As COVID-19 cases continue to rise in Australia, there have been mixed messages about whether or not Australians under the age of 60 should get the AstraZeneca vaccination.

Confusingly, the federal government and the Australian Technical Advisory Group on Immunisation (ATAGI) seem to have held contradictory stances. ATAGI still recommends that the Pfizer vaccine is preferred over AstraZeneca for adults aged 60 years. However, it is now saying:

“In the context of a COVID-19 outbreak where the supply of Comirnaty (Pfizer) is constrained, ATAGI reinforces adults younger than 60 years old who do not have immediate access to Comirnaty (Pfizer) should re-assess the benefits to them and their contacts from being vaccinated with COVID-19 Vaccine AstraZeneca, versus the rare risk of a serious side effect.”

This advice falls short of a recommendation for younger adults to take AstraZeneca, which perpetuates uncertainty around the vaccine’s safety. This mixed messaging is understandably confusing and, unfortunately, introduces anxiety around getting vaccinated.

Therefore, today we are demystifying this AstraZeneca debate by answering the following questions:

  1. the individual’s age
  2. the community’s COVID-19 exposure

Firstly, the greater an individual’s age, the more likely they are to experience adverse symptoms from COVID-19 infection. Therefore, the elderly benefit more from being protected by vaccination than younger people. Secondly, higher community COVID-19 exposure means individuals are more likely to contract and spread the virus. Hence, being vaccinated in areas of high COVID-19 exposure is more beneficial.

ATAGI looked at three exposure scenarios when they formed their advice, low, medium and high.

    • Low exposure = First wave of COVID-19 in Australia.
    • Medium exposure = Second wave of COVID-19 in Victoria.
    • High exposure = Europe in January 2021.

In the high COVID-19 exposure scenario, there are approximately 2,110 COVID-19 hospitalisations per 100,000 people aged 60+ (2.11%). Thus, the benefit of the AstraZeneca vaccine in effectively protecting people from COVID-19 is very high, particularly in this demographic. Even in the low COVID-19 exposure scenario, approximately 27.5 COVID-19 hospitalisations occur per 100,000 people aged 60+. As a result, ATAGI unequivocally recommends that the benefits of the AstraZeneca vaccine outweigh the potential risks for people aged 60+.

Beyond the elderly and those who live in high COVID-19 exposure areas, people with pre-existing medical conditions and Aboriginal and Torres Strait Islanders may also be more vulnerable to COVID-19 and benefit more from vaccination. Furthermore, people working in aged care also experience greater social benefits from vaccination. As a result, the federal government prioritised these groups in the first phase of the vaccine rollout.

What are the risks of AstraZeneca?

As with any vaccination, the AstraZeneca vaccine has the potential for adverse side effects. Fortunately, the majority of these side effects are mild and resolve within a few days. For example, the most common AstraZeneca side effects include tiredness, headache, muscle pain, fever and chills.

However, the AstraZeneca vaccine is also associated with a rare and potentially dangerous side effect. Thrombosis with thrombocytopenia syndrome (TTS) is a severe reaction triggered by the immune system’s response to the AstraZeneca vaccine, usually after the first dose. TTS involves blood clots and low blood platelet levels, which arise 4-42 days post-vaccination and can cause hospitalisation and death in extreme cases.

The good news is that cases of TTS are very rare. Since the beginning of Australia’s vaccine rollout, only six deaths from TTS have occurred from 6.8 million AstraZeneca doses (to the 4th August 2021). The specific risk of TTS for an individual depends on their age. TTS prevalence is higher among younger people (40-49-year-olds are the most susceptible) and is lower among the elderly (60-69-year-olds are the least susceptible). Nevertheless, the probability of TTS is at most 0.005% (for 40-49-year-olds), which is comparable to the likelihood of being struck by lightning.

Therefore, whilst TTS is a severe risk, it is very improbable. Nevertheless, the federal government and ATAGI must consider it in its cost-benefit analysis of the AstraZeneca vaccine.

How do the benefits stack up against the risks?

Evaluating the benefits and risks of the AstraZeneca vaccine depends on the individual’s age and the level of COVID-19 exposure in the community. By comparing the prevalence of TTS with hospitalisations, ICU admissions and deaths caused by COVID-19 in specific age groups and exposure settings, we can identify the communities where the benefits of AstraZeneca outweigh the risks.

For example, in a low exposure scenario, the benefits of COVID-19 protection by the AstraZeneca vaccine outweigh the risks of TTS only for people 60 years and older. This finding is consistent with ATAGI’s health guidance that the benefits of the AstraZeneca vaccine outweigh the risks with vaccination for people 60 years and over. Therefore, ATAGI recommends Pfizer over AstraZeneca for people younger than 60 years because the risk of TTS outweighs the benefits of being vaccinated. This judgement is due to the number of hospitalisations, ICU admissions and deaths being comparatively less than TTS cases in low COVID-19 exposure settings.

However, as NSW continues to see cases rise and Victoria enters its sixth lockdown, it is clear that the COVID-19 exposure level is increasing, and the health guidance should adjust accordingly.

At the moment, ATAGI’s guidance states that Australians under the age of 60 can still get the AstraZeneca vaccine with informed consent (i.e. deciding with a complete understanding of the potential risks and benefits).

However, the Australian Medical Association president, Dr Omar Khorshid, still finds this problematic given the growing number of cases. Dr Khorshid objects that ATAGI’s preference towards Pfizer deters Australians from getting vaccinated with AstraZeneca, even when the benefits may outweigh the risks in medium and high exposure scenarios.

Whether ATAGI will change its health guidance to more firmly recommend AstraZeneca is still uncertain. However, if cases continue to rise and the government struggles to increase the supply of alternative vaccinations, the country may move towards the high COVID-19 exposure scenario. In this case, the benefits of the AstraZeneca vaccine will vastly outweigh the risks for almost all Australians.

[Disclaimer: The content of this blog post is provided for information only and should not be taken as medical advice. People should consult their GP for medical advice on vaccinations and make their own decisions on their course of action.]

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