Man: Pfizer only.

If they try to give me Johnson & Johnson,
I'll tell them to give me COVID instead.

Narrator: The internet seems to know
exactly which vaccines are the best

and the worst.

M: Moderna?

More like mediocre, average.

We don't do average.

Mira Fricke: Humans love comparing.

No wonder we're also doing it
with the COVID-19 vaccines.

The problem is you can't
compare vaccines that easily.

And doing so might even
be harmful in a pandemic.

N: We tend to look at these numbers -

efficacy rates -

because they measure
how likely you are to get COVID-19

after you've been vaccinated.

MF: The problem is that these numbers
were not created equal.

Instead, they are determined by when
and where the efficacy trials took place.

Carlos Guzmán: I think

that simple comparisons
of vaccine efficacy out of context

can lead to very wrong conclusions.

There are key differences
in the study population for example,

age, gender, genetic environmental factor,
preexisting conditions.

N: So how do efficacy trials work?

Participants are split into two groups.

One group gets the vaccine;
the other, a placebo.

They then go about their lives as usual.

After a certain period of time,

researchers count
how many of them caught COVID-19.

If all participants who got sick
came from the placebo group,

and zero from the vaccine group,

the vaccine would be 100% effective.

And if exactly the same number of people
from both groups got sick,

the vaccine efficacy would be zero

because the risk of getting infected
didn't change with the vaccine.

But the chance of the participants
getting the disease during a trial

corresponds with the overall
infection rate in their environment.

CG: There are also differences in terms
of presence or absence of virus variant

that are neutralized more
or less efficiently by the antibodies

stimulated by the type protein
of the original SARS‑CoV‑2 virus

that is the one that was included
in current vaccines.

MF: So while we think
we know which vaccine is best,

our opinions have actually been influenced
by circumstantial factors.

N: Let's look at an example.

The Moderna and Pfizer trials
were performed mostly in the US

and before the arrival
of more infectious variants,

like the one from the UK or South Africa.

The AstraZeneca or Johnson & Johnson
trials, on the other hand,

were either conducted later

or in countries

where more infectious variants emerged
and became dominant in infections.

MF: So efficacy rates
will never be exactly the same

in a real world setting,

and they can change over time.

CG: For example, recently we have
the report from Qatar,

where 50% and 45% of the infection

are caused by the South African
and the British variant.

This study showed us that the efficacy
of the BioNTech/Pfizer vaccine

drops to 89% and 75% for infection caused
by the British and South African variant.

MF: But maybe there's been too much
fixation on effectiveness all along.

N: Effectiveness is usually the metric
for the best possible outcome:

no symptoms at all.

Instead, we could look at

how the vaccines prevent hospitalization
and death from COVID-19,

because all these vaccines
do that equally well.

MF: Now there is one other aspect
that influences how we judge vaccines:

side effects.

N: Reports of rare blood clots
have made headlines

and got people worried.

The EU also decided
not to renew its contracts

with AstraZeneca and Johnson & Johnson.

All this might give the impression
that some vaccines are worse than others.

MF: But again, it's not that simple

because everyone's individual risk
of getting infected

influences the assessment
of how beneficial each vaccine is.

N: Let's look at an example
with the AstraZeneca vaccine

and assume moderate infection rates
of 55 cases per hundred thousand.

Out of 100,000 people under the age of 29,

around two will develop a rare blood clot
after the AstraZeneca vaccine,

but none would have needed intensive care
with a COVID-19 infection.

But someone over the age of 60

is much more likely to end up
in intensive care with COVID-19

and less likely to develop
a rare blood clot.

MF: That's why some governments
recommend the AstraZeneca vaccine

only for people aged 60+.

But this assessment changes
if the infection rates are higher.

N: Let's look at the same calculation
but with higher infection rates.

Here, 401 cases per hundred thousand.

Now everyone is more likely to end up
in intensive care with COVID-19

than to develop a blood clot
after a vaccine.

In this scenario, the benefit
of getting the AstraZeneca vaccine

outweighs the risk of rare blood clots
for all age groups.

CG: And of course,

for a preventive intervention aimed
at healthy individuals, like vaccine,

it is crucial

that the risk-benefit balance

is acceptable for different population,
groups or even individuals.

MF: So are some vaccines
worse than others?

If we just look at side effects,

some perform slightly better,

at least from what we know so far.

But that's only one aspect

and shouldn't be the only one we consider.

CG: I think that the key issue

is that the best vaccine
or vaccination program

is the one that allows us
to prevent disease and death.

And of course, to reduce the direct
and indirect consequences -

negative consequences -

on the bad damage.

MF: Any vaccine that received
emergency approval from the WHO

protects against severe cases of COVID-19.

They prevent deaths
and help end this pandemic.

N: So as long as vaccines are scarce,

there's a pretty good argument
to take whichever one is available to us,

because if we insist
on getting a specific vaccine,

we might prolong this entire pandemic,

and that can cost lives.

Subtitles by Maurício Kakuei Tanaka
Review by Carol Wang