Name: What Does a 95% Effective Vaccine Really Mean?
Uploaded: 2021-05-24T06:20:39.000Z
Duration: 4 min 45 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

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This episode was filmed on March 19th, 2021.

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If I offered you a choice between two things,

and one of them was 95% effective and the other was 66% effective,

you might wonder why I was even bothering.

This is playing out right now with our vaccines for COVID-19.

For the sake of ease, we're going to compare the three vaccines

authorized for use here in the USA, but this basic logic applies to all of them,

like Sputnik V or the vaccine made by AstraZeneca.

The vaccine made by BioNTech and distributed here in the US by Pfizer

And the one by Moderna clocks in at a similar-sounding 94%.

the pharmaceutical division of Johnson & Johnson,

has a 66% efficacy rate, though their data is still awaiting peer review.

But this is not a matter of you having a 5% chance of getting sick

That's just not how the numbers are calculated.

So let's look at how it does work, why we can't compare these numbers,

and why that's ok -- it's still a good thing that we have these vaccines.

This efficacy number comes from phase III clinical trials.

Specifically, it's from their primary endpoints,

meaning the criteria they set ahead of time to determine a vaccine's efficacy.

Phase three trials recruit thousands of volunteers,

some who receive the vaccine and some a placebo.

To calculate the overall efficacy of the vaccine,

they take the number of cases of illness in the placebo group

and calculate how much less likely the vaccinated participants were to get sick.

So, let's look at the BioNTech-Pfizer numbers as an example.

The vaccine and placebo groups had about 21,000 participants each.

There were 8 positive COVID-19 cases in the vaccine group -- that's not 5% of all participants!

Meanwhile, 162 of those who didn't get the vaccine got sick.

So 162 minus 8 equals 154 more unvaccinated people

who got sick compared to the vaccine group.

So it's pretty simple, but the complications arise

when you start comparing one trial to another.

Because each clinical trial arrived at that efficacy number slightly differently.

Moderna and BioNTech looked for any symptomatic COVID-19 after two doses

of vaccine or placebo shots, and counted those as cases in their trials.

By contrast, the Janssen trial looked at efficacy

And they also had different criteria for when a case counted.

Like, Moderna started counting cases that were contracted

14 days after participants got the second shot, while it was only 7 days for Pfizer.

They had similar efficacy numbers, but under the hood,

Not only that, but the trials all happened in different times,

in different places, with different strains of the virus going around.

This is why you can't compare the numbers from one clinical trial to another,

which is hard to do when all the vaccines are all being invented at the same time.

Talking about these numbers in the same breath is comparing apples to oranges,

BioNTech's 95% is different from Janssen's 66%...

So, if you're thinking, “but 95% still sounds better,”

well, there are a few more important points.

All three of these vaccines were 100% efficacious at preventing death in the trials.

Less death and less burden on our hospitals are among the main reasons

health officials are urging people to get whichever vaccine they're offered.

At least as long as they don't have related allergy or other health concerns.

And finally, there's a difference between what happens

in a controlled trial setting and the real world.

Think of efficacy as how well the vaccine works in trials,

and effectiveness as how it works in the real world.

We know one of those, but not the other -- yet.

There are several variables that are likely to affect how things play out.

Like higher or lower proportions of vaccinated people in various communities,

whether more contagious variants become dominant,

and racial and other inequities in vaccination rates.

And in the meantime, I'll be on the edge of my seat,

and wearing a mask if that seat is in public.

But who knew you needed so much math to understand clinical trials?

Well, it turns out if you want a more complex understanding of the world,

you need an appreciation for the numbers that underlie it all.

From fundamentals to stats to complex numbers,

they've got a course to help you learn all about it.

If you want to check it out, you can go to Brilliant.org/SciShow.

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