What Is the Delta Variant?

A COVID-19 mutation, also known as the B.1.617.2 variant

All viruses develop mutations and variants, and the SARS-CoV-2 virus, which causes COVID-19 infection, is no different. Since the start of the pandemic, multiple variants have developed.

One of these variants is B.1.617.2, also known as the Delta variant. This variant was initially discovered in India in December 2020 before it spread across the globe.

Young woman getting vaccinated

Marko Geber / Getty Images

Where Delta Has Spread

After starting in India in 2020, the B.1.617.2 variant became the dominant strain in that country and globally until the worldwide surge of the Omicron variant (B.1.1.529) in late 2021.

The Delta variant was detected in people in over 130 countries during its spread, including the United States, Canada, and England.

Spread in the United States 

The Delta variant was first found in the United States in March 2021 and was later found in all 50 states. By August 7, 2021, the Delta variant was responsible for more than 90% of U.S. COVID-19 infections until the Omicron variant emerged as the dominant variant in December 2021.

Why Do Viruses Mutate?

It's common for all viruses to mutate. When a virus enters the body, it begins to make copies of itself. Sometimes, mistakes (mutations) are made during the copying process that can make it easier for the virus to invade a cell. When this same mutation continues to further replicate, a variant of the virus forms.

Is the Delta Variant More Contagious?

The Delta variant is more contagious than the original COVID-19 virus. In fact, it is estimated to be at least 50% more contagious than the original strains.

The infectiousness of a virus is measured by an R number, or the number of people an infected person will give the virus to. For example, if the R is 1, an infected person is likely to give the virus to one other person. An R of 5 means an infected person is likely to transmit it to five other people, and so on.

Research has shown that the Delta variant has an estimated R of 5.08 compared to the original strain's estimated R of 2.79.

Risk of Reinfection

There is currently limited data on breakthrough infections, especially regarding the Delta variant. These are COVID-19 infections in people who have been vaccinated against COVID-19 or who have had COVID-19 before.

However, one study of a COVID-19 outbreak in July 2021 showed that of the 469 cases, 74% occurred in fully vaccinated persons.

Is the Delta Variant More Severe?

Research is still ongoing, but in a Scottish study, people infected with the Delta variant experienced a higher hospitalization rate than those infected with previous strains of COVID-19.

It's unclear if the Delta variant directly causes more severe sickness, but the fact that it is more contagious than previous variants is confirmed. This is particularly concerning for those with underlying medication conditions that put them at a higher risk of developing severe sickness.

Some of these conditions include:

  • Cardiovascular disease
  • Diabetes
  • Chronic respiratory disease
  • Cancer

Will Vaccines Work Against the Delta Variant?

It appears that those who have been fully vaccinated against COVID-19 infection should be at reduced risk of becoming infected with the Delta variant.

A study published in The New England Journal of Medicine found the Pfizer/BioNTech and AstraZeneca vaccines to be 88% and 67% effective, respectively, against the Delta variant after two shots.

Other studies suggest that one dose of the Moderna vaccine is 72% effective at preventing symptomatic disease caused by the Delta variant. In addition, the Johnson & Johnson vaccine is up to 67% effective in preventing hospitalization.

Research also suggests that a booster shot of the Moderna or Pfizer/BioNTech vaccine has continued effectiveness in preventing symptomatic disease by the Delta variant.

Are Kids More at Risk for the Delta Variant?

There is limited research on whether kids are more at risk for the Delta variant. In addition, there is no strong evidence that the variant makes kids sicker than previous ones.

As Delta is a more contagious strain, kids continue to be at risk of catching it like anyone else. In countries such as the United Kingdom, kids were found to have higher infection rates.

Kids too young to get vaccinated against COVID-19 are obviously at greater risk as well.

Preventing Transmission

Precautions to prevent transmission of the Delta variant are the same for the original COVID-19 virus.

  • If you are not vaccinated against the virus, you should:
  • Stay 6 feet apart from others who don’t live in your household.
  • Wear a mask that covers your mouth and nose.
  • Practice good hand hygiene by washing hands often or using hand sanitizer regularly.

If you are fully vaccinated and have received one booster shot, the guidance is different. The Centers for Disease Control and Prevention (CDC) says that wearing a mask and physical distancing are no longer necessary in certain counties where the spread of COVID-19 is contained. However, local regulations should be followed.


The B.1.617.2 variant, otherwise known as the Delta variant, was globally the dominant COVID strain from August 2021 to December 2021 until the surge of the Omicron variant.

The Delta variant is more contagious than previous variants, but it's unclear if it causes more severe sickness. Research shows that fully vaccinated people are at a reduced risk of developing symptomatic disease and hospitalization by the Delta variant.

A Word From Verywell

The B.1.617.2 variant is more contagious than previous variants. Getting vaccinated is the best way to protect yourself against this and other virus strains.

Check with a healthcare provider to ensure the vaccine is safe for you to get. Increasing vaccination rates in the United States and around the world is the best way to decrease the spread of COVID-19.

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.

15 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. SARS-CoV-2 variant classifications and predictions.

  2. World Health Organization. WHO Director-General's opening remarks at the media briefing on COVID-19.

  3. Yale Medicine. 5 things to know about the Delta variant.

  4. Centers for Disease Control and Prevention. COVID data tracker.

  5. Centers for Disease Control and Prevention. Omicron variant: what you need to know.

  6. Grubaugh ND, Petrone ME, Holmes EC. We shouldn't worry when a virus mutates during disease outbreaksNat Microbiol. 2020;5(4):529-530. doi:10.1038/s41564-020-0690-4

  7. Liu Y, Rocklöv J. The reproductive number of the Delta variant of SARS-CoV-2 is far higher compared to the ancestral SARS-CoV-2 virusJ Travel Med. 2021;28(7):taab124. doi:10.1093/jtm/taab124

  8. Brown CM, Vostok J, Johnson H, et al. Outbreak of SARS-CoV-2 infections, including COVID-19 vaccine breakthrough infections, associated with large public gatherings — Barnstable County, Massachusetts, July 2021. MMWR Morb Mortal Wkly Rep. 2021;70(31):1059-1062. doi:10.15585/mmwr.mm7031e2

  9. World Health Organization. Coronavirus disease (COVID-19).

  10. Lopez Bernal J, Andrews N, Gower C, et al. Effectiveness of Covid-19 vaccines against the B.1.617.2 (Delta) variant. N Engl J Med. 2021;385(7):585-594. doi:10.1056/NEJMoa2108891

  11. Nasreen S, Chung H, He S, et al. Effectiveness of COVID-19 vaccines against symptomatic SARS-CoV-2 infection and severe outcomes with variants of concern in OntarioNat Microbiol. 2022;7(3):379-385. doi:10.1038/s41564-021-01053-0

  12. Bekker LG, Garrett N, Goga A, et al. Effectiveness of the Ad26.COV2.S vaccine in health-care workers in South Africa (The sisonke study): results from a single-arm, open-label, phase 3B, implementation study. The Lancet. 2022;399(10330):1141-1153. doi:10.1016/S0140-6736(22)00007-1

  13. Accorsi EK, Britton A, Fleming-Dutra KE, et al. Association between 3 doses of mRNA COVID-19 vaccine and symptomatic infection caused by the SARS-CoV-2 Omicron and Delta variantsJAMA. 2022;327(7):639-651. doi:10.1001/jama.2022.0470

  14. Centers for Disease Control and Prevention. How to protect yourself & others.

  15. Centers for Disease Control and Prevention. When you've been fully vaccinated.