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Consider a hypothetical scenario: two people, both fully vaccinated, wanting to meet for dinner.
In our hypothetical, they see no one else but each other – perhaps for a romantic date alone.
How much risk from COVID-19 do they face?
To answer this question we need to appreciate the multi-layered protection offered by vaccine.
And to know if it’s a risk worth taking, say risk communications experts, we need to think carefully – about what we would truly regret.
If two people who are fully vaccinated meet, they essentially have three layers of vaccine protection.
First, both are substantially less likely to bring the virus to the meeting because they are less likely to be infected.
AstraZeneca’s vaccine offers between 60 and 67 per cent effectiveness against infection, Pfizer’s 90 per cent and Moderna’s 95 per cent, per analysis of a range of studies by the Murdoch Children’s Research Institute.
Second, if either person is infected but vaccinated, they are less likely to pass the virus on compared to an unvaccinated but infected person.
UK data suggests if the infected person is vaccinated with two doses of Pfizer’s jab, they are half as likely to pass Delta on to their contacts; for AstraZeneca, they are about 25 per cent less likely.
Combined, these two layers produce a very substantial reduction in transmission risk: a 79 per cent reduction for AstraZeneca and a 93 per cent reduction for Pfizer, according to analysis led by the Doherty Institute.
“If you have combinations of brands it will be somewhere in between,” says University of NSW infectious diseases modeller Associate Professor James Wood. “The simple message is: vaccines work really well against transmission and severe disease.”
Third, the uninfected person enjoys protection from their infected friend because they too are vaccinated.
Additional defences, like masks and handwashing, cut that further. An editorial just published in the British Medical Journal, written by Bond University’s Professor Paul Glasziou, suggested the best evidence was that mask-wearing, for example, cut the relative risk by 10 per cent.
However, this overall risk is an average. Some people will be at lesser or greater risk of infection – and some will be at less or greater risk of a bad outcome should they fall sick.
“We’re faced with a classic problem. Risks are quantified on a population level,” said Dr Claire Hooker, a specialist in infectious diseases risk communication at the University of Sydney.
The numbers are an average, and “do not tell you as an individual, where you’re going to land on the bell curve,” she said.
For example, all vaccines do less well at protecting the most vulnerable and the elderly from infection.
The effectiveness of vaccines also changes over time. A person two weeks out from their second jab has peak protection; Pfizer’s vaccine falls from 93 per cent effective to 53 per cent after four months.
Then there is the question of serious illness and hospitalisation.
AstraZeneca offers between 88 and 94 per cent effectiveness, and Pfizer between 75 and 100 per cent.
The protection also varies by age and length of time since vaccination, but by far less than protection against infection.
For people aged between 40 and 64, Pfizer offers 95 per cent effectiveness against serious illness or hospitalisation – even after five months.
But for people over 65, that falls to 90 per cent. For people over 65 classed as ‘extremely vulnerable’ that falls all the way down to 71 per cent.
Working out whether these risks are worth it is tricky, and needs clear thinking, said Dr Hooker.
“Would it be an area of insane regret for me if I brought COVID home to my elderly mother, and she became critically ill?”
If we wanted to keep risk at zero, we could all never leave the house. But most people want to return to something approaching normal life, said Dr Hooker.
“I’ve bought theatre tickets. I cannot wait. I’m going to go and not worry this will mean my children, who are going, will get COVID. I accept the degree of risk, and the fact luck will be involved. My decision quality of life, and things that give me joy.”
Liam Mannix’s Examine newsletter explains and analyses science with a rigorous focus on the evidence. Sign up to get it each week.
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