Covid 19 coronavirus: The vaccines – and why humans are really rubbish at calculating risk

If you’re reluctantly taking a less-preferred vaccine, it might not work as well.

If my GP called me now and told me he had a spare Covid-19 vaccine, you wouldn’t be able to slide a bus ticket between “I have a spare vaccine” and “I’ll be there in five minutes”.

Others might be more cautious, picky or even “vaccine elitist”. How choosy might we be? A study published in March in Public Policy Forum, a non-profit Canadian think tank for public-private dialogue, asked 2500 people what they thought of the Moderna, Pfizer and AstraZeneca vaccines. Of the first two, respectively 78 per cent and 77 per cent of participants said they’d take them if offered (Moderna was slightly more preferred). The AstraZeneca vaccine came in third at 70 per cent.

Interestingly, Pfizer’s vaccine was seen as the most effective (84 per cent), followed by Moderna’s (82 per cent), then AstraZeneca’s (76 per cent) and that was mirrored for perceptions of safety. Based on these results, it has been argued that about 10 per cent of people might decline their less-preferred vaccine if offered it.

Why? All three require two doses, so I don’t think it’s got anything to do with that. People like the Pfizer vaccine because they believe that it’s more effective and safer than others. And the Pfizer and Moderna vaccines are more effective – about 95 per cent effective at preventing symptomatic infection, versus 76 per cent for AstraZeneca. All are close to 100 per cent effective for preventing serious infection outcomes.

There has been a lot of coverage about the outcomes of clinical trials that clearly position AstraZeneca as less effective, so I don’t think we should pillory people if they worry it’s less effective – it is, on average.

There has also been a lot of media attention on the potential for clotting with AstraZeneca’s vaccine (and also the Janssen vaccine made by Johnson & Johnson). This has been intriguing to watch because it’s a classic question about levels of risk: if you’re standing in a New York supermarket queue with nine others, the odds of one having Covid are between 10 per cent and 19 per cent. If you then catch Covid, your odds of dying are between 1 per cent and 2 per cent – 1400 times more likely than dying during a skydive. If you receive the AstraZeneca vaccine, your chance of dying from a blood clot is a smidgen over one in a million. But humans are really rubbish at, first, calculating risk in ways that means something to us and, second, weighing relative risks, particularly when we can easily bring images of blood clots to mind.

People who are worried aren’t in the best space to weigh up things such as relative and absolute risks, and that is particularly true of those who are vaccine hesitant. If you worry you’re not getting the best vaccine, it means you’re worrying when you otherwise wouldn’t. But it’s worse than that. Research shows the efficacy of things such as the seasonal flu vaccine are affected by mood. People who are happy, or primed to be happy by watching a funny movie beforehand, produce a better response to vaccinations. This means you may have a less-effective response to a less-preferred vaccine because you’re in a more negative mood.

This also illustrates something broader about the decisions we make as consumers. Specifically, the “tyranny of choice” – the more things we have to choose between, the less happy we are with the particular thing we choose. In part, because we worry that we’re making the wrong decision. You don’t worry that you bought the best pair of shoes if there was only one style to choose from. This is a new position to be in regarding vaccines. I mean, can anyone tell me who produces the seasonal flu vaccine we get?

Finally, because we’re bad at judging risk, we worry this is a life-and-death decision and that makes us worry even more that we have to make the right decision. In fact, it doesn’t matter – all the vaccines mentioned are effective at reducing the risk of dying of Covid symptoms to nearly zero.

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