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Home Knowledge Center Coronavirus (COVID-19) Resource CenterWhat is the Delta Variant?

Coronavirus (COVID-19) Variants

Find up-to-date information on the COVID-19 variants.

Vaccines play a key role in protecting against COVID-19 variants

We’ve put together commonly asked questions to help you understand what the COVID-19 variants are, how they spread, and how getting vaccinated can help protect us all.

Delta Variant

What is the Delta variant and why is it causing a spike in cases?

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Ebola, the common cold, the seasonal flu, and smallpox, and is almost as infectious as the chickenpox3 according to unpublished data from the CDC.

Does Delta’s contagiousness change our exposure risk?

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.

I’m fully vaccinated – do I need to be concerned?

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.

Does the Delta variant cause a more severe infection than the original strain?

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.

Omicron Variant

What is the Omicron variant?

It is a mutation of the COVID-19 virus and is being called a “variant of concern.” Viruses constantly change through mutation and sometimes these mutations cause a new variant of the virus. It was first detected in South Africa in November 2021.

Researchers say Omicron is 20 times more transmissible than the original strain of COVID-19 and two-and-a-half times as transmissible as Delta, even if you are vaccinated.

As of December 20, 2021, Omicron has been detected in most states and territories and is spreading rapidly.

Is the Omicron variant more severe than other COVID-19 variants?

Early findings suggest that Omicron might be less severe than the Delta variant, but more data is still needed. Studies are ongoing about how the Omicron variant behaves. It is important to remember that all variants of COVID-19 can cause severe disease or death.

That is why preventing the spread of the virus and reducing your risk of exposure to the virus is so important. New variants like Omicron are a reminder that the COVID-19 pandemic is far from over. It is essential that you get vaccinated and continue to follow existing advice on preventing the spread of the virus, including physical distancing, wearing masks, regular handwashing, and keeping indoor areas well ventilated.

How can I protect myself and my family against the Omicron variant?

Vaccines remain the best way to reduce your risk of severe illness, hospitalization, and death from COVID-19.  People who are not fully vaccinated should take steps to protect themselves, including wearing a mask indoors in public.

1 Variant Proportions, CDC, accessed September 17, 2021

2 5 Things to Know About the Delta Variant, Yale Medicine accessed September 17, 2021

3 The Delta Variant Isn't As Contagious As Chickenpox. But It's Still Highly Contagious, NPR accessed September 17, 2021

4 When You’ve Been Fully Vaccinated, CDC 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  accessed September 17, 2021

6CDC, “Science Brief: COVID-19 Vaccines and Vaccination”,, last accessed 9/17/2021.

7The Lancet, “SARS-CoV-2 Delta VOC in Scotland: demographics, risk of hospital admission, and vaccine effectiveness”,, last accessed 9/17/2021. 

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