It's understandable some people are feeling uncertain about getting a COVID-19 vaccine when available. We’re answering some common questions about the Delta variant and vaccine effectiveness.
Black and white microcopy image of the SARS-CoV-2 virus.

Are COVID-19 vaccines effective against the Delta variant? We explain. Image recorded at our Australian Centre for Disease Preparedness. It is a transmission electron micrograph of several SARS-CoV-2 particles sitting on the surface of host cell filopodia. The particle at the end point of the right hand filopodium shows the distinct peplomer spikes.

Australia is having a major challenge with the Delta variant of concern right now. 

It’s understandable that some people are wondering how effective a COVID-19 vaccine will be. Especially when we hear about new variants emerging that are more problematic than ever before.

Vaccination is one of the best protections we have against COVID-19 right now. We looked at this in-depth in our previous vaccine explainer.

Are COVID-19 vaccines effective against virus variants like Delta?

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Dr S.S. Vasan is our COVID-19 project leader. Last year he led the preclinical trials of the Oxford-AstraZeneca vaccine at our Australian Centre for Disease Preparedness (ACDP) in Geelong. Since the beginning of the pandemic, Vasan has been closely monitoring new variants of concern of SARS-CoV-2 (the virus that causes COVID-19) and researching whether vaccines will work against them.

We asked Vasan to answer some common questions about virus variants and vaccines.

What is a SARS-CoV-2 variant?

Simply put, when the virus changes from its original genetic makeup, we call it a variant.

To detect variants, we compare the virus genomes against the original or reference isolate called ‘Wuhan-Hu-1’ and see what’s changed.

Even a single mutation technically makes it a ‘variant’. Viruses like SARS-CoV-2 need to make copies of themselves in order to survive. When they make those copies, sometimes the copies might have errors. Imagine using a photocopier to make a copy of a copy of a copy and so on. Eventually, some letters might look a little different where the ink has smudged, or part of the copy is missing, or new letters have even been added in somehow.

Because mutations are especially frequent for RNA viruses like SARS-CoV-2, we’re looking at tens of thousands of variants. But most of them are not concerning – only a handful are currently of interest or concern (more on this below).

How much more contagious is Delta than previous SARS-CoV-2 variants? 

The Delta variant of concern is the most important to date. 

According to the World Health Organization (WHO), there is increasing evidence of greater transmissibility and secondary attack rate (disease spreading to those close to the person who’s infected).

The Delta variant has spread to at least 135 countries, including Australia.

And what is Delta Plus?

Some Delta isolates have a mutation called K417N, which is also present in the Beta and Gamma variants of concern. Some media outlets have dubbed this ‘Delta Plus’. Scientists are studying this mutation’s impact on vaccines and antibody therapies.

Is Delta more deadly?

The WHO says this variant of concern has an increased risk of hospitalisation. This is unsurprising because increased transmissibility could go hand-in-hand with case severity until most of the world’s population is vaccinated.

A study from February to June in Canada, yet to be peer-reviewed, compared non-variants of concern with Delta. It found people infected with the Delta variant were:

  • 105% more likely to be hospitalised
  • 241% more likely to be admitted to an intensive care unit
  • 121% more likely to die from the disease.

For the other three variants of concern, these values were 52%, 89% and 51% respectively. This shows the Delta is the most problematic variant of concern to date.

Does Delta affect younger people more?

We’re starting to see this, but comprehensive evidence will take time.

One of the largest studies of its kind in India, yet to be peer-reviewed, showed that mortality increased by almost 40% in the second wave. This was particularly in the younger patients of age less than 45 years.

It’s especially sad to see media reports of younger people in Australia dying of COVID-19. This is why vaccinations are so important to protect our entire population.

Are the Pfizer and AstraZeneca vaccines effective against the Delta variant?

Yes, both vaccines are effective against the Delta variant. A peer-reviewed study in The New England Journal of Medicine showed that after two doses:

  • Pfizer-BioNTech vaccine is 85.3 to 90.1% effective against symptomatic disease caused by Delta 
  • Oxford-AstraZeneca vaccine is 61.3 to 71.8% effective against symptomatic disease caused by Delta

If you get infected after you’re vaccinated, it is likely to be mild rather than severe disease. Therefore, vaccination is absolutely worth it – both to protect yourself and to reduce transmission to our family and community.

If you’re waiting for your second dose, will your first vaccine provide any protection against Delta?

Absolutely! The New England Journal of Medicine paper reported 25.2 to 35.7% effectiveness after one dose of either vaccine against the Delta variant.

So even one dose of Pfizer or AstraZeneca will give you some protection against Delta. And set you on the path to getting even better protection from your second dose.

Do variants happen in populations where the disease is spreading fast?

The more a virus is able to replicate and spread in a population, the greater the likelihood of mutations of consequence. Where the environment permits highly transmissible variants, we also expect disease severity to go up.

But if we halt transmission, we can suppress the spread of variants. This is why vaccinations and lockdowns are an essential part of a pandemic response. They drive transmission down, and drive the virus evolution towards less severe disease outcomes.

What is a ‘variant of concern’ and a ‘variant of interest’? 

National ‘concern about a variant’ of SARS-CoV-2 is often justified. But that doesn’t necessarily make it a ‘variant of concern’ to the WHO and the rest of the world.

The definitions we use in Australia are consistent with the US Centers for Disease Control (CDC) and the WHO.

Definition of ‘variant of interest’ (sometimes called a ‘variant under investigation’)

  • Changes to receptor binding (the way the virus attaches to cells)

  • Reduced antibody neutralisation

  • Reduced efficacy of treatments

  • Potential diagnostic impact

  • Predicted increase in transmissibility or disease severity.

These four variants – Eta, Iota, Kappa, Lambda – are of interest. This is one step below concern. 

A ‘variant of concern’ has a greater impact across all these measures:

  • Changes to receptor binding, often targeted by vaccines

  • Significantly reduced antibody neutralisation

  • Reduced vaccine or treatment effectiveness

  • Diagnostic detection failures

  • Evidence of increased transmissibility and more severe disease (in terms of hospitalisations or deaths).

Out of tens of thousands of variants, only four are currently of concern to the WHO: Alpha, Beta, Gamma and Delta.

There is one step higher than a variant of concern – called a ‘variant of high consequence’. Thankfully, we don’t yet have one for SARS-CoV-2.

Will we need booster shots to keep up with the variants? If so, how soon?

Research is ongoing on this topic, called ‘vaccine matching’. If we take both doses of either vaccine, we should be okay for at least a year, based on neutralisation efficacy studies to date.

New guidelines will emerge based on how the virus evolves, and how the vaccines are performing.

What about other variants of concern – do the vaccines work against those too?

Some variants like the D614G (dubbed the G-strain) attract a lot of media attention. But they don’t necessarily affect vaccines, as my team was the first to demonstrate last year.

Others like Beta affect many first-generation vaccines. There are also newer vaccines, such as the Indian Institute of Science’s warm vaccine Mynvax, which withstood all four variants of concern in our laboratory tests.

So we do have positive news. We don’t yet have a SARS-CoV-2 variant of high consequence that significantly reduces the effectiveness of prevention or medical countermeasures.

Hopefully most people will be vaccinated before we face that situation. Getting a COVID-19 vaccine is one of the best protections we currently have.

But you’ll be safe knowing the vaccines available in Australia are effective at protecting against severe disease from the Delta variant.


  1. Why not make a conventional vaccine like all others using some of the dead virus, instead of pushing the experimental mRNA shot? Knowing how RNA DNA and protein work together and need each other, we shouldn’t be altering them. Yeah the modified protein created by the modified RNA injected working with your DNA makes your immune system hyper to reduce symptoms of covid, the mprotein and mRNA will inevitably likely produce mDNA, yeah… these are the questions with unknown answers right now. Surly we have plenty samples of the virus now to be able to make an actual proper / safer vaccine.

    1. The “established whole microbe approach” includes inactivated, live attenuated, and viral vector vaccines (, so yes Australia does have Oxford-AstraZeneca that fits the latter category.

      mRNA is newly approved but researched and studied for decades, and held to the same rigorous safety and effectiveness standards as all other types of vaccines (

      1. Not sure if you will know, but any idea why are we only using the mRNA vaccines now? Why not 5 years ago?

        1. Great question. “Their application used to be restricted by the instability and inefficient in vivo delivery of mRNA but recent technological advances overcame these issues.”

          Scientists have been working on it for decades, just as they have been on foldable smartphone screens before they are “suddenly” available.

          1. The breakthrough is likely heavily influenced by the enormous global funding made available. Vaccines have “failed” to emerge in the past as commercial considerations intervened. With covid that intervention has not happened.

      2. Pretty certain I read somewhere AZ is an mRNA vaccine. It is just produced differently to Pfizer?

        1. Hi Anthony, the AstraZeneca is actually a “viral vector” vaccine, rather than an mRNA vaccine. The World Health Organization made a great explainer about vaccine types here:

          Team CSIRO

    2. I’m searching online for “mDNA” but just get results for “MDMA” 😀 Oh… here we go… MDNA is mitochondrial DNA. I don’t think there’s such a thing as messenger DNA.

      1. Yes AZ is a viral vectored vaccine. messengerRNA (mRNA), not to be confused with microRNA (miRNA), is a key part of the mechanism whereby the instructions in our DNA are converted into proteins. is a good resource on transcription followed by translation. Apologies jonmay217 and Anthony Rogan, I could not keep with recent comments until now due to heavy work load. Another good resource for popular science articles is CSIRO’s own Double Helix magazine (

  2. Easily understood writing. Thanks.

  3. Very informative,. Thank you.

  4. thanks useful factual easy to understand

    1. Updated figures just reported by the BBC reconfirms that having two doses of Covid vaccine remains the best way to protect against the Delta variant. 2.5 million tests results from 743,526 participants in the UK’s Covid-19 household-infection survey shows the Oxford-AstraZeneca jab is 71% protective and the Pfizer-BioNTech is 93$ effective against symptomatic infection two weeks after the second dose.

      1. I thought I read somwhere that although initially Pfizer is more effective than AZ, with time the effectiveness of both become closer together. Is this correct?

        1. Dear Brian Bycroft, firstly I apologise for the delayed response due to heavy work load. The study you are referring to is this one which showed that the gap between Pfizer and Oxford-AstraZeneca is less (78% versus 61%) 90 days after the second dose. This also shows that, as a population, we have to continue to be vigilant in terms of following personal protection, hand hygiene and social distancing — in combination with getting both doses of either vaccine as soon as practicable.

          1. As I believe immunity declines with age so at what age does the immune system start to decline in a healthy person

            For example what would the efficacy be for a 70 -75 year old reasonably active and double vaccinated with AstraZeneca (4 months ago? )

  5. What are the long term effects of these vaccines? Thalidomide was initially thought save, and look at all the deformities it caused in babies. Many drugs that were rushed out have proven to have long term side-effects. Will this vaccine cause Parkinsons or other major/minor illness after 10 years or so? I am not an anti-vaxer, my concern is the long term effects on my health. I know getting vacced now will help protect me from the seriousness of Covid and death from Covid, which justifies the now, but what about later?

    1. Thalidomide Trust, is a registered charity, supporting a unique community of people who are living with disabilities as a result of their mothers taking the drug thalidomide during the first three months of pregnancy.

      And they STRONGLY recommend taking the vaccine.

      Vaccine testing is being falsely equated with thalidomide development 50 years ago! See

      “Thalidomide was trying to solve the problem of sleeplessness and was marketed with zero data, no efficacy or safety or randomized trials. Covid vaccines are trying to solve the problem of a life-threatening pandemic that has killed hundreds of thousands of people and there are many randomized trials showing the vaccines are highly effective. The fact that people were efficient and fast does not mean that safety steps were skipped”. 

      1. I lived through the thalidomide disaster. In the UK the drug was promoted as an anti morning sickness drug, not as a sleep drug.

        1. I got the information about different vaccination as between the age of’s really helpful for all of us during this pandemic period.

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