The Delta Variant: What You Need to Know

An infectious disease specialist discusses the dangers of the more transmissible coronavirus variant and the best way to protect ourselves.

After months of declining infection rates in the United States, COVID-19 cases are on the rise again due to the highly contagious Delta variant.

“Delta currently accounts for 99% of new U.S. COVID-19 cases and is the variant that is increasing the fastest throughout the country,” says Dr. Roy M. Gulick, chief of the Division of Infectious Diseases at NewYork-Presbyterian/Weill Cornell Medical Center and professor of medicine at Weill Cornell Medicine. “We’re seeing a rise in cases, hospitalizations, and deaths, and the unvaccinated are especially vulnerable.”

On July 27, the CDC updated its guidance for fully vaccinated people given new evidence on the transmissibility of the Delta variant, adding a recommendation for fully vaccinated people to wear a mask in public indoor settings in areas of substantial or high transmission. Fully vaccinated people may choose to wear a mask in these settings regardless of the level of transmission, particularly if they are immunocompromised or at increased risk for severe disease from COVID-19, or have someone in their family who is.

Dr. Gulick spoke with Health Matters about the Delta variant — the danger it poses, why it’s more transmissible, and how we can defeat it.

Dr. Roy Gulick

Dr. Roy M. Gulick

Health Matters: What is the Delta variant?
Dr. Gulick: Like other viruses, coronaviruses, including SARS-COV-2, the virus that causes COVID-19 can change (or mutate) over time. This leads to different viral variants, and these have emerged in different parts of the world. The variants originally were named for their place of origin and with letters and numbers, but the World Health Organization now names them with Greek letters.

The Delta variant was originally called B.1.617.2 and was first identified in India in late 2020 and is spreading rapidly throughout the world. It is now the dominant viral strain in the United Kingdom and the U.S.

The CDC has labeled the Delta variant as a “variant of concern.” What does that mean?
SARS-COV-2 variants can have different properties than the original virus. Variants of concern are those viruses that can be transmitted between people more easily, cause worse COVID-19 illness, are less susceptible to antibodies resulting from natural infection or vaccination, are less susceptible to COVID-19 treatment or prevention strategies, or are more difficult to detect.

The Delta variant is more transmissible and may be associated with more severe COVID-19 illness, and this has led to a COVID-19 surge in unvaccinated populations.

What makes a variant more transmissible?
Some viral variants can grow better in human cells and make greater quantities of the virus — the higher levels of virus can increase the efficiency of transmitting to other people. The Delta variant is about 50% more transmissible than the Alpha variant (originally identified in the United Kingdom) and about twice as contagious the original SARS-CoV2 strain. One study found that people with Delta may carry 1,000 times more virus  in their nose and mouth than with the original strain – more virus means easier transmission to others.

Do the vaccines offer protection against the Delta variant?
Full vaccination with the mRNA vaccines (Pfizer and Moderna) protects well against the Delta variant and the other common variants. (In fact, a recent study found that two doses of the Pfizer vaccine were 88% effective at protecting against symptomatic disease caused by Delta.) The Johnson & Johnson vaccine likely also offers protection, though possibly at a lower rate. According to the CDC, more than 97% of those hospitalized with COVID-19 infection and more than 99% of COVID-19 deaths currently are in unvaccinated people.

How do we stop the Delta variant from spreading?
Vaccines are the best tool we have to end the COVID-19 pandemic. People who can’t get vaccinated or don’t respond to vaccinations (for example, people with compromised immune systems) should continue to wear a mask, wash their hands, and practice social distancing.

We saw the CDC make the recommendation that people who live in areas of high transmission mask indoors. This is a reaction to the increasing spread of the Delta variant and the fact that after months of declining cases, U.S. COVID-19 cases, hospitalizations, and deaths are increasing again, especially in states and areas with low vaccination rates.

Viral variants can occur when the virus is transmitted and reproduces itself. COVID-19 vaccines are key to controlling the pandemic and to prevent future variants from developing. Current vaccines are highly effective and not only prevent infection, but they also effectively reduce transmission to others.

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