6 Things to Know About the COVID-19 Delta Variant

Doctor William C Lloyd Healthgrades Medical Reviewer
Medically Reviewed By William C. Lloyd III, MD, FACS
Written By Susan McBratney, PhD on December 22, 2021
  • three-dimensional model of the coronavirus
    What to Know About the Delta Variant of COVID-19
    COVID-19 Delta is a SARS-CoV-2 “variant of concern.” Variants of concern have a collection of random mutations that cause the virus to act differently than the original novel coronavirus and can be more contagious or resistant to treatment. The Delta variant started in India and spread quickly across the globe. The first cases in the United States were confirmed around June, and within a month it was causing all new U.S. coronavirus infections and COVID-19 illness.

    (Variants of concern are denoted by the Greek alphabet—Alpha, Beta, Gamma, Delta, Epsilon, and so on—rather than the country from which they are initially discovered.)
  • teenage boy explains symptoms to his doctor during the Covid-19 pandemic
    1. COVID-19 Delta variant is extremely contagious.
    The COVID-19 Delta variant, also designated B.1.617.2, is estimated to be 60% more contagious than the Alpha variant, which itself is 50% more contagious than the original virus that started the pandemic. Based on case rates here and in the United Kingdom, the Delta variant is 2 to 4 times more contagious than the original virus. That means one infected person can potentially infect six susceptible people (such as unvaccinated people and vaccinated, but immunocompromised people).
  • Heart rate monitor, patient and doctors in background in intensive care unit
    2. Delta variant illness appears to be similar to earlier versions of the virus.
    The exponential growth of the Delta variant caused a rise in COVID-19 hospitalizations, which is expected based on the shear number of infections. But does the Delta variant cause more severe COVID-19 than earlier versions of the virus? There's no clear answer to that question.

    In a Scottish study of about 20,000 COVID infections (mainly unvaccinated individuals), the Delta variant doubled the risk of hospitalization compared to the Alpha variant. However, the overall trend in U.S. hospitalizations is not significantly between the Delta variant and earlier versions.

    The Centers for Disease Control and Prevention (CDC) reports the Delta variant "might cause more severe illness than previous strains in unvaccinated people."
  • teen girl wearing face mask smiles as she take a selfie while getting vaccine outdoors at pop-up vaccine clinic
    3. Vaccination is highly effective at preventing severe disease and hospitalization caused by the Delta variant.
    The difference in protection among unvaccinated, partially and fully vaccinated people is much larger with the Delta variant than Alpha variant. In other words, even a partially vaccinated individual is more vulnerable to the Delta variant than to the Alpha and other variants of concern. Likewise, people who recovered from COVID-19, but have not been vaccinated against it, have a higher risk of reinfection with Delta compared to Alpha and older versions of the virus (if the first infection occurred more than six months earlier).

    Being fully vaccinated offers the most protection, but vaccinated people can still get infected and sick from the Delta variant because vaccination is not 100% effective. Vaccination helps prevent COVID-19 Delta hospitalization or death, which appears to be 10 times more likely for unvaccinated than fully vaccinated individuals, according to several studies summarized by the CDC.

    A booster shot of an mRNA-type COVID-19 vaccine strengthens immunity to the Delta variant. The U.S. Food and Drug Administration (FDA) also authorized an additional COVID-19 vaccine dose for immunocompromised people.
  • nurse wearing personal protective equipment provides oxygen mask to a pediatric male patient lying in a hospital bed; concept of coronavirus and covid 19
    4. The Delta variant is affecting children.
    When the Delta variant took over England, there was a steep rise of cases in children. This was due, in part, to the fact that children younger than 12 are not yet able to receive the COVID-19 vaccine. The number of U.K. cases caused by the Delta variant was higher in younger (ages 5 to 49) vs. older (ages 50 or older) age groups, but cases rose at a similar rate in both groups.

    Most children develop mild (or no) symptoms, but some children require hospitalization, even intensive care. The Delta variant does not appear to cause more severe disease in children, compared to the Alpha variant or the original virus. Rather, the rise in U.K. and U.S. hospitalizations may be because there are simply more Delta infections occurring in the large pool of unvaccinated children.
  • young woman with eyes closed lying under blanket on couch, suffering with runny nose, headache, tissues lying around
    5. COVID-19 Delta symptoms are similar to earlier versions of the virus.
    The most common symptom in people with a confirmed COVID-19 infection is headache—according to data collected from a COVID-19 symptom study app since May 2021, when Delta was the most common circulating variant in the United Kingdom. COVID-19 Delta symptoms, including sore throat, runny nose, and fever may be easily mistaken for those of flu or the common cold (however, it is rare to develop a fever with a cold). COVID-19 testing is the only way to know for sure that you have COVID-19.
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    6. The Delta variant may be resistant to targeted COVID-19 treatments.
    Laboratory-made antibodies that target SARS-CoV-2 have emergency use authorization under certain circumstances for preventing severe COVID-19. Additional laboratory studies show that these targeted monoclonal antibody treatments, particularly bamlanivimab, are less efficient at blocking the Delta version of SARS-CoV-2’s spike protein compared to the spike protein from the original virus.
Delta Variant of COVID-19: 6 Things to Know

About The Author

Susan McBratney has been a staff writer and medical editor at Healthgrades since 2009. She previously worked in basic science research and literature curation before switching gears to write and edit medical content in the consumer-focused digital health field.
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  3. Bernal JL, Andrews N, Gower C, et al. Effectiveness of COVID-19 vaccines against the B.1.617.2 variant. medRxiv 2021.05.22.21257658; doi: https://doi.org/10.1101/2021.05.22.21257658 
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  6. Does the Johnson & Johnson vaccine protect against the delta variant of COVID? Nautilus. https://coronavirus.nautil.us/johnson-and-johnson-vaccine-against-delta-variant/ 
  7. Five things we know about the Delta variant (and two things we don't). Gavi, The Vaccine Alliance. https://www.gavi.org/vaccineswork/five-things-we-know-about-delta-coronavirus-variant-and-two-things-we-still-need 
  8. Planas D, Veyer D, Baidaliuk A. et al. Reduced sensitivity of SARS-CoV-2 variant Delta to antibody neutralization. Nature (2021). https://doi.org/10.1038/s41586-021-03777-9
  9. McMorrow, Meredith (July 29, 2021). "Improving communications around vaccine breakthrough and vaccine effectiveness". p. 15. Retrieved July 30, 2021. https://context-cdn.washingtonpost.com/notes/prod/default/documents/8a726408-07bd-46bd-a945-3af0ae2f3c37/note/57c98604-3b54-44f0-8b44-b148d8f75165.#page=1

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Last Review Date: 2021 Jun 24
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