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The Australian Technical Advisory Group on Immunisation (ATAGI) loves a preference, and this single word “preferred” has driven much of the uncertainty around what any given individual should actually do.

ATAGI holds that AstraZeneca is not the preferred vaccine for people aged under 60, but a preference is not a directive. It’s not even a recommendation. It’s slightly stronger in meaning than the serving suggestion on the box of a Lean Cuisine.

ATAGI’s use of waffly language is deliberate. If we’re being generous then that makes sense because it is difficult to make strict prescriptions about the delivery of a new vaccine we’re still learning about in a dynamic setting where one’s risk of infection can change in a day. Also if one month’s firm, confident recommendation turns out to be incorrect, there won’t be much trust in the next recommendation.

The new position held by ATAGI is that an interval of four to eight weeks between doses of AstraZeneca is preferred (there’s that word again) in an outbreak setting. The efficacy of the AstraZeneca vaccine is greatest with a >12-week interval between doses (>80% efficacy) and drops when the doses are closer together (55% efficacy for <6-week interval). Efficacy refers to the reduction in symptomatic cases of COVID-19 among vaccinated people vs the number of cases in non-vaccinated people in a given time period.

Studies have found a relationship between clinical trial efficacy and the level of an individual’s COVID-19 antibodies and eventually we will be able to accurately assess someone’s immunity with a blood test, as we currently do for diseases such as hepatitis B and rubella. 

The key question now: is it better to have protection sooner at the expense of protection longevity or do you wait longer with less protection and get better longevity? Also, what’s the difference between waiting four weeks or eight weeks for the booster? A lot can happen in four weeks and this gaping window opens another front of uncertainty.

In short, the efficacy of an eight-week booster is about 60%, which is not that different to the four-week interval’s 55%. If someone has already decided to get an early booster, there isn’t much to be gained by quibbling over a measly 5% efficacy. ATAGI could help offset a lot of uncertainty by just calling it four weeks for an early booster.

As for whether it’s better to get earlier or longer lasting protection, that seems to be a question the people of Greater Sydney are answering with their feet. Most people want any vaccine as soon as possible and the likely availability of future boosters or revaccination with an mRNA vaccine means there isn’t much incentive to hold out for longevity.

Another recommendation from ATAGI is that people under 60 in an outbreak setting who cannot access an mRNA vaccine “reassess the benefits to them” by being vaccinated with AstraZeneca. Again, not prescriptive language but the decision for people aged 20-50 can be reduced to two numbers. The risk of thrombosis with thrombocytopenia syndrome (TTS, a treatable condition) from AZ is 3 in 100,000. The chance of dying from COVID if infected is 120 in 100,000. I know what my preference is.

Mitchell Squire is a GP and a visual satirist for Crikey.