What do we know so far about the Delta variant of COVID-19?

This article was published on
July 1, 2021

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The Delta variant was first detected in India in December 2020 and has since been verified in 77 countries today, rising from 62 countries earlier in June according to the World Health Organization.

The Delta variant was first detected in India in December 2020 and has since been verified in 77 countries today, rising from 62 countries earlier in June according to the World Health Organization.


What our experts say

The variant B.1.617.2 , also known as the Delta variant, continues to spread rapidly to more countries around the world, and is potentially more dangerous due to:

  1. Its ability to spread faster between individuals, with varying estimates, up to it being 50 percent more transmissible than the Alpha variant. 
  2. The likelihood of hospitalization is double compared to Alpha variant, according to a recent study published by The Lancet.
  3. Its main symptoms may be different from previously prevalent variants, which can make it harder for the general public to identify mild infections and self-isolate. 

However, similar to other circulating variants, the available, authorized vaccines offer a degree of protection against the Delta variant. The same study published by The Lancet suggests that the Pfizer-BioNTech vaccine starts to show strong protection against infection by this variant after 29 days from the first dose, and up to 97 percent protection after two weeks from the second dose. 

A recent study published as a pre-print in Public Health England, shows that individuals who receive two doses of any vaccine have 94 percent protection against being hospitalized due to the Delta variant, and are 80 percent less likely to develop symptomatic disease compared to individuals who have not been vaccinated.

Data from both of these sources have found that having only one dose of either the Pfizer-BioNTech or the Oxford-AstraZeneca vaccine is less protective against the Delta variant, than the Alpha variant. 

The recommendations to protecting oneself from the Delta variant remain the same as previous variants of the virus:

  1. Get vaccinated when you can, and commit to getting all of the required doses for that vaccine.
  2. Wearing a mask that covers your nose and mouth protects you and others.
  3. Maintain social distancing from individuals outside of your household.
  4. Avoid crowded and poorly ventilated areas. 
  5. Use soap and water to wash your hand frequently and use hand sanitizer if these are not available. 

All tests taken for the Delta variant, including ones that come back positive, are swabbed from the "nasopharyngeal region" in our noses. There is currently no current research that says the Delta variant is not being detected in the nasopharyngeal region. However, as the virus mutates, the part of the virus that COVID-19 tests target might change characteristics. Those changes can affect the accuracy of testing and lead to more false-negatives (test results that show negative when the person is actually infected with the virus).

Since the virus is expected to mutate, the FDA has authorized COVID-19 tests that target multiple mutations. Tests that look at specific parts of the virus only lead to higher risk of false negatives.

It should be noted that no test is 100 percent accurate and they all lead to some false positives and some false-negative results. This is why consecutive tests are recommended to confirm the results. 

Context and background

The Delta variant was first detected in India in December 2020 and has since been verified in 77 countries today, rising from 62 countries earlier in June according to the World Health Organization. 

Its ability to transmit quickly between individuals and countries is causing a shift in the prevalence of disease in some regions, perhaps most notable in the United Kingdom where it is now responsible for 91 percent  of COVID-19 cases in the country. Additionally, data from the Zoe Covid Symptom Study, where participants use a mobile app to report their symptoms, suggest that the symptoms of COVID-19 infections in the United Kingdom are changing; cough and unique symptoms like loss of smell are becoming rare, while headaches, fever, a sore throat and a runny nose are now the most frequent symptoms. This adds to the potential of this variant to spread as it can be easily disregarded by an individual as a common cold, where patients are less likely to self-isolate. 

All viruses are subject to change in their characteristics, due to repeated mutations in their genes during the process of replication. Scientists attribute this variant’s ability to transmit faster to changes in the proteins on the surface of the virus, known as spike proteins, which allow it to bind more efficiently to the cell mechanism it uses to enter human cells. Additionally, a separate study has suggested that the variant may have a greater ability to latch on to human cells, thus it has the potential to infect more cells and overwhelm the immune system. 

The variant is now rising in other European countries such as Denmark, and World Health Organization  officials are fearful of it becoming the dominant strain in Europe soon, and hoping that increasing vaccinations can help keep it at bay. 

There is a similar rise in the United States, where the Center for Disease Control and Prevention is now tracking the spread of the Delta variant as a variant of concern. Alpha remains the dominant variant, however, data from nationwide sampling conducted by the genomics company Helix, suggests that the Delta variant is rising fast in California, replacing the Alpha variant, which now accounts for 42 percent of cases after having previously accounted for 70 percent of cases, in April. 

Outbreaks of the virus have caused new lockdowns in Australia, delayed plans to re-open in Ireland and has caused travel restrictions from the United Kingdom.

While factors like restricted travel, access to vaccines, and increasing vaccination rates can help control the spread of the Delta variant, scientists are closely watching it for its potential to become the dominant strain globally, and fearful of the consequences of its spread to countries with less resources, and less effective disease surveillance.   


  1. About Variants of the Virus that Causes COVID-19 (United States Centers for Disease Control and Prevention)
  2. SARS-CoV-2 Delta VOC in Scotland: demographics, risk of hospital admission, and vaccine effectiveness (The Lancet)
  3. Effectiveness of COVID-19 vaccines against hospital admission with the Delta (B.1.617.2) variant (Public Health England)
  4. Five things we know about the Delta variant (and two things we don't) (Gavi)
  5. Global Variants Report (United States Centers for Disease Control and Prevention)
  6. What makes the Delta covid-19 variant more infectious? (Technology Review)
  7. About the Zoe Covid Symptom Study (FAQs)
  8. Delta coronavirus variant: scientists brace for impact (Nature)
  9. The Delta Variant: What Scientists Know (The New York Times)
  10. SARS-CoV-2 Viral Mutations: Impact on COVID-19 Tests (US FDA)
  11. Genetic Variants of SARS-CoV-2 May Lead to False Negative Results with Molecular Tests for Detection of SARS-CoV-2 - Letter to Clinical Laboratory Staff and Health Care Providers (US FDA)
  12. SARS-CoV-2 E Gene Variant Alters Analytical Sensitivity Characteristics of Viral Detection Using a Commercial Reverse Transcription-PCR Assay (Journal of Clinical Microbiology)
  13. FDA: New COVID-19 Variants May Cause False Negatives on Diagnostic Tests (VeryWellHealth)

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