- [Narrator] Throughout the pandemic,

athletes have been the most tested population

on the planet, sometimes providing examples

of emerging theories about COVID-19.

And as countries around the world continue

to fight back COVID-19,

attention has turned to Tokyo

where the Olympics are under way.

One challenge some athletes are facing:

testing positive for the virus,

despite having been vaccinated.

This isn't the first we've heard of this.

- What are chances of getting COVID-19

after receiving a vaccine?

Well, you've got a better chance

of buying a winning lottery ticket

but it happened to Warriors player Damion Lee.

- [Reporter 1] He will be out at least two weeks.

It was not a false positive as first thought,

even though Walman had been vaccinated.

- [Reporter 2] Yankees GM Brian Cashman says

that three players tested positive

and three others are likely infected.

About 85% of the team is vaccinated,

including those who have the virus.

- [Narrator] These examples are known

as breakthrough infections,

which occur when someone tests positive

for COVID-19 at least two weeks after getting

their final shot.

Do these breakthrough cases mean the vaccines are failing?

- No, the vaccines are not failing.

The vaccines are working extremely well,

and as expected.

They do protect the majority of recipients

from severe disease.

- [Narrator] That's what vaccines are designed to do:

Prevent death and severe disease.

But most vaccines, including those created

to fight COVID-19

don't completely protect you from infection.

So it's not all that surprising

that breakthrough infections are showing up.

As of July 12th, more than 159 million people in the US

have been fully vaccinated.

CDC data suggests just under 5,500

have had breakthrough infections,

resulting in hospitalizations or deaths.

That's one in approximately 29,000 people

who have been vaccinated.

- Breakthrough infections are something

that we want to monitor

but in terms of their overall influence in the pandemic,

they play a much smaller role

than transmission among people who haven't been vaccinated.

- [Narrator] But these cases raise questions

about our immunity to the virus

and hint at a future in which it isn't gone completely

and we learn to live with it.

That's due in part to variants.

Research shows that variants,

including Delta, can partially evade the immune response

from prior infection and vaccination.

- We mount a really good immune response

against the virus that our body's trying to recognize.

Your body is really good

at recognizing and neutralizing those specific threats.

But when the virus starts to change,

sometimes it doesn't recognize the virus as well.

And so that's how you sort of see it chip away

at that immune response.

That's one of the reasons that health officials

are really sort of concerned

about this global vaccination drive

in order to prevent the virus from spreading,

both to save lives

and to prevent it from further mutating

and evading immune response.

- [Narrator] The Delta variant

is the most contagious version

of the virus to be identified,

but research suggests that full vaccination

is still protective against severe disease and death:

the outcomes that have made COVID-19 so devastating.

What dictates whether someone is more likely

to get infected, even if they're vaccinated?

Dr. Hatziioannou says there are four main variables.

First is the amount of virus that is circulating

in your community.

- So if a great number of people around you are infected,

then the possibility

of you getting exposed obviously increases.

If you're in close proximity with people

that are infected,

particularly those that are unvaccinated

and have generally higher viral loads,

then the probability of you getting infected increases.

- [Narrator] The second is tied to vaccine uptake.

- So if a large proportion of the population is vaccinated,

then your virus transmission,

virus loads, everything decreases.

So the chances of spreading the virus

amongst this population obviously decreases.

- [Narrator] That's because vaccinated people act

as a kind of shield,

even when they do get infected.

A recent CDC study found that vaccinated people

carried less virus and potentially didn't spread it

as much as unvaccinated people.

Cases were also shorter and less severe.

Vaccines help create a kind of immune memory

of what a virus looks like,

helping the body fight it off more quickly

when it spots it.

That makes it harder for the virus to spread overall.

Roughly half of all Americans are fully vaccinated

but in some states and globally,

the vaccination rate is much lower,

giving the virus more opportunity to spread and mutate.

That's why having large gathering,

like concerts or the Olympics can be so challenging

from a public health perspective.

Third, individual behavior matters.

- So as measures have been abandoned,

such as masking and social distancing,

when you don't have a significant number

of the population vaccinated,

then the ability of the vaccines to protect you

from getting infected decrease.

- [Narrator] Finally, even after vaccination,

individual immune systems vary in their ability

to prevent and fight off infection.

Older and immunocompromised people seem

to be more susceptible to breakthrough cases,

and those tend to be severe.

- That's why health officials

are considering additional doses right now,

primarily for people who have a compromised immune system

and might not have produced a good immune response

after two doses of the vaccine.

- [Narrator] Breakthrough cases might be asymptomatic

or mild, so people may not know to get tested.

But that's not the case for athletes.

- We're actually seeing breakthrough infections happen

a lot more amongst sports teams,

like baseball or for the Olympics

because those are the people

that get tested pretty regularly,

even if they're vaccinated.

- [Narrator] At the Olympics,

organizers of the games are scrambling

to deal with a rising load of athletes and officials

who are testing positive upon arrival in Japan,

some with breakthrough infections.

The vaccination rates are low in Japan.

Cases there are rising

and organizers didn't require those participating

to get vaccinated.

Dr. Hatziioannou and many other experts remain concerned

that holding a large-scale international event prior

to reaching a critical mass

of vaccinated individuals has the potential

to contribute to the virus's spread around the world.

- The Olympics is a special event

and it's truly remarkable

that it brings all these people

from all these different countries together

but it also poses a perfect ground

to mix variants and spread the virus

that will then go back to each athlete's country.