(soft music)
- [Narrator] We know that COVID-19 spreads
through close contact,
making crowded areas, potentially deadly.
But when some members
of a crowd are immune,
that begins to change.
This is called herd immunity.
It's what's protecting
much of the population
from diseases like measles and polio.
And it's what's driving
vaccine efforts today.
- The notion is,
there'll be enough people in
that community who're immune,
so that when you go to
pass the disease on,
you won't be able to pass it on
in a way that it will propagate.
- [Narrator] Economies around the world
can then safely reopen.
And some say it could put
an end to the pandemic.
But the path to herd immunity
includes big hurdles.
And experts say it
could take years or more
if we get there at all.
We'll explain.
When scientists want to determine
how fast a virus might spread,
they look at its reproduction number.
This represents how many
people could catch the virus
from a single infected person
when no one in the population is immune.
- That helps us to understand
how much additional immunity
or other measures do we
need to implement in place
to start to bring this up
epidemic under control,
as opposed to having it expand over time.
- [Narrator] For example,
for the polio virus,
one infected person will pass the disease
to five to seven people on average.
And from there, the disease
will spread exponentially.
For the novel coronavirus,
scientists say an infected
person will likely spread it
to two to four other people.
- That number can go down
as more people become
immune or also in response
to measures like wearing
masks, social distancing.
- [Narrator] When enough people
in a population gain immunity,
the chances of infecting others goes down.
And once it falls below a certain point,
it becomes harder and harder
for the virus to spread.
As a result,
the majority of the
community becomes protected,
not just those who are immune.
- Over time, the virus dissipates
and really essentially goes away
or circulates at very low
levels in that community.
- [Narrator] Scientists
say there are two ways
a population can reach herd immunity.
Through broad infection or
more commonly vaccination.
To get there through infection,
the majority of the
population needs to get sick
from a virus and develop
molecular defenses
like antibodies that
fight off the disease.
But this method would be deadly.
Experts estimate that less than 5%
of people worldwide have had COVID-19.
We get scientists say,
to reach herd immunity,
that number would have
to be between 60 and 70%.
- If we need to get to
60 or 70% infections,
that's obviously more
than 4 billion people getting infected.
And that would be tens of
millions of people dying,
it would be hundreds of millions
of people getting very, very
sick, it would be devastating.
It would be devastating for
the entire global population,
would be devastating for the economies
and it is wholly unnecessary.
- [Narrator] There's also no guarantee
that broad infection
would lead to eradication.
- We don't have many examples
of complete immunity to viruses
through natural infection
because people continue to
get born in a population.
So every year you have a new
set of babies who are born
and they're going to be
susceptible to that infection.
- [Narrator] Experts say
the more efficient way
to herd immunity, is through vaccination.
That's how scientists eradicated smallpox.
- Because of vaccines,
childhood mortality has
dropped dramatically.
Kids no longer die of measles.
They don't die of a lot of the diseases
that kids used to die of.
Obviously smallpox was one
of the horrible scourges
of humanity throughout
the entire human history.
We don't talk about smallpox anymore.
Because our vaccination
campaign ultimately led
to the eradication of smallpox.
- [Narrator] But experts
say they don't know enough
about the coronavirus to
predict the likelihood
of reaching herd immunity.
And that there are large
barriers to getting there.
Even with a vaccine.
One challenge is that
scientists don't yet know
how long natural immunity might last.
- There's a big difference
between being immune
for six months and being
immune for 10 years.
- [Narrator] For other
respiratory viruses, like the flu,
antibodies tend to decrease over time.
Scientists have observed this
in some patients recovering from COVID-19.
But there isn't enough data yet
to be certain about what
that means for immunity.
How long a COVID-19 vaccine
will last is still unclear,
but it may be measured in months or years,
rather than a lifetime.
Another challenge,
not all vaccines in
development will work the same.
And their effectiveness could differ.
- The better a vaccine is
in terms of the level of protection
and the duration of protection,
the further it gets you towards
that point of herd immunity.
But even a less than perfect vaccine,
if given to enough
people in the population,
can help reduce the spread.
- [Narrator] Scientists
are working quickly
to test dozens of vaccines
already in development.
But it could take years
to figure out which
ones are more effective.
So even if people get vaccinated,
some could be more susceptible
to catching the virus.
But even as cases surge around the world,
some experts are optimistic.
- We suspect that people
who have gotten COVID-19 remain protected
for many months, at least
because we haven't seen a large
number of repeat infections.
We also know that it's possible
to give people multiple vaccinations.
So I'm hopeful that we can
develop over time a vaccine that
even if we do need to give multiple doses,
will be highly effective
in generating the levels
of immunity that we need
to beat this disease.
- [Narrator] If this happens,
global herd immunity may be possible.
But until then, health
experts have stressed
that measures like social distancing
and wearing masks will be key
to slowing down the spread.
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