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?

To request a transcript please contact us.

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. What is the recommended time frame between first and second dose of the covid vaccine? The recommendations seem to vary across countries, states and vaccine availability.

    1. Hi Maiky. Pfizer is approved for 3 weeks between doses in Australia while AZ is approved for 4-12 weeks. Additional clinical studies of the AstraZeneca showed that when a longer interval between doses was used, the efficacy increased. But in an ongoing COVID-19 outbreak, like those in Vic, NSW and ACT, it may be more important to get a second dose as soon as possible to get some protection. That’s why the Australian Technical Advisory Group on Immunisation (ATAGI) recommends a shorter interval for the AZ vaccine for people living in outbreak situations.

      Team CSIRO

  2. I tested positive for delta variant in July O havent been tested so ce then .I’m in Fiji and they no longer testing people.How long do I have to wait to get vaccine as I have been told that I now will have antibodies present for Covid 19.

    1. As soon as you can

  3. I have double dose of astrazenica, will it improve my immunity or reduce severity of reaction to virus if I also get double dose of Pfizer as well?

    1. I have this same question. I feel like I have gotten the short end of the stick after getting AstraZeneca and I am nervous about becoming infected even after 2 doses, given the substantially lower level of protection with AZ. When will the people who received AZ have the opportunity to gain a level of immunity comparable to Pfizer, or even better, Moderna recipients?

    2. Hi Bill, the Australian Technical Advisory Group on Immunisation (ATAGI) just put out this statement on boosters/additional shots that might help: It says that at the moment, two doses of Pfizer or AZ have been shown to protect against severe COVID-19. There’s no recommendation yet on a booster shot but we expect there’ll be more news on this in the next few months.

      Team CSIRO

    3. Dear Bill and Pax, both vaccines do protect the individual from severe disease and help prevent the transmission to a large extent. 90 days after the second dose Pfizer and Oxford-AstraZeneca are 78% and 69% effective against Delta. Please see this preprint

      Note that these two vaccines are not strictly comparable because Pfizer is mRNA and Oxford-AZ is viral vectored. The latter does induce robust and polyfunctional CD4+ and CD8+ T-cell responses, and there are now studies looking at Oxford-AZ prime followed by an mRNA booster ( We don’t recommend mixing vaccines unless the ATAGI specifically studies all the evidence and revises its guidance, but I am sharing this to reassure you that both vaccines are good and nobody should feel getting “short end of the stick”.

  4. A family member has said they want to wait for the protein-based vaccine being developed in Adelaide by VAXINE. It’s a legitimate vaccine but it’s only now about to start Phase 3 trials. Any information about the chances of this getting through the TGA, and any kind of time frame? Obviously I’m trying to push the “sooner-is-better” Phizer,, Moderna or AZ, but I’m hitting a brick wall!

    1. Hi Tracey, unfortunately we don’t have information on that one. The National Centre for Immunisation Research and Surveillance has a webpage monitoring vaccine candidates here that might be useful: But the earlier people get vaccinated, the sooner they will be protected. All the best talking to your family member.

      Team CSIRO

  5. Good one Vasan. It’s nice to see you addressing peoples concerns in the comments. Thanks for your knowledge and patience. I note that now that there’s more discussion on there likely being no need for a booster in younger people who have received two AZ doses. Can you comment on what age is younger? If I have had two AZ shots and in my mid 30’s would there be any benefit in getting a second mRNA vaccine, Moderna or Pfizer if it became available?

    1. Thank you Jesse for your kind words. I apologise for the late response due to heavy workload. At present we have been having considerable discussions in the relevant WHO expert advisory groups on the need for additional booster doses (above and beyond the two-dose ‘primary schedule’). There is likely to be a need for it in the future, for both adults and younger people, but we are not there yet. What is a priority for Australia is to get the overwhelming population vaccinated with two doses of either vaccine as soon as practicable. The issue of additional boosters is currently not a priority to the WHO unless the vast majority of the world is able to get at least one and ideally two doses. We cannot protect ourselves and aspire for movement of people and goods unless the rest of the world is also vaccinated, so it’s not just a question of vaccine equity but also in Australia’s public health and economic interest. So at present subsequent doses are not needed until the ATAGI reviews all the data and revises its guidance in the future. Such a booster could well be recommended with the same vaccine or with a mRNA vaccine, and the evidence base is still emerging (see for instance We have to be patient and wait for evidence. Unfortunately public health professionals cannot keep up with unrealistic expectations set by 24×7 news and social media. I hope you will agree. Best wishes.

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