What is ATAGI’s role? ATAGI can only give general advice to the government for the whole population.

Its task is to think about the whole population as if it were merged into a single person,or in the case of AstraZeneca,a series of people of different age ranges. It then has to formulate advice based on population-based averages of the benefits and risks of getting the AstraZeneca vaccine,which has a number of limitations.

It’s important to understand context plays a key role in formulating this advice to the government. The risk of the blood clotting and bleeding condition,called thrombosis with thrombocytopenia,from the AstraZeneca vaccine is slightly higher for younger people.

This is only part of what’s driven the advice for Pfizer to be the preferred vaccine for those under 60.

In fact,the risk of dying from this condition is incredibly rare whatever your age.

What has been the bigger driver of the advice is the fact you’re less likely to develop severe disease from COVID if you’re younger,which means the corresponding benefits of vaccination are much lower if you take a narrow view of the benefits of the vaccine being solely the prevention of severe disease.

How did ATAGI draw its conclusions on AstraZeneca?

ATAGI initially said Pfizer was the preferred vaccine for under-50sin April,and then changed this to under-60sin June.

There are several assumptions in ATAGI’s advice which need to be understood.

Firstly,it calculated the risks and benefits of AstraZeneca across three scenarios — low,medium and high exposure risk. ATAGI has presented its advice assuming a low amount COVID circulating in the community,which has been the case until Sydney’s latest outbreak.

A low amount of COVID in the community means there’s a low chance of severe COVID,which is even smaller for younger people. This means there’s less of a benefit of being vaccinated for younger people,which is what has driven the advice for the Pfizer vaccine to be preferred for younger people.

However,the problem with this low prevalence assumption is we’re vaccinating to protect us not just right now,but also against thefuture risk of COVID,and future lockdowns,like the situation we’re seeing in Sydney now.

Once you’re in this situation,even if ATAGI changes its recommendations in response to more COVID circulating,which it did on Saturday,in some sense the horse has already bolted.

Another assumption implicit in ATAGI’s advice that itprefers under-60s get Pfizer,is that Pfizer is available and you have the option to get it now.

However,given the limited supply of Pfizer vaccine,the decision to hold off on the AstraZeneca vaccine is not one to get Pfizer,it is one to hold off on getting vaccinated at all. This leaves you exposed and vulnerable to COVID. This is an important distinction to make,which of course will change as we get more Pfizer vaccine.

Another major limitation in the ATAGI advice is the panel,in dealing with population-level data,takes a very narrow view of the benefits of vaccination:the prevention of severe disease.

It doesn’t take into account other benefits that may be relevant to many people. It doesn’t take into account the prevention of long COVID;the benefits of being vaccinated allowing travel and other freedoms;and,most glaringly,the importance many people place on getting vaccinated to protect their loved ones and the community.

These may weigh heavily on individuals but aren’t taken into account when you look at the risk-benefit calculation from a narrow perspective.

So what’s the bottom line on AstraZeneca?

We must remember the AstraZeneca vaccine is a fantastic vaccine.

It’s safe and effective,and two doses offer almost complete protection against severe disease and death from COVID,includingthe Delta variant.

It does carry a small risk of the blood clotting and bleeding condition,but this risk is incredibly small. COVID is much more of a threat to your health than the vaccine,as we are seeingin NSW right now.

If you’re under 60 years of age,the decision to have the AstraZeneca vaccine is one only you can make. But if you do make it,you should understand the benefits go beyond just preventing severe disease.

Hassan Vally is Associate Professor in Epidemiology at La Trobe University. This article was first published on The Conversation.