(Coronavirus (COVID-19) Delta Variant

Coronavirus (COVID-19) Delta Variant

Find up-to-date information on the COVID-19 Delta variant.

Vaccines play a key role in protecting against the Delta variant

We’ve put together commonly asked questions to help you understand what the Delta variant is, how it spreads, and how getting vaccinated can help protect us all.

According to the Centers for Disease Control (CDC), more than 99% of all new cases1 in the United States are caused by the Delta variant, a highly contagious strain that was first identified in India last year. Dr. F. Perry Wilson, a Yale Medicine epidemiologist, estimated that this strain of the COVID-19 virus is roughly 50% more contagious than the previously dominant Alpha strain.2 This mutation, while alarming, is not surprising, as viruses will continue to mutate in order to replicate and survive.

The Delta variant appears to be more transmissible than many other viruses, including those that cause MERS, SARS, Ebola, the common cold, the seasonal flu, and smallpox, and is almost as infectious as the chickenpox3 according to unpublished data from the CDC.

Getting vaccinated remains the best way to protect yourself and others from the virus. And given the Delta variant’s transmissibility and current nationwide vaccination rates, we all must take greater precautions to stay safe.

The CDC recommends that everyone—vaccinated or not—should wear a mask indoors or in public places in areas that are experiencing high transmission rates.4 This is because the Delta variant produces exponentially higher viral loads in infected people. Masks add an added layer of protection on top of vaccination under conditions where Delta may be circulating.

While incidence rate of COVID-19 infection, hospitalization, and death is higher in unvaccinated than vaccinated people,5 we are still learning how the vaccines work against the Delta variant, according to Yale Medicine®. Understanding the transmissibility of this COVID-19 strain, the threshold to stop community spread is much higher than before. That means that even areas with higher vaccination rates may be susceptible to outbreaks, and emerging data from the CDC shows that vaccinated individuals may be able to spread the virus to others.6

We are still learning more, but data from recent outbreaks and breakthrough infections make clear that the COVID-19 vaccines offer significant protection—as they were designed to—against severe illness and death. For now, wearing a mask remains important, especially for immunocompromised people and adults in contact with young children, older adults, or those vulnerable to the virus.

This is an important question that scientists are working to understand. An article in The Lancet® stated that early data showed that the Delta variant was twice as likely as the earlier Alpha to result in hospitalization in unvaccinated individuals,7 but other data has shown no significant difference. The information could change as we learn more.

1 Variant Proportions, CDC, https://covid.cdc.gov/covid-data-tracker/#variant-proportions accessed September 17, 2021

2 5 Things to Know About the Delta Variant, Yale Medicine https://www.yalemedicine.org/news/5-things-to-know-delta-variant-covid accessed September 17, 2021

3 The Delta Variant Isn't As Contagious As Chickenpox. But It's Still Highly Contagious, NPR https://www.npr.org/sections/goatsandsoda/2021/08/11/1026190062/covid-delta-variant-transmission-cdc-chickenpox accessed September 17, 2021

4 When You’ve Been Fully Vaccinated, CDC https://www.cdc.gov/coronavirus/2019-ncov/vaccines/fully-vaccinated.html accessed September 17, 2021

5 Monitoring Incidence of COVID-19 Cases, Hospitalizations, and Deaths, by Vaccination Status — 13 U.S. Jurisdictions, April 4 - July 17, 2021, CDC https://www.cdc.gov/mmwr/volumes/70/wr/mm7037e1.htm  accessed September 17, 2021

6CDC, “Science Brief: COVID-19 Vaccines and Vaccination”, https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/fully-vaccinated-people.html, last accessed 9/17/2021.

7The Lancet, “SARS-CoV-2 Delta VOC in Scotland: demographics, risk of hospital admission, and vaccine effectiveness”, https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)01358-1/fulltext, last accessed 9/17/2021.