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 detected in the U.S. in March 2021. 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.

A clinical study published in The New England Journal of Medicine suggests that one dose of the Johnson & Johnson vaccine is up to 85% effective against the Delta variant. In addition, the Novavax vaccine demonstrated 82% clinical efficacy against the Delta variant.

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.

The Food and Drug Administration (FDA) has authorized the Pfizer BioNTech and Moderna vaccines for emergency use authorization (EUA) in children ages 6 months and older and the Novavax vaccine for EUA in children ages 12 years and older.

Additionally, updated bivalent boosters from Pfizer-BioNTech and Moderna received EUA from the FDA.

The CDC recommends one bivalent booster shot 2 months after completing the primary series or last booster for all people over the age of 5, no matter the type of vaccine series initially received.

Children ages 5 years are only eligible to receive the bivalent Pfizer booster. Everyone ages 6 years and older can choose to get the Pfizer or Moderna bivalent booster.

In limited situations, people ages 18 years and older who have completed the primary COVID-19 vaccination and have not received any previous booster doses may receive a monovalent Novavax booster dose if they cannot receive an mRNA vaccine.

Preventing Transmission

In August 2022, The Centers for Disease Control and Prevention (CDC) updated COVID-19 guidance to prevent transmission of the Delta variant and other variants. The CDC recommends:

  • COVID-19 primary series vaccines for everyone ages 6 months and older, and COVID-19 boosters for everyone eligible ages 5 years and older.
  • If you were exposed to the virus that causes COVID-19, wear a high-quality mask for 10 days and get tested on day 6.
  • If you test positive for COVID-19, self-isolate at home for at least five days. You are likely most infectious during these first 5 days. Wear a high-quality mask when you're around others at home and in public through day 10.


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.

16 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. Sadoff J, Gray G, Vandebosch A, et al. Safety and efficacy of single-dose Ad26.COV2.S vaccine against Covid-19N Engl J Med. 2021;384(23):2187-2201. doi:10.1056/NEJMoa2101544

  12. Novavax. Novavax announces positive results of COVID-19 vaccine in pediatric population of PREVENT-19 Phase 3 clinical trial.

  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. Stay Up to Date with COVID-19 Vaccines Including Boosters.

  15. Centers for Disease Control and Prevention. Interim Clinical Considerations for Use of COVID-19 Vaccines Currently Approved or Authorized in the United States.

  16. Massetti GM, Jackson BR, Brooks JT, et al. Summary of guidance for minimizing the impact of COVID-19 on individual persons, communities, and health care systems - United States, August 2022. MMWR Morb Mortal Wkly Rep. 2022;71(33):1057-1064. doi:10.15585/mmwr.mm7133e1

By Julie Scott, MSN, ANP-BC, AOCNP
Julie is an Adult Nurse Practitioner with oncology certification and a healthcare freelance writer with an interest in educating patients and the healthcare community.