Blood clot safety advice changes Aussie COVID-19 vaccine plans

This article was published on
April 9, 2021

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Australia's COVID-19 vaccine rollout has hit a major hurdle with the Government announcing it will accept the advice of the Australian Technical Advisory Group on Immunisation (ATAGI) and prioritise the Pfizer vaccine over the AstraZeneca vaccine for Australians under 50.

Australia's COVID-19 vaccine rollout has hit a major hurdle with the Government announcing it will accept the advice of the Australian Technical Advisory Group on Immunisation (ATAGI) and prioritise the Pfizer vaccine over the AstraZeneca vaccine for Australians under 50.


What our experts say

Context and background


Media briefing

Media Release

Expert Comments: 

Dr Jose Perdomo

In my opinion this is the correct decision. Even though the risk of serious thrombosis following AstraZeneca’s vaccination is extremely low, I believe that restoring public confidence in vaccination was a key consideration by ATAGI.

The fact that people under 50 will have a choice (and this is a challenge, since we don’t know the number of Pfizer vaccines available in Australia) is likely to reduce vaccine hesitancy in this population. This is essential if the final aim of the vaccination program, i.e. achieving herd immunity againstCOVID-19, is to be reached in a reasonable timeframe.

Overall, ATAGI’s is a cautious approach, but there were strong reasons, both medical but mainly social, to make this decision

Dr Jane Williams

This could be a good and timely opportunity to donate Australia’s AstraZeneca supplies to neighbours such as PNG, where COVID infection is prevalent, and the benefits of this COVID-19 vaccine may outweigh the risks.

Benefit-harm ratios are difficult to calculate because they rely on value judgements about which benefits and harms matter. We are in the fortunate position of negligible community transmission because our borders are closed and people entering Australia have to undertake what can be a very onerous quarantine.

Many Australians wishing to return can’t do so, and Australians wanting to travel to see family or work can’t do so. Delays to vaccine rollout will presumably necessitate ongoing border closure. If we accept that this is an avoidable harm, it should arguably be included in a benefit-harm calculus.

Similarly, if Australia is unable to produce the vaccine preferred for under50s, there’s a risk that supplies of Pfizer intended for countries with less capacity to pay will be diverted to Australia. This is also a harm.

Dr Roger Lord

Yesterday the European Medicines Agency (EMA) reported that a possible link might exist between vaccination with the AstraZeneca COVID-19 vaccine and development of an immune response leading to the development of unusual blood clots which resemble an atypical heparin-induced-thrombocytopenia. Identification of specific risk factors associated with the event are currently not possible.

This adverse event is rare with 169 cases of cerebral venous sinus thrombosis (CVST) and 53 cases of splanchnic vein thrombosis reported from the 34 million people vaccinated in the European Union and UK since 4 April 2021. A higher incidence of the adverse event in younger people, particularly females subsequently prompted Britain’s vaccine advisory board to recommend that the AstraZeneca COVID-19 vaccine not be provided to those under the age of 30.

Recommendations from the EMA review has now been considered by the Australian Technical Advisory Group on Immunisation (ATAGI) and the Therapeutic Goods Administration (TGA) which have recommended the Pfizer COVID-19 vaccine is now preferred for adults aged under 50 years. It is difficult to ascertain how much of the advice provided represents independent assessment of safety guidelines or a consensus with the direction provided by European regulators. No indication is provided by the Australian regulator as to why the AstraZeneca vaccine should not be used for individuals under 50 while Britain’s regulator has set this limit much lower at 30 years.

These significant differences between regulators in the absence of any new information suggest vaccine safety may not be the principal focus of discussions. Switching individuals from the AstraZeneca vaccine to the more costly Pfizer COVID-19 vaccine may also prove problematic with five cases of severe allergic reaction per one million doses being reported. These allergic reactions, like the unusual blood clots, are extremely rare and specific risk factors with use unknown.

Professor Ross Gordon

The announcement that the AstraZeneca vaccine is now not advised for those aged under 50 years old has certainly put a major dent in the vaccination strategy here in Australia. From a behavioural sciences perspective, people appreciate timely, clear, transparent, easily understood and honest information on which to base their health decisions. Unfortunately, this has been lacking so far in the Australian vaccination strategy.

While the recommendation on the AstraZeneca jab is understandable and based on proceeding with the utmost caution – there is concern that it could lead to increased vaccine hesitancy. To address this, those in charge of the strategy should communicate clearly with the public regarding when they might get the vaccine, where, how, which type of vaccine they will receive (Pfizer or AstraZeneca) and any risks involved. Furthermore, it will be important that the promises made to the public from the communication of this information are met. This will necessitate swift negotiation and procurement of other approved vaccines for those aged under 50 (e.g. Pfizer), better planning and coordination of vaccine rollout, improved infrastructure, and effective communications with stakeholders (e.g. GPs, pharmacies, hospitals, clinics) and the public. Involving behavioural scientists, along with other experts in this approach can help Australia’s vaccination strategy get back on track.

Associate Professor Hassan Vally

It's really important that the latest developments regarding the AstraZeneca vaccine and blood clots are put into perspective. It is certainly a significant setback and one we would prefer we didn’t have to deal with, however, we have to be clear that this is an extremely rare side effect and once again the Australian government has taken an extremely cautious approach to protect our health.

In making the adjustments to the vaccine rollout and making Pfizer the preferred vaccine for those under 50 years of age, the goal is to mitigate the risk of this blood clotting syndrome even further which looks to mainly affect those in the younger age groups.

Professor Nikolai Petrovsky

Last night the Prime Minister announced that the Australian government was withdrawing the recommendation that the CSL/Astra Zeneca adenovirus vector vaccine be given to Australians under 50 years of the age, bringing Australia in line with previous decisions taken by regulators in Germany, Norway, Canada and other countries where this vaccine is being used.

This action is overdue given the strong evidence from overseas that the CSL/Astra Zeneca vaccine causes a rare but potentially fatal side effect, called vaccine-induced thrombosis with thrombocytopenia, in otherwise young healthy individuals. Instead, the government is now recommending that those under the age of 50 receive the Pfizer mRNA vaccine, which to date has not been associated with such a serious side effect.

While the link between the CSL/Astra Zeneca vaccine and this syndrome is apparently no longer in dispute, the reason why this particular vaccine is causing this problem remains unknown. It has yet to be determined whether this syndrome is being driven by the novel Chimpanzee adenovirus being used for this vaccine, or by some other ingredient in the vaccine -  hopefully this question will now be the subject of intense scientific investigation.

Independent vaccine experts including myself have repeatedly told the government that it was extremely risky to put all their eggs in the one basket in backing just a few select vaccines associated with CSL.  This was a particularly high risk given the selected vaccines were based on completely novel technologies never before tested in humans. This advice was unfortunately ignored.

The absence of a successful and diversified vaccine industry in Australia is a clear government policy failure that requires immediate attention. It is only by Australia developing such a diversified local vaccine development and manufacturing capacity that we can stop our dependence on the uncertain supply of overseas produced pandemic vaccines, thereby ensuring Australians are at the front rather than the back of the line when it comes to access to the very best vaccines.

Dr Rob Grenfell

Based on the European Medicines Agency finding a possible link between the AstraZeneca vaccine and a rare thrombotic event, ATAGI has recommended Australia consider an alternative vaccine for people under 50 years old.

As with all medicines, we need to carefully consider the risks of vaccinating alongside the risks of not vaccinating. Looking at the European and UK figures, it looks like the possible risk of this blood clot condition is five in a million.

For context, the risk of dying from a motor vehicle accident in Australia in 2018 was five in 100,000.

The risk of death from COVID in people over 30 years old is one in 900, and for those over 50 it’s one in 90. And ‘long COVID’, where debilitating symptoms last for months after initial infection, is a real concern.

Tens of millions of doses of the AstraZeneca vaccine have been given worldwide so far, and from that, we’ve seen a substantial decrease in serious illness from COVID-19.

What’s important is that we do find a way to continue vaccinating Australians against COVID-19, whether that’s using the AstraZeneca vaccine for people over 50 and those at higher risk of COVID-19 under 50; using the Pfizer vaccine where available; or even through the Novavax vaccine expected to be reviewed by the TGA later this year.

Professor Bruce Thompson

The recent announcement by ATAGI recommending that the Pfizer vaccine is preferred over the AstraZeneca vaccine in people under the age of 50 years is significant. The recommendation is in response to the potential risk of the syndrome called Thrombosis with Thrombocytopenia. This is a very cautious recommendation based on the relative risk of a significant adverse event compared with the relative benefit of the medication.

To put this into context, all drugs have risks and side effects. For example, we have seen in recent media reports, that blood clotting is a well described side effect for the contraceptive pill but again it is very rare. For many drugs, a sheet of paper is included in the box that provides the consumer with a pile of information about the medication, how it works and potential side effects. For many routinely used drugs, there is a long list of side effects, which in the main are very rare.

For the case of the AstraZeneca vaccine, the rare chance of blood clotting will now be added to the current list of side effects which include sore arm, headache, lethargy etc. The yet to be fully proven potential side effect of blood clotting in the AstraZeneca vaccine however is more serious, as it has been linked to an increase in mortality. As such the ATAGI have recommended that the AstraZeneca vaccine is given to adults over the age of 50 years who have a greater risk of the serious consequences of COVID including hospitalisation and death. As a person who is greater than 50 years of age (just….), I am very happy to still have the AZ vaccine as the chance of blood clotting is very rare. The good thing is that I can now look out for it.

Dr Indu Singh

Europe halted the AstraZeneca vaccine after rare cases of blood clotting issues with low platelets, the blood cells responsible for stopping bleeding by clotting the blood. On 2nd April the TGA received one report of a case of blood clotting 7 days after the AstraZeneca vaccine. An 82-year-old woman also died in Queensland on 8th April.

Yesterday, the Australian Technical Advisory Group on Immunisation recommended no AstraZeneca vaccine for adults under 50 years.  As there is an increased risk of severe outcomes from COVID-19 in older adults, they will benefit from vaccination. However, the rate of adverse effects is too low and pre-existing conditions could be a factor. Personally, I believe the impact of the vaccine on decreasing coronavirus infectivity out ways any risks.

Platelets stop bleeding by sticking together to clot blood after damage to blood vessels. Platelet function can easily get disturbed, by certain antibodies, clotting blood inside intact blood vessels.

The vaccines produce antibodies against the COVID-19 virus. In rare cases, these antibodies may decrease platelets or disturb their function leading to clotting of blood inside intact vessels called thrombosis.

Look out for warning signs like severe-persistent headache, abdominal pain, vomiting or confusion, 4-20 days after vaccination and maintain social distancing and precautions after vaccination as well.

Prof Adam Finn

There is a lot that remains unclear about the cases of thrombosis and thrombocytopenia being intensively studied by regulators and widely reported in the media. We need to know more about the people affected and we need to understand exactly how the illnesses came about, while many other questions remain unanswered at this time.

However, there are some things that are very clear. The first is that these cases are very rare indeed. The second is that the vaccines that are available and in use in the UK prevent COVID very effectively. The risks of COVID-19 are real, especially for the middle-aged and elderly people alongside those with medical conditions and occupations in health and social care who are currently being offered first and second vaccine doses.

Although numbers of cases are currently falling, making it less likely that people will get infected in the coming days and weeks, as the lockdown is relaxed we can expect transmission rates to go back up again. The risks of death and serious illness from COVID-19 are therefore much greater for all these people than any possible risks due to vaccination. In short, if you are currently being offered a dose of Oxford-AstraZeneca vaccine, your chances of remaining alive and well will go up if you take the vaccine and will go down if you don’t.

We should learn a lot more about all this in the near future as information comes in from the many countries around the world now giving this vaccine to very large numbers of people. While this happens, we need to make our policy decisions carefully to ensure we maximise public health benefits and protect the public as effectively and as efficiently as possible.


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