Making a decision in Melbourne about Astra Zeneca? It is all about exposure.

The best vaccine is the one you had weeks before being exposed.

3 Sept 2021

The two major states in Australia -- New South Wales and Victoria -- are now facing escalating Covid-19 outbreaks, and are both actively promoting fast vaccination as a means out of a health crisis.

Vaccines are available, but the question is how fast can they be delivered into peoples arms,

Currently vaccination delivery to younger people is a major priority, as the elderly are already largely protected by an ongoing program of immunisation that has until recently given  preference to the people most vulnerable to bad disease outcomes.

There's plenty of AstraZeneca vaccine, and late arriving supplies of the messenger RNA alternatives from Pfizer now being scavenged by the Federal government from diverse sources.

They are being bought into the country as fast as possible, as the young have been told that Pfizer is the preferred vaccine choice for them.

Not surprisingly, many younger people are having difficulties deciding about whether risks of COVID-19 infection are bad enough to justify taking on the more plentiful AstraZeneca vaccine.

This viewpoint was encouraged by expert medical advice that AstraZeneca is preferred vaccine for the young (which might have been true if outbreaks had been kept under control. But now it has become clear that they have not been controlled.)

Intensity of virus exposure is the key. It's going up exponentially fast.

Infection trajectories in New South Wales 2021

The difficult part of this choice is that the balance of risks and benefits depends upon the numbers of active coronavirus infectious cases a person comes in contact with -- that is intensity of exposure. So far there has been so little active community infection that many people say "why take a risk with a vaccine?".

But the major Delta variant outbreaks in New South Wales, and now Victoria, upset these complacent  risk calculations.

Experience in Sydney hotspots is valuable local information.

The worst hotspots of the current Sydney Delta variant COVID-19 outbreak give a simple pathway for  better understanding the balance of risks in choosing an AZ jab in the arm. 

These unfortunate hotspots forebode what's going the happen in terms of exposure to virus in many other areas of Eastern Australia over the next two months.

The ever reliable Covid-19 data website gives useful numbers on these pockets of disease.

At the moment one of the worst spots is Canterbury Bankstown local government area (LGA) which is clocking at 4664 total cases so far.

The Canterbury Bankstown city website tells us the population of the local government area is 394380 people in 2021.

This gives an outbreak infection rate of near 1200 per 100,000 people so far (3 Sept 2021). (Note this is a minimal value for infection rate as the Sydney outbreak is far from over and vaccination protection is currently incomplete and new cases per day are still accelerating. It is not unreasonable to predict it will rise above 2000 per 100,000 people.).

This tweet by Adrian Esterman helps to frame the credibility of this judgement:

What does such a disease intensity mean in terms of risk?

Fortunately the Australian government has provided a simple table of information for people to use this hotspot number to help them decide about AstraZeneca vaccination:

(Risk Profile for Az Vaccine under various scenarios pdf)

The table compares the risks of vaccination with the benefits assuming that a "high exposure" is present in the community. 

At 1200 cases per 100,000 people, Canterbury Bankstown is now entering this high exposure zone of risk.

The balance of risk depends on people's age.

Even for the youngest adults, the table shows that immunisation with AstraZenaca vaccine poses a small (but usually manageable risk of a blood clot) as compared with the more frequent active infection chances of entering the intensive case unit, and a much more likely stay in hospital with nasty COVID-19 damaging lungs, heart or kidneys.

As far as the Pundit is concerned,  The take home message is don't delay taking the AstraZenaca vaccine, or a second AZ jab,  if it's one you can quickly get an appointment for. 

The best vaccine is the one you had weeks before being exposed.