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Man: Pfizer only.

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

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

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and the worst.

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M: Moderna?

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More like mediocre, average.

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We don't do average.

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Mira Fricke: Humans love comparing.

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

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

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And doing so might even
be harmful in a pandemic.

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N: We tend to look at these numbers -

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efficacy rates -

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

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after you've been vaccinated.

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

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

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Carlos Guzmán: I think

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that simple comparisons
of vaccine efficacy out of context

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can lead to very wrong conclusions.

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There are key differences
in the study population for example,

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age, gender, genetic environmental factor,
preexisting conditions.

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N: So how do efficacy trials work?

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Participants are split into two groups.

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

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They then go about their lives as usual.

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After a certain period of time,

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researchers count
how many of them caught COVID-19.

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

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and zero from the vaccine group,

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the vaccine would be 100% effective.

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

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the vaccine efficacy would be zero

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

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

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corresponds with the overall
infection rate in their environment.

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

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that are neutralized more
or less efficiently by the antibodies

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

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that is the one that was included
in current vaccines.

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

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our opinions have actually been influenced
by circumstantial factors.

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N: Let's look at an example.

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The Moderna and Pfizer trials
were performed mostly in the US

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and before the arrival
of more infectious variants,

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like the one from the UK or South Africa.

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

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were either conducted later

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or in countries

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where more infectious variants emerged
and became dominant in infections.

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MF: So efficacy rates
will never be exactly the same

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in a real world setting,

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and they can change over time.

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

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where 50% and 45% of the infection

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are caused by the South African
and the British variant.

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

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

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

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

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no symptoms at all.

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Instead, we could look at

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

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because all these vaccines
do that equally well.

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

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side effects.

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N: Reports of rare blood clots
have made headlines

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and got people worried.

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The EU also decided
not to renew its contracts

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with AstraZeneca and Johnson & Johnson.

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

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MF: But again, it's not that simple

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because everyone's individual risk
of getting infected

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influences the assessment
of how beneficial each vaccine is.

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

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

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

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

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

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But someone over the age of 60

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

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and less likely to develop
a rare blood clot.

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

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only for people aged 60+.

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But this assessment changes
if the infection rates are higher.

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

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Here, 401 cases per hundred thousand.

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

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than to develop a blood clot
after a vaccine.

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In this scenario, the benefit
of getting the AstraZeneca vaccine

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

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CG: And of course,

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

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it is crucial

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that the risk-benefit balance

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is acceptable for different population,
groups or even individuals.

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MF: So are some vaccines
worse than others?

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If we just look at side effects,

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some perform slightly better,

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at least from what we know so far.

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But that's only one aspect

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and shouldn't be the only one we consider.

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CG: I think that the key issue

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is that the best vaccine
or vaccination program

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

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

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negative consequences -

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on the bad damage.

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MF: Any vaccine that received
emergency approval from the WHO

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protects against severe cases of COVID-19.

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They prevent deaths
and help end this pandemic.

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N: So as long as vaccines are scarce,

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

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because if we insist
on getting a specific vaccine,

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we might prolong this entire pandemic,

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and that can cost lives.

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Subtitles by Maurício Kakuei Tanaka
Review by Carol Wang