The statistical secrets of Covid-19 vaccines


It’s an old question in the world of public health – whether all of these numbers are helping people or overwhelming them. But more and more, it seems that more information about vaccines is alleviating the hesitation rather than exacerbating it. It depends on how people view their risk profile. “If you have Covid, you have 100% Covid, and if you don’t, it’s 0% Covid,” says Olliaro. “You have to take into account the perspective of the individual within the community.”

One of the the hallmark of the pandemic is that it affects different groups of people in different ways. In the United States, the poor and people of color are much more likely to get sick and die from Covid-19 than whites and the rich. The elderly are more at risk than the young.

And like any other medical procedure ever, the vaccines themselves come with risks and benefits. The J&J and AstraZeneca vaccines have been associated with very rare but severe blood clots, which led to a hiatus in use of the J&J vaccine in the United States last month. People with severe allergies may be more likely to experience anaphylactic shock from the two-dose mRNA-based vaccines.

All of these complications create a haze around the decision-making space, making some people’s risk-benefit calculations more complex – or creating a space for people who perceive themselves to be at low risk for Covid-19, or who are more concerned. by the side effects that they need to be, to think that it’s okay not to get vaccinated. “Most people don’t sit there with numbers worrying about the decimal point, thinking, ‘I’m going to weigh the benefit / risk ratio,’” says Alexandra Freeman, executive director of the Winton Center for Risk & Evidence Communication at the University of Cambridge. But just because most people don’t do math doesn’t mean they don’t bite the bullet. As Freeman says, “a risk is very subjective”.

So, OK, let’s talk about these blood clots. Freeman’s group put together a bunch of infographics that woven a few of these threads into a useful tapestry. Instead of comparing the risk of contracting Covid to the risk of getting the vaccine – a problem of apples to oranges – they instead released a document by comparing the potential risk of a blood clot from the AstraZeneca vaccine to its actual benefit, the number of ICU admissions linked to Covid avoided by its use. And then they broke that down by age group and risk of exposure. (In real life, the risk of exposure would be different from country to country and even from profession to profession … and the group assumed an efficacy of 80% for the vaccine in all domains, simplification needed … and they used a fixed 16 week time frame as all of this risks evolve over time as infection rates go up and down. Statistics!)

Out of 100,000 people at low risk of exposure, they calculated, the AstraZeneca vaccine could cause blood clots in 1.1 people and prevent just 0.8 ICU admissions. If you’re the only type of person looking for number one, that seems like a reason to avoid the AstraZeneca vaccine – and indeed, European regulators have limited its use. Fortunately, there are all of these other vaccines.

At the other extreme, among people who for whatever reason have a high risk of exposure – a lot of infections plaguing their county, say – in those 60 to 69 years old, the vaccine might cause just 0.2 cases of blood clots (which seem to affect mostly younger people), but excluding 127.7 people in intensive care. This makes a difficult case. In most of the Winton Center groups, the risk of the AstraZeneca vaccine pays off.

Again, however, the The United States and Europe have ceded the power to evaluate these vaccines to the companies that made them. Each used slightly different protocols and different populations. A multi-arm study of each of them could have ironed out these statistical problems. WHO in fact ad such a test in 2020; nothing seems to have come out of it.



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