugs designed to treat COVID-19 infection can be less effective at fighting this variant. The Delta variant’s mutated surface has the potential to allow the virus to tightly attach to target receptors in the body, which is what makes the virus highly contagious.

Complete Vaccination Is Essential

Research suggests those who are fully vaccinated are not as protected against the Delta variant as they may be against other strains of COVID-19.

Specifically, recent reports indicate that following the two doses of the Pfizer and AstraZeneca vaccines, recipients were respectively afforded 88% and 66% protection against symptomatic disease from the Delta variant. These results represent a slight decrease in efficacy when compared to rates seen with the less fatal variants, like the Alpha variant. 

However, a single dose of either vaccine was only 33% effective at protecting people from symptomatic disease from the Delta variant. One study found that people who received only one of the two recommended doses for the AstraZeneca and Pfizer vaccines had antibody responses that “barely inhibited variant Delta.”

There is a growing concern that this variant may be responsible for breakthrough infection in fully-vaccinated people. Still, a two-dose regimen offers significantly better protection than partial or no vaccination.

Young People Are Taking the Biggest Hit

The Delta variant appears to be disproportionately spreading in people 12–20 years old. But thanks to the May authorization of the Pfizer vaccine in ages 12-15, this group can now be protected.

Obviously, that still leaves an exposed population: children under 12 years of age. We all are patiently awaiting the results of randomized placebo studies involving children younger than 12 to determine both the safety and efficacy of the vaccines in this group.


The prudent way to combat the Delta variant, and any other strains of COVID-19, is through vaccination. The message is simple: Get fully vaccinated against this evolving virus. It will help protect those who can't yet.

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.

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