Why we decided to put the Pfizer vaccine ahead of AstraZeneca for under 50s

This is an edited version of the Twitter feed of Professor Allen Cheng, a member of the Australian Technical Advisory Group on Immunisation, explaining the AstraZeneca decision.

It has been a long week but I will try to explain the Australian Technical Advisory Group on Immunisation’s statement published on Thursday night.

As with all medical treatments, when we have a choice we need to consider the risks and the benefits. In this case, we’re thinking about the risk of a side effect due to vaccination and the benefit of a reduced risk of COVID-19.

Professor Allen Cheng.Credit:Justin McManus

A rare but serious clotting disorder (thrombosis with thrombocytopenia) has been reported after the AstraZeneca vaccine. One case has been reported in Australia to date from about 420,000 AstraZeneca vaccine doses, which ATAGI reported on Good Friday.

While DVTs (deep clots) in general are common and don’t seem to be increasing following vaccination, emerging evidence suggests that this unusual disorder is probably caused by vaccination.

However, it appears to be quite rare – UK data suggest it is occurring at the rate of one case per 200,000 to 250,000 vaccines. Other estimates put the risk as somewhat higher, but still pretty rare.

While there are more cases reported in younger people and women, it isn’t clear if this just reflects the populations that received the vaccine first (especially healthcare workers).

The other side of the equation is the potential benefit of the vaccine in preventing COVID-19.

The risk of severe COVID-19 is strongly linked to age. The risk of death from COVID-19 rises roughly three-fold for every 10 years of age. A 50-year-old is roughly 10 times more likely to die from COVID-19 than a 30-year-old. So the benefit of getting vaccinated (and not getting COVID-19) is much higher for older people than younger people.

Thus, the benefit in preventing COVID-19 through vaccination is greater with age, and the risk of this clotting condition possibly decreases with age.

So how did we come up with 50 years? And why did the UK pick 30 years as a threshold?



If there was a lot of COVID-19 about, then the benefit in preventing COVID-19 would outweigh the risk for almost all adults, except for very young adults. This is pretty much the situation in Britain at the moment.

In Australia, we don’t have COVID-19 in the community at the moment, but we recognise that the risk of incursion is ever present. So the balance of the risks and benefits are different.

This is helpfully explained by this infographic from Cambridge (for reference, the “low exposure risk” corresponds roughly to the risk during the second wave in Victoria).

There are a few caveats here. If we could work out who did and who didn’t get this clotting condition, we might be able to advise younger people better about their personal risk.

If we had a large outbreak, then this risk benefit analysis would change, and we’d have to reconsider this advice. The risk-benefit balance would also be different in countries with even larger outbreaks than the UK.

We also carefully used the word “prefer” (Pfizer over AstraZeneca) in younger people. We respect patient autonomy – that people have a choice about the vaccines and treatments they get.

If a younger person said that they were happy to take a one in 200,000 risk of clotting for the benefit of getting protected from COVID-19 earlier, then as long as this was an informed decision, we should respect that choice.

What if you’ve already had your AstraZeneca vaccine? The good news is that 199,999 out of 200,000 people won’t get anything more than a sore arm and a fever for a day or so, and you’ll be protected against getting COVID-19 after a few weeks.

But if you get severe persistent headaches or other unusual symptoms between four and 20 days after vaccination, seek medical attention (this is different to the common side effects after vaccination, which usually only last a day or so).

The other obvious question is about alternatives to AstraZeneca. I’m not privy to the discussions, but I do know that the Australian government have been in constant communications with vaccine manufacturers.

It’s no secret that there is global competition for available vaccines, and we have secured enough Pfizer for 40 per cent of the population over the remainder of the year. We have access to some more vaccine via COVAX, and Novavax hopefully coming later in the year subject to regulatory approvals and supply.

There’s no question that this decision will slow things down – having onshore capacity to produce vaccine is very valuable.

So over the next few days, Commonwealth and state governments will be working out how the program will look in the coming weeks and months. But because we’re thankfully not dealing with ongoing COVID-19 outbreaks, we can make this choice to take a safer path.

Professor Allen Cheng is a member of the Australian Technical Advisory Group on Immunisation, an infectious disease physician and director of the Infection Prevention Healthcare Epidemiology Unit at Alfred Health. He tweets at @peripatetical.

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