in India in October of 2020

has quickly spread around the world

and been detected in nearly 100 countries

within 10 months.

As of July, more than 80% of known COVID cases

in the US are caused by the Delta variant.

- The Delta variant is currently the greatest threat

in the US to our attempt to eliminate COVID-19.

- The Delta variant is both more transmissible

and possibly more severe.

- [Narrator] Studies have shown that some COVID vaccines

are effective at preventing symptomatic illnesses

caused by the strain.

Nevertheless, as of July,

cases are on the rise in countries

where a large part of the population is vaccinated.

Even countries that have been previously spared

from the pandemic are now battling

a ferocious wave of infections.

We look at some moment in key countries

to understand why the deadly outbreak of Delta

is not just threatening the economic recovery

but also the return to normal around the world.

First, we go back to October of 2020.

Health authorities say the Delta variant

was first detected here in the state

of Maharashtra in India.

By the spring, the number of cases in India

began ticking up.

Then from March to May,

infections jumped 20-fold,

plunging the country in disarray.

- [Betsy] It ripped through the country.

Thousands and thousands of new infections every day.

Hospitals were completely overwhelmed

and they were running out of oxygen

for patients who needed help breathing.

(speaking in foreign language)

- [Narrator] Public health experts blamed

India's deadly wave on several factors.

There were political rallies for state elections

that drew large crowds.

There was also a religious festival in April

when millions of Hindus gathered to bathe

in the Ganges River.

And then a few days later,

the government relaxed safety measures

like mask wearing and social distancing.

But scientists say

what really helped accelerate the case load

was Delta variant's molecular composition

and its nine key mutations in the protein spikes

that surround the virus.

Mutations on these spikes allow the varies

to invade human cells more easily,

making the Delta variant about 50%

more transmissible than others.

By June 1st, the variant wasn't only in India

but detected in more than 60 countries.

And the United Kingdom

was one of the first Western countries

to see an alarming increase in cases

as Delta became the dominant strain.

- We're seeing cases growing by about 64% per week

and in the worst affected areas,

it's doubling every week.

- [Narrator] What surprised health officials

was that around 75% of the adult British population

had already received at least one dose of a vaccine

and about 50% was fully vaccinated.

- The UK's a really unfortunate situation.

There are good levels of vaccination there

but the variant took off anyway.

- [Narrator] And that was partly because vaccines

were reacting differently to Delta

than they had to earlier strains.

By mid-June, studies showed

that one dose of some vaccines

was 17% less effective

at preventing symptomatic illnesses caused by Delta.

But two doses provided good protection.

- The more the Delta variant is allowed to spread,

the greater the risk of mutations,

which would evade the vaccines or evade treatments.

And we'd kind of almost be back to the beginning.

- [Narrator] By late June,

Delta was detected in countries

that had been slower to vaccinate,

in part because they had been spared the brunt

of the pandemic

and cases began rising in these places.

Australia and Thailand imposed lockdowns.

Even Japan decided to ban spectators at the Olympics

because of a new wave of infections.

- There are many, many, many countries

that don't have access to vaccine yet.

Not many people in the population are vaccinated.

And this is the huge concern about this variant.

That it's just going to rip through these countries.

- [Narrator] Meanwhile, in the US,

as people headed into their 4th of July weekend,

the Delta variant had become the dominant strain.

People were traveling again

and many stopped wearing masks

after the CDC said it wasn't necessary

for those who were fully vaccinated.

The variant has mostly spread to areas

where vaccination rates have been slow.

In some parts of the country,

like in Los Angeles, authorities have asked people

to wear masks in indoor public places,

regardless of vaccination status.

But some public health experts say the appetite

for another lockdown is low.

- It's difficult to ask people

to once again adopt these measures.

The message is everybody needs to get vaccinated.

- [Narrator] So on July 4th, Biden urged people

to do just that

as the administration planned

to make it easier for people to get their shots,

like sending more mobile clinics to festivals

and sporting events.

- While the virus hasn't been vanquished,

we know this,

it no longer controls our lives.

It no longer paralyzes our nation

and it's within our power to make sure it never does again.

- [Narrator] And that means it's a race against time

to vaccinate more people around the world

before the coronavirus mutates against.

(lively music)