OK, has been a long week but will try to explain the ATAGI statement published tonight
Like 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.
A rare but serious clotting disorder (thrombosis with thrombocytopenia) has been reported after the AZ vaccine. One case has been reported in Australia to date from about 420,000 AZ vaccine doses which ATAGI noted reported on Good Friday health.gov.au/news/atagi-sta…
While DVTs (deep clots) in general are common and don't seem to be increased following vaccination, emerging evidence suggests that this unusual disorder is probably caused by vaccination.
However, it appears to be quite rare - UK data suggest that it is in the rate of 1 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 (esp healthcare workers)
The other side of the equation is the potential benefit of the vaccine in preventing COVID.
The risk of severe COVID is strongly linked to age. The risk of death from COVID rises roughly three-fold for every 10 years of age. A 50 year old is roughly 10 times more likely to die from COVID than a 30 year old. 
So the benefit of getting vaccinated (and not getting COVID) is much higher for older people than younger people.
Thus, the benefit in preventing COVID through vaccination is greater with age, and 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 about, then the benefit in preventing COVID would outweigh the risk for almost all adults, except for very young adults. This is pretty much the situation in the UK at the moment.
In Australia, we don't have COVID 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 AZ) 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 1 in 200,000 risk of clotting for the benefit of getting protected from COVID earlier, then as long as this was an informed decision, we should respect that choice.
What if you've already had your AZ 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 after a few weeks.
But if you get severe persistent headaches or other unusual symptoms between 4 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 AZ. 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% 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 outbreaks, we can make this choice to take a safer path.

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More from @peripatetical

10 Apr
Hi @bruce_haigh - I can't help with the politics, but I might be able to help with the maths.
As a person in their 70s in Australia, if you had gotten COVID last year, on average you'd have a 38% chance of being hospitalized, a 7% chance of going to ICU, and a 10% chance of dying.
Obviously not everyone got infected, but during 2020, 6 in every 100,000 (1 in 16,000) people in their 70s needed ICU for COVID, and 8 in every 100,000 (1 in 12500) people died from COVID. This was obviously higher in Victoria than elsewhere, and who knows what 2021 will bring.
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Vaccines can protect people both directly and indirectly. If you get an effective vaccine, you directly benefit. You have a reduced risk of getting the disease.
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I'm seeing commentary asking why Australia isn't just rolling out the vaccine prior to regulatory approval, or why the TGA just doesn't register the vaccine based on approvals in other countries.
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A thread about the SARS-CoV-2 vaccines in development. There are a lot of candidate vaccines in development, but we still have a long way to go.
Vaccines work by training the immune system on a virus or a component that doesn't cause disease, so that it can respond more efficiently when it sees the real thing.
For SARS2, there are many different approaches being tried. These can be classed into 4 main groups - inactivated/live attenuated, protein subunits, viral vectors and nucleic acid vaccines.
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Data suggest that viral load does start to increase and is maximal slightly before the onset of symptoms, although there is clearly a lot of variation between patients.
Although it can be hard to pinpoint the exact time of transmission, it also appears that transmission can occur from people with infection before the onset of symptoms.
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