Exploring the Delta variant

Written by: Sarah Lambert and Rosie Lobley

Download the latest Vaccine Tracker here

What is a variant? On average, the SARS-CoV-2 virus has accumulated around one to two genomic letter mutations per month.1 These mutations create distinct genetic lineages, or variants, of the virus.

While many variants are genetically unremarkable and do not differ drastically from each other, some mutations can lead to strains of the virus which are particularly harmful – either in terms of heightened transmissibility or increased resistance to antibodies. Where a single genetic strain is linked to a significant increase in cases or clusters, it is identified as a ‘Variant of Interest’ (VOI); if a VOI displays evidence of increased transmissibility or increased severity, or if it is shown to be less susceptible to treatment or vaccination than other variants, it is upgraded to a ‘Variant of Concern’ (VOC).2


How do the Variants of Concern differ from previous variants? Alpha, Beta, Gamma, Delta

The World Health Organization (WHO) has identified four key Variants of Concern, officially referred to as the Alpha, Beta, Gamma and Delta variants. These highly infectious mutant strains of the SARS-CoV-2 virus are primarily characterised by the N501Y mutation, which affects the DNA of the spike protein covering the surface of the coronavirus, causing it to bind more tightly to receptors in the body.3 Variant B.1.617.2, which is also known as the Delta variant, appears to be a significant contributor to the widespread and deadly ‘second wave’ of COVID-19 in India.3


How has this variant contributed to the second wave of the COVID-19 pandemic in India?

India is experiencing a sizeable second wave of COVID-19 across all states, with daily case numbers reaching 400,000 by mid-May 2021 – around four times that recorded during the peak of the first wave in the country.4,5 The emergence of the Delta variant, combined with relaxation of pandemic restrictions, may be partially responsible for the recent rise in COVID-19 cases; although research in this area remains limited.5

Notably, there is currently no strong evidence to show whether patients with the Delta variant develop more severe symptoms of COVID-19 infection.5 In fact, the fatality rate (calculated by the total deaths divided by the total infections) is low compared to that experienced in the first wave; however, this is currently being eclipsed by the sheer number of infections occurring.5 Although India has been quick to roll out its vaccination programme, the proportion of vaccinated individuals remains low due to the size of the overall population.5

India is experiencing a second wave of COVID-19 across all states, with daily case numbers reaching 400,000 by mid-May

Although it was first detected in India, the World Health Organization has now reported cases of the Delta variant in 85 countries across the world.6


Countries, territories and areas with reported cases of the Alpha, Beta, Gamma and Delta variants of COVID-19, as of 22 June 2021 (some yet to be validated by the WHO)

Alpha (170), Beta (119), Gamma (71), Delta (85), Not Specified (9)
The designations employed and the presentation of the map above do not imply the expression of any opinion whatsoever on the part of the World Health Organisation concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Dotted and dashed lines on maps represent approximate border lines for which there may not yet be full agreement.


By 26 May 2021, Public Health England had identified more than 7,000 cases of B.1.617 (the parent lineage of B.1.617.2, the Delta variant) in the UK, including both B.1.617.2 and B.1.617.3.7 The Delta variant is now officially considered a Variant of Concern in the UK, based on evidence which suggests this variant is at least as transmissible as variant B.1.1.7, the Alpha variant, which was first detected in the UK.8


Are current COVID-19 vaccines effective against the Delta variant?

It has been reported that the B.1.617.2 variant could reduce the efficacy of COVID-19 vaccines by affecting the SARS-CoV-2 spike protein interaction with the CR3022 antibody, which neutralises the virus; while increasing the interaction with the ACE2 receptor.3 However, Public Health England states that there is insufficient evidence to indicate that any of the variants recently detected in India cause more severe disease or render vaccines any less effective.9

There is insufficient evidence to indicate that any of the variants detected in India render vaccines any less effective

In a preprint published on 22 May 2021, Public Health England reported that two doses of the Comirnaty (Pfizer-BioNTech) or Vaxzevria (Oxford-AstraZeneca) COVID-19 vaccine were highly effective against symptomatic cases of the B.1.617.2 variant.8

Find out how the different vaccines work here


What are the key risks specifically associated with the Delta variant?

While variant B.1.617.2 is not associated with heightened fatality rates, its increased rate of transmission carries a number of potential harms. A growing majority of cases of COVID-19 in the UK, as tracked by Imperial College London’s Real-time Assessment of Community Transmission (REACT) programme, are occurring in children and young adults, who are less likely to have been vaccinated.10 If the rise in transmission continues unchecked, infection will spread to older and vulnerable patients, as vaccines cannot guarantee 100% resistance to the virus – and this in turn will lead to a rise in hospitalisations, increasing pressure on healthcare systems and diverting resources from other sectors.10

If the rise in transmission continues unchecked, infection will spread to older and vulnerable patients

In India, where only 4% of the total population is vaccinated,11 the risks are more pressing. The longer the SARS-CoV-2 virus is allowed to proliferate, the more it will mutate, creating a ‘cycle of variants’12 whereby ever-more virulent strains develop, eventually becoming effectively resistant to existing treatment or vaccinations.


    1. Duchene S, et al. Virus Evolution 2020; https://bit.ly/367IC4T
    2. World Health Organization. Tracking SARS-CoV-2 variants; updated 15 June 2021. Available at: https://bit.ly/2V2ud7S [Accessed June 2021].
    3. Ranjan R, et al. medRxiv 2021; doi.10.1101/2021.04.17.21255665 [Preprint].
    4. World Health Organization. COVID-19: India. Updated 29 June 2021. Available at: https://bit.ly/3hbWt0n [Accessed June 2021].
    5. Ranjan P, et al. bioRxiv 2021; doi:10.1101/2021.04.03.438113 [Preprint].
    6. World Health Organization weekly epidemiological update on COVID-19. 22 June 2021. Available at: https://bit.ly/3dUQZ8r [Accessed June 2021].
    7. Public Health England. Variants of concern or under investigation. 26 May 2021. Available at: https://bit.ly/3Aoaks8 [Accessed June 2021].
    8. Lopez Bernal J, et al. Public Health England. Effectiveness of COVID-19 vaccines against the B.1.617.2 variant (preprint). Available at: https://bit.ly/3yfFpMD [Accessed May 2021].
    9. Public Health England. Confirmed cases of COVID-19 variants identified in UK: 7 May 2021 update. Available at: https://bit.ly/3dEEhui [Accessed May 2021].
    10. Imperial College London. Coronavirus infections rising exponentially in England – REACT study. 17 June 2021. Available at: https://bit.ly/3qHNtTJ [Accessed June 2021].
    11. India overtakes US in total jabs given, now world no. 2. Times of India. 29 June 2021. Available at: https://bit.ly/2UdoU53 [Accessed June 2021].
    12. Delta COVID variant may by edging race against vaccines. Guardian. 27 June 2021. Available at: https://bit.ly/3dCaF0v [Accessed June 2021].

Download the latest Vaccine Tracker here