Viktor Forgacs
Viktor Forgacs

EXPERT Q&A: COVID-19 Delta variant is more infectious, but is it more lethal?

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There is a lot of confusion circulating online and in the media about the mortality rate of the Delta variant of the SARS-CoV-2 virus, and how effective vaccines are at preventing Delta and deaths from this strain. It is clear that Delta is more infectious than earlier strains, but is it more or less lethal? An issue causing confusion is that there are several different data sources on Delta and the vaccines' efficacy against it - some data is based on antibody responses in the lab, some on real-world situations.  The vaccines also perform differently depending on whether you're looking at preventing infection, hospitalisation, or death. And mortality data can come from countries with high rates of vaccination, or low rates of vaccination, producing very different results. Some commentators have recently gone as far as to say that the Delta variant should be allowed to run its course and treated like other seasonal illnesses such as influenza. The AusSMC has asked Australian experts to address some of the most burning questions about the Delta strain.

Organisation/s: Australian Science Media Centre

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Expert Reaction

These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.

Professor Christine Jenkins is Head of the Respiratory Group at The George Institute for Global Health, Professor of Respiratory Medicine at UNSW Sydney and Clinical Professor at the University of Sydney

Is the Delta variant more or less deadly than earlier variants? What’s the evidence for this? If it is less deadly, should we be less concerned?

At present, evidence suggests the Delta variant is no more deadly than the variants that have preceded it. The context of the spread of Delta is very different to the context in which the original variants (Wuhan, Alpha) circulated in 2020 – in unvaccinated populations with little knowledge about prevention or treatment. In the 2021 context of vaccine roll-out and more effective strategies for treating COVID-19, it is extremely challenging to determine if the Delta variant is more lethal, as overall the mortality rates (case-fatality rate) from COVID 19 are declining in countries where there is good access to healthcare.

The Delta variant causes higher viral loads in infected people during the asymptomatic phase, making it far more contagious. It is also more readily taken up into airway cells, a consequence of its slight molecular change. It may also spread more readily by aerosol transmission - as a result it infects people who are mobile and congregating closely in groups or indoors in crowded spaces. Invariably this means that younger adults are more exposed to it, as older adults and those with underlying chronic diseases are typically more cautious about their movements and social contacts in the pandemic.

Additionally, more older adults have been vaccinated as they qualified in the first tier of people to whom vaccination was offered. Consequently, the younger adult cohort is more exposed to the Delta variant, more easily acquiring infection and potentially becoming ill. We are seeing a greater proportion of young people in the hospital admission statistics, but it is still mainly older people who die from COVID-19.

Last updated: 28 Feb 2024 11:01am
Declared conflicts of interest:
None declared.
Jaya Dantas is Professor of International Health in the School of Population Health at Curtin University

Is the Delta variant more or less deadly than earlier variants?

The Delta variant is 1) more contagious than other strains 2) leads to community clusters & local outbreaks 3) unvaccinated are at risk 4) vaccination is the best protection 5) we have still much to learn about Delta as it evolves and spreads.

What's the evidence for this?

This fast-spreading variant saw a dramatic wave of COVID-19 cases in India, Indonesia and is responsible for the rise in cases in the UK, Israel and USA that overwhelmed hospitals and crematoriums and has since been found in 110 countries. It's estimated to be 55 per cent more transmissible than Alpha and almost twice as infectious as the original strain that began spreading worldwide in early 2020.

If it is less deadly, should we be less concerned?

It is highly contagious, with vertical spikes in cases in many countries. Early data from Scotland found COVID-19 patients infected by Delta were 1.8 times more likely to be hospitalised than those with an alpha infection. Other evidence found Delta had some propensity to evade antibody-based treatments and that it potentially increased the risk of reinfection in people who have recovered from COVID-19 caused by another strain.

Last updated: 22 Jul 2021 4:38pm
Declared conflicts of interest:
None declared.

Associate Professor Jill Carr is from the Virus Research Laboratory in the College of Medicine and Public Health at Flinders University

Is the Delta variant more or less deadly than earlier variants? What’s the evidence for this? If it is less deadly, should we be less concerned? 

At present, the media is constantly calling this ‘more transmissible, deadly’ etc, which is very emotive language.

It is a viral variant of concern. It may have some properties that make it more ‘fit’ but it also may have some properties that might benefit the community (e.g. quicker to reach detection level = shorter asymptomatic phase = less spread; more virus in the nose = maybe less in the lower lung = more severe disease…. we don’t really know).

For me, the thing of most concern is its ability to evade our immune system and vaccine response. There is data to suggest reduced neutralisation by antibodies, but overall vaccines still protect against the Delta strain – importantly, they protect against severe disease. So it's one to watch but no need for panic.

How do vaccination rates influence the statistics we're seeing on the impact of the Delta strain in countries like the UK, India or the US?

If vaccination rates are not high enough, any variant that is present will spread. In the USA, the majority of hospital presentations with COVID-19 are in the unvaccinated population.

How effective are the various vaccines against Delta (both at the 1 and 2 Jab point)? 

Vaccine efficacy is reportedly similar for Delta and Alpha strains, and comparable to initial strains i.e. lower for the first dose; higher at the second; not fully protective against infection but highly effective at preventing severe disease.

Last updated: 22 Jul 2021 4:39pm
Declared conflicts of interest:
None declared.
Professor Brian Oliver leads the Respiratory Molecular Pathogenesis Group at the University of Technology Sydney and the Woolcock Institute.

There is a lot of confusion with the Delta strain of COVID-19, based on:

  1. People using the data from one area and applying it to another (with different populations, different susceptibilities, different rates of vaccination or types of vaccination, and different health care systems)
  2. People using old data (remembering that with strains like Delta, from week-to-week there is new data available), and sometimes people using the data in the wrong way (the data are expressed using a variety of different statistical terminology)
  3. People being misquoted.
  4. Not understanding what the source of the data actually is, so for example, is this a real-world study of a population, a study of the effectiveness of antibodies in people that is used to infer effectiveness of a vaccination, or a study on cells or animals. Unfortunately, all of these types of study tend to be mixed together in media reports, and in my opinion they are not the same.

If we use the preprint article of Stow and colleagues (See here) from the 14th of June which has been widely quoted in the media as an example of a real-world study in the UK. This showed that after one dose of either the AZ or Pfizer vaccinations, the vaccines are 75 per cent effective at preventing hospitalisations, and if you have two doses they are 94 per cent effective. However, if you look at the ability of the vaccinations to prevent symptoms, they are less effective, with a likelihood (odds) of 0.69 with one dose of either vaccine, and 0.2 with two doses.  What this means is that with one dose of a vaccine you are 1.45 times less likely to have a symptomatic infection, and with two doses, five times less likely to have a symptomatic infection.  As you can see, rephrasing the numbers in an attempt to make something more understandable can change the meaning if not quoted correctly.

As an example of how quickly data changes with Delta, if we want to quote the latest data from the UK with vaccination efficacy, which is Technical briefing 17 from Public Health England, on the 25th of June (10 days after the above preprint) https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1001354/Variants_of_Concern_VOC_Technical_Briefing_17.pdf. In that briefing, one dose of either the AZ or Pfizer vaccinations the vaccines are 80 per cent effective at preventing hospitalisations, and if you have two doses they are 96 per cent effective.  

The most recent technical briefing from Public Health England (9th July, Technical briefing 18), https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1001358/Variants_of_Concern_VOC_Technical_Briefing_18.pdf
does not have vaccine efficacy data, but does have case fatality rates of 1.9 per cent for the initial stain (alpha) and 0.2 per cent for the Delta Strain.  It is this comparison which has led to Delta being described as less dangerous than the alpha strain, but of course, many more people are vaccinated [in the UK], and hospitals there are better equipped to treat COVID-19 patients.

If we use a study by French researchers published in the journal Nature, (https://www.nature.com/articles/s41586-021-03777-9), the study states 'Sera from individuals having received one dose of Pfizer or AstraZeneca vaccines barely inhibited variant Delta', while two doses of the vaccine were 95 per cent effective.  What the investigators did was to take serum from people that had been vaccinated and see how good the antibodies were at inhibiting the Delta stain in a petri dish.  The issue with trying to interpret this data is that it is clearly different from the PHE studies above.  When an individual has had one dose of a vaccine, the antibodies they produce are still being optimised, and the second dose refines the antibody quality and provides long-lasting immunity.  There is nothing wrong with the science behind the French study, but what is likely happening is that in people with one vaccination that become infected with the Delta strain, their immune system is primed and can produce a stronger response than is estimated by just looking at the antibody effectiveness alone.

When it comes to understanding if the Delta strain is more transmissible than the Alpha strain, the prominence of the Delta strain first seen in India and then the UK and other countries is hard to misinterpret and indicates that this is a much more transmissible strain of COVID-19.

Last updated: 22 Jul 2021 4:41pm
Declared conflicts of interest:
None declared.

Dr Stuart Turville is an Associate Professor in the Immunovirology and Pathogenesis Program at UNSW Sydney's Kirby Institute

Is the Delta variant more or less deadly than earlier variants? What’s the evidence for this? If it is less deadly, should we be less concerned?

The WHO had the estimates at 2x fold increase in transmission with this variant compared to earlier variants. Is related to its increased infectivity per particle and that the viral loads when people present look to be higher. There is a paper that has recently come out to suggest the latter. This is one study and moving forward others may or may not confirm this observation.

How do vaccination rates influence the statistics we're seeing on the impact of the Delta strain in countries like the UK, India or the US?

The determination of disease severity for Delta is complex as we are now dealing with countries with large vaccine uptake in the elderly  population where the disease severity can be high. The UK for instance is reporting low disease severity with Delta, but this is within a population where over 50 per cent are vaccinated. South Africa and the countries with low vaccine uptake (South Africa is presently only at  2.9 per cent fully vaccinated) are seeing greater frequency of deaths compared to the UK, but it is too early to say that disease severity and accumulative deaths are in excess of that observed with earlier variants. Yet to be peer reviewed studies have specifically looked at severity and suggest that it is two fold higher compared to other variants. This is one study in pre-print and not peer reviewed and moving forward others may or may not confirm this observation.

How effective are the various vaccines against Delta (both at the 1 and 2 jab point)?

The AstraZeneca and Pfizer vaccines after the second dose work very well against this variant. See this paper in NEJM, published on July 21st, 2021: https://www.nejm.org/doi/full/10.1056/NEJMoa2108891

In countries with high vaccine coverage (UK, USA and Israel), it will be important to observe the presentation (or more importantly lack thereof) of severe disease and deaths. As mentioned above the data from the UK looks very encouraging but it is still very early at the moment. I was asked about this in an article but prefaced that the PHE data from England was very promising (this is context dependent data though) but it was too early to know.

Last updated: 22 Jul 2021 4:40pm
Declared conflicts of interest:
None declared.
Dr Roger Lord is a senior lecturer (Medical Sciences) with the Faculty of Health Sciences at The Australian Catholic University and Visiting Research Fellow with The Prince Charles Hospital (Brisbane)

According to the WHO, the Delta variant of COVID-19 has been detected in 96 countries and spreads more readily than other variants. The higher rate of transmission of Delta has led to the prediction by the European Centre for Disease Control that by the end of August, 90 per cent of the cases in the European Union will be the Delta variant.

Higher levels of transmission are also likely to lead to mutations of the variant.

The rate of spread and mortality of the variant will depend on the level of vaccination in a population together with any immunity from previous infections. Where these are low, as in the case for Australia, both transmission and mortality will be higher and is why stringent lockdowns are now being enforced in 3 states.

Available COVID-19 vaccines are less effective against infections from the Delta variant.
Available data for the Pfizer/BioNTech vaccine shows a 64 per cent effectiveness following 2 doses at preventing symptomatic illness compared to 94 per cent against older variants. [Data from Public Health England, published 14th June 2021]

Despite this lower rate of effectiveness, vaccination will prevent significant rises in deaths and hospitalisation rates. A high level of vaccination in the community is the key to prevent significant transmission and severe illness caused by the Delta variant.

Last updated: 22 Jul 2021 4:40pm
Declared conflicts of interest:
None declared.
Prof Bruce Thompson is the Head of the Melbourne School of Health Sciences at The University of Melbourne

Is the Delta variant more or less deadly than earlier variants? What’s the evidence for this? If it is less deadly, should we be less concerned?

The data on this is not entirely clear. However, based on Australia’s data, 9.8 per cent of patients (covidlive.com.au) with COVID-19 are currently in hospital with an escalating pneumonia illness that is life threatening. 1.6 per cent of patients with COVID-19 are in the intensive care unit. To be in the intensive care unit there is a very high chance of mortality. The difference this time with the delta variant, compared to previous variants, is that there is a more even distribution of age in those that are in hospital. For example, in the ICU in NSW there is a teenager. That is not good. The problem with the very high rate of hospitalisation of this condition is that if we let the virus go, then 10 per cent of the total population will end up in hospital. Currently we don’t have that capacity.

How do vaccination rates influence the statistics we're seeing on the impact of the Delta strain in countries like the UK, India or the US?

Vaccination does a number of things.
1. The chance of dying from COVID-19 is significantly reduced.
2. The chance of going to hospital is significantly reduced.
3. The chance of contracting COVID-19 is significantly reduced.
4. If you have contracted COVID-19, then the chance of passing it on to others is also significantly reduced.

So, the upshot is that in a vaccinated population, the virus has less chance to spread by opportunities for people to contract it, and if they contract the virus they are much less likely to spread it. Importantly COVID-19 in a vaccinated person is a much milder disease.

Last updated: 22 Jul 2021 4:41pm
Declared conflicts of interest:
None declared.

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