What is the Delta variant and why is it so dominant?

July 13, 2021


In our latest expert webinar, Professor Tim Spector, ZOE COVID Study lead, and Professor Wendy Barclay, Head of the Department of Infectious Disease at Imperial College London, answered all your questions about the Delta variant. 


What is the Delta variant?

Ever since SARS-CoV-2, the virus that causes COVID-19, first emerged in 2019 it’s been evolving to become better adapted to human hosts. These new versions are known as variants.  

“The virus has gradually transitioned from the original virus that was found in Wuhan - and wasn’t well adapted to human hosts - to a series of variants which emerged in different parts of the world,” explains Wendy. 

Some of the variants became known as ‘variants of concern’ because they had worrying properties such as faster transmission, increased disease severity, or the potential to evade vaccines. 

The Delta variant is the fourth and most recent ‘variant of concern’. It was first identified in India in October, spreading to other countries around the world, and is now the dominant strain of the SARS-CoV-2 virus in the UK. 

“Data from the ZOE COVID Study is telling us that there are currently about 33,000 estimated symptomatic cases of COVID-19 in the UK. 97% of those are likely to be Delta,” says Tim. 


Why has the Delta variant become so dominant?

Natural selection ensures that viruses with an advantage over previous strains, such as faster transmission between people, eventually become the dominant strain. This is exactly what has happened with the Delta variant in the UK.

Wendy explains that mutations in the Delta variant’s spike proteins help it spread more easily, giving it an advantage. 

“The spike proteins stick out from the surface of the virus particle and help it latch onto and enter our cells. Any mutation that helps the virus do that more efficiently is going to drive a virus that can transmit from one person to the next better,” she says.

Estimates suggest that the Delta variant is 40-60% more transmissible than the Alpha variant (previously known as the Kent variant). As a result, the Delta variant has outcompeted the Alpha version to become the UK’s dominant COVID-19 strain. 

Is the Delta variant more dangerous than previous variants?

It’s difficult to compare the severity of the Delta variant to previous strains of the virus. This is because many of the people who are most at risk from COVID-19 are now vaccinated, including older people and those with underlying health conditions, so most of the new infections are in younger people who are generally healthier. 

Concerningly, early research suggests that people who catch the Delta variant are more likely to require hospitalisation than those infected with the Alpha variant. But, as Wendy explains, this data doesn’t tell the whole story. 

“Although we’re seeing more people going to the hospital if they catch the Delta variant, at the moment there isn't any evidence that their outcome, once they’re in the hospital, is worse. In fact, there's evidence that fewer of the people in hospital need extra oxygen or end up in intensive care compared with previous waves,” she says.  


Are vaccines working against the Delta variant?

“We can see from the data that is coming from Public Health England and Public Health Scotland that the vaccines all still working really well for Delta in protecting people from severe disease,” Wendy says.

However, due to changes in the virus spike protein which render the protective antibodies generated from vaccination less effective, vaccines do not offer full protection from catching a COVID-19 infection or mild disease. 

“People can still get infected with Delta if they've had a vaccine and particularly if they’ve only had a single dose of vaccine, where protection drops to around 50%,” she warns.

Although the vaccines offer good protection against severe disease from the Delta variant, we also don’t know whether they will prevent long COVID, potentially condemning thousands of people to lingering health problems in the future.

What’s next for COVID variants?

There are already several versions of the Delta variant in the UK, known as Delta plus, which have additional mutations in their spike proteins. 

“Interestingly, the Delta Plus variants, like the Alpha Plus variants we saw a few months ago, don't actually seem to be much better than the original Delta,” says Wendy. 

“What concerns me more is that there are whole areas in the rest of the world where they don't have vaccines, and there is lots of virus circulating in people with no protection at all. And, as a result, there will almost certainly be other variants with constellations of mutations emerging that could make them more effective and make them the next threat.”

The battle against COVID-19 and its many variants is certainly not over. We’re continuing our research into the effects of the COVID-19 pandemic as the vaccine rollout progresses and new variants emerge. 

You can play your part by downloading the ZOE COVID Study app and taking just a minute every day to log your health.

Stay safe and keep logging.


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