SARS-CoV-2 Delta Variant (B.1.617.2): What we know so far

The World Health Organization (WHO) has identified several variants of the SARS-CoV-2 virus. Some of these are designated as Variants of Concern (VOCs), while some others are designated as Variants of Interest (VOIs). WHO designates SARS-CoV-2 variants using letters from the Greek alphabet to avoid confusion, and prevent stigmatising states from where the variants first emerged.

Till date, the WHO has identified four Variants of Concern- Alpha, Beta, Gamma, and Delta respectively. This article will briefly discuss the Delta variant.

Background Information

The Delta Variant (B.1.617.2) was first documented in India in October 2020.

It was designated a Variant of Interest (VOI) on 4 April 2021, and a Variant of Concern on 11 May 2021.

It has been reported from 85 countries so far.

Key Messages:

Transmissibility (How easily the variant can be transmitted from person to person)

Compared to previously circulating/co-circulating variants, the Delta Variant has increased transmissibility and secondary attack rate.

A study in Japan showed that the Delta Variant was associated with 1.23 times higher transmissibility than Alpha Variant.

Disease severity

The severity of disease caused by Delta Variant is not confirmed. It is suspected that compared to previously circulating/co-circulating variants, the Delta Variant may be associated with increased risk of hospitalization.

A study from Singapore showed that infection with Delta Variant was associated with higher odds of oxygen requirement, ICU admission, or death [adjusted odds ratio (aOR) 4·90, 95% CI 1.43-30.78]. Additionally, the aOR for pneumonia was 1.88 times higher (95% CI 0·95-3·76) for those infected with Delta compared to infection with non-VOC SARS-CoV-2 lineages.

Risk of Reinfection (Risk of getting infected again after previous infection)

Following infection with SARS-CoV-2, the immune system develops the ability to neutralize virus. Neutralizing activity varies by virus strain/variant, and a reduction in neutralizing activity has been reported for the Delta Variant, suggesting an increased risk of reinfection. However, evidence is evolving, and this may change once more evidence is available.

Impacts on Diagnostics

So far, no impact has been reported on current diagnostic methods. That is, existing methods to diagnose SARS-CoV-2 infection will detect the Delta Variant as SARS-CoV-2 infection.

Impacts on vaccine efficacy/effectiveness

This pertains to protection against infection and disease. What we want to know is if the available vaccines protect against infection/disease due to the Delta Variant.

There is limited evidence based on research involving only two vaccines- Pfizer/BioNTech-Comirnaty, and AstraZeneca-Vaxzevria.

Technical details of research

One study is from the UK, and investigated vaccine effectiveness following full courses of both Pfizer BioNTech-Comirnaty and AstraZeneca-Vaxzevria vaccines against symptomatic disease due to the Delta variant; VE against Delta, while slightly reduced, was maintained for both vaccines (88% for Pfizer BioNTech-Comirnaty and 67% for AstraZeneca-Vaxzevria).

In the follow-up study on the effectiveness of these vaccines against severe disease (hospitalization) due to Delta variant among ≥16 year olds in the UK, investigators estimated overall vaccine effectiveness (VE) against hospitalization. VE estimates against hospitalization due to Delta and Alpha variants ≥ 14 days post second dose was estimated to be 96% (95% CI: 86-89%) and 95% (95% CI: 78-99%) respectively, for Pfizer BioNTechComirnaty and 92% (85% CI: 75-97%) and 86% (95% CI: 53-96%) respectively, for AstraZeneca-Vaxzevria.

Single dose effectiveness against hospitalization ≥ 21 days after immunization remained high for Pfizer BioNTech-Comirnaty at 94% (95% CI: 46-99%) against Delta and 83% (95% CI: 62-93%) against Alpha. Effectiveness of one dose of AstraZeneca-Vaxzevria against hospitalization was similar for Delta and Alpha variants, but reduced relative to two doses at 71% (95% CI: 51-83%) and 76% (95% CI: 61-85%), respectively.

A second study from Scotland by Sheikh et al. applied a test negative case-control design to a large COVID-19 surveillance platform and found that two doses of Pfizer BioNTech-Comirnaty were 83% (95% CI: 78-87) and 79% (95% CI: 75-82%) effective against symptomatic disease and infection due to Delta, respectively, ≥ 14 days after receipt of second dose in persons 15 years and older. These estimates were somewhat reduced compared to VE estimates against Alpha: 92% (95% CI: 88-94%) and 92% (90-93%) for symptomatic disease and infection, respectively.

The study also showed reduced effectiveness of two doses of AstraZeneca-Vaxzevria against Delta compared to Alpha with VE estimates of 61% (95% CI: 51-70%) and 60% (95% CI: 53-66%) against symptomatic disease and infection ≥ 14 days after second dose, respectively, compared to corresponding estimates of 81% (95% CI: 72-87%) and 73% (95% CI: 66-78%) against Alpha.

Single dose effectiveness against Delta was similar to that of Alpha with low VE for both vaccines and for both symptomatic disease and infection ≥28 days after immunization with VE estimates ranging from 18% to 39%. In a separate cohort analysis, single dose effectiveness against hospitalization ≥28 days after immunization among SARS-CoV-2 positive individuals was estimated for Pfizer BioNTechComirnaty and AstraZeneca-Vaxzevria vaccines combined; VE was estimated to be 62% (95% CI: 42-76%) and 72% (95% CI: 57-82%) against Delta and Alpha, respectively, demonstrating lower protection against Delta compared to Alpha (though confidence intervals overlap, indicating no statistical significance).

Together, these studies suggest moderately reduced VE at preventing symptomatic disease and infection due to the Delta variant as compared to Alpha. While the Scotland study suggests there could be reduced effectiveness of vaccines against hospitalization due to Delta as compared to Alpha, confidence levels overlap and VE for individual vaccines was not estimated.

Plain language summary

Preliminary evidence from England and Scotland suggests that people infected with the Delta variant are about twice as likely to end up in hospital, compared with those infected with Alpha variant.

People who have had one vaccine dose are 75% less likely to be hospitalized, compared with unvaccinated individuals, and those who are fully protected are 94% less likely to be hospitalized.

Delta variant is moderately resistant to vaccines, particularly in people who have received just a single dose. A single dose of either AstraZeneca’s or Pfizer’s vaccine reduced a person’s risk of developing COVID-19 symptoms caused by the Delta variant by 33%, compared to 50% for the Alpha variant. A second dose of the AstraZeneca vaccine boosted protection against Delta to 60% (compared to 66% against Alpha), while two doses of Pfizer’s vaccine were 88% effective (compared to 93% against Alpha).

Impacts on Neutralization by vaccine

Virus neutralization studies have been conducted with three vaccines- PfizerBioNTech-Comirnaty, AstraZeneca-Vaxzevria, Bharat-Covaxin.

One study reported no/minimal loss of virus neutralization with Bharat-Covaxin.

Some other studies reported No/Minimal to moderate loss of virus neutralization with PfizerBioNTech-Comirnaty, and Bharat-Covaxin.

Substantial loss of virus neutralization was reported with a single dose of AstraZeneca-Vaxzevria. Please note that these were laboratory studies, and may not accurately reflect vaccine behaviour in the real world.

Geographical variations in spread

Cases of the Delta variant in the United Kingdom are doubling roughly every 11 days. But countries with ample vaccine stocks should be reassured by the slower increase in hospital admissions. 

In the USA, communities with high proportions of African American and Hispanic individuals, where vaccination rates tend to be low, could be especially hard hit by Delta. The Delta variant is spreading faster in US counties where less than 30% of residents have been fully vaccinated, compared to the counties with vaccination rates above that threshold.

The Delta variant poses the biggest risk to countries that have limited access to vaccines, particularly those in Africa, where most nations have vaccinated less than 5% of their populations.

Prevention and control

Vaccination remains the best tool for combatting a Delta surge as the Delta variant poses a relatively low threat to fully vaccinated people.

Authors of a recently published study suggested that a combination of public health and social measures (PHSM) (e.g., masking, physical distancing, lockdowns, testing) should be implemented alongside a vaccine strategy to improve population and health system outcomes.

Useful Links:

Link to WHO video on Delta Variant:—delta-variant-and-vaccines

Link to Weekly Epidemiological Report (22 June 2021):—22-june-2021


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