In early March, more than 6,000 doses

were supposed to be shipped to the city of Detroit, Michigan.

But the mayor said, no thanks.

"Moderna and Pfizer are the best.

And I am going to do everything I can

to make sure the residents of the city of Detroit get the best."

He was referring to these numbers: the vaccines' "efficacy rates."

The vaccines from Pfizer/BioNTech and Moderna have super high efficacy rates:

95 and 94 percent.

But Johnson & Johnson? Just 66%.

And if you only look at these numbers, it's natural to think that these vaccines

are worse than these.

But that assumption is wrong.

These numbers are arguably not even the most important measure

of how effective these vaccines are.

To understand what is, you first have to understand what vaccines are even supposed to do.

A vaccine's efficacy rate is calculated in large clinical trials,

when the vaccine is tested on tens of thousands of people.

Those people are broken into two groups:

half get the vaccine, and half get a placebo.

Then, they're sent out to live their lives,

while scientists monitor whether or not they get Covid-19 over several months.

In the trial for Pfizer/BioNTech, for example, there were 43,000 participants.

In the end, 170 people were infected with Covid-19.

And how those people fall into each of these groups determines a vaccine's efficacy.

If the 170 were evenly split,

that would mean you're just as likely to get sick with the vaccine as without it.

So it would have a 0% efficacy.

If all 170 were in the placebo group, and zero people who got the vaccine were sick,

the vaccine would have an efficacy of 100%.

With this particular trial, there were 162 in the placebo group,

and just eight in the vaccine group.

It means those who had the vaccine were 95% less likely to get Covid-19:

The vaccine had a 95% efficacy.

Now, this doesn't mean that if 100 people are vaccinated, 5 of them will get sick.

Instead, that 95% number applies to the individual.

So, each vaccinated person is 95% less likely than a person without a vaccine

to get sick, each time they're exposed to Covid-19.

And every vaccine's efficacy rate is calculated in the same way.

But each vaccine's trial might be done in very different circumstances.

"So, one of the biggest considerations here, when we look at these numbers,

is the timing in which these clinical trials were performed."

This is the number of daily Covid-19 cases in the US since the pandemic began.

The Moderna trial was done completely in the US, here, in the summer.

The Pfizer/BioNTech trial was primarily based in the US, too, and at the same time.

Johnson & Johnson, however, held their US trial at this time,

when there were more opportunities for participants to be exposed to infections.

And most of their trial took place in other countries, primarily South Africa and Brazil.

And in these other countries, not only were case rates high,

but the virus itself was different.

The trials took place as variants of Covid-19 emerged,

and became the dominant infections in these countries;

variants that are more likely to get participants sick.

In South Africa, most of the cases in the Johnson & Johnson trial were that of the variant,

not the original strain that was in the US over the summer.

And despite that, it still significantly reduced infections.

"If you're trying to make one-to-one, head-to-head comparisons between vaccines,

they need to have been studied in the same trial, with the same inclusion criteria,

in the same parts of the world, at the same time."

"If we were to take Pfizer and Moderna's vaccines,

and redo their clinical trial at the same time that we saw J&J's clinical trial,

we might see quite different efficacy numbers for those."

These efficacy numbers really just tell you what happened in each vaccine's trial,

not exactly what will happen in the real world.

But many experts argue this isn't even the best number to judge a vaccine by anyway.

Because preventing any infection at all is not always the point of a vaccine.

"The goal of a vaccine program for Covid-19 is not necessarily to get to 'Covid zero,'

but it's to tame this virus, to defang it,

to remove its ability to cause serious disease, hospitalization, and death."

It helps to look at the different outcomes of an exposure to Covid-19 like this:

The best-case scenario is, you don't get sick at all.

The worst case is death.

In between, there's being hospitalized, severe-to-moderate symptoms,

or having no symptoms at all.

In the absolute best circumstances, vaccines give you protection all the way to here.

But realistically, that isn't the main objective of Covid-19 vaccines.

The real purpose is to give your body enough protection to cover these possibilities,

so if you do get an infection, it feels more like a cold

than something you'd be hospitalized for.

And this is one thing that every one of these Covid-19 vaccines do well.

In all these trials, while some people in the placebo groups were hospitalized,

or even died from Covid-19,

not one fully vaccinated person, in any of these trials,

was hospitalized or died from Covid-19.

"One thing that I wish that mayor would have understood,

was that all three vaccines have essentially 100% effectiveness in protecting from death."

The mayor of Detroit did backtrack, and said he'd start taking Johnson & Johnson doses,

because it's still "highly effective against what we care about most."

Efficacy matters. But it doesn't matter the most.

The question isn't which vaccine will protect you from any Covid infection,

but which one will keep you alive?

Or out of the hospital?

Which one will help end the pandemic?

And that's any of them.

"The best vaccine right now for you is the one that you're offered."

"With each shot that goes into someone's arm, we get closer to the end of this pandemic."