There have been 64,579 new cases of the Delta variant in the week prior to 15 September 2021, the UK government reports. The variant continues to be the dominant strain in the UK.


Elsewhere, cases of the Delta variant of concern are rising. According to data from the GISAID COVID tracking initiative, the last four weeks has seen 50,971 new cases in the USA, and 16,452 in Germany.

A new review of all coronavirus variants, published in Nature Reviews Genetics on 17 September, suggests that the different coronavirus vaccines have varying effectiveness against Delta. The scientists say that studies in the UK have shown the Pfizer vaccine is approximately 85 per cent effective against the Delta variant, while the AstraZeneca vaccine is around 60 per cent effective.

The review also mentions new mutations of the Delta variant that have led to a sub-strain called the Delta Plus variant.

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The prevalence of Delta among the UK population is particularly concerning as a study has found that catching the variant has double the risk of hospitalisation compared with catching the Alpha (originally 'Kent') variant.

Researchers at Public Health England (PHE) and the MRC Biostatistics Unit at University of Cambridge published their findings in The Lancet Infectious Diseases.

The experts used data from over 40,000 individuals with confirmed COVID-19, however the large majority of the patients were unvaccinated. Of all the Alpha and Delta cases included, 24 per cent had partial protection from one coronavirus vaccine dose, while only 2 per cent were fully vaccinated. The paper does not identify which vaccines were involved in the study.

Is the Delta variant more transmissible?

Experts who have been studying the Delta variant, also referred to as B.1.617.2, say that it poses a significantly increased risk to individuals.

Commenting on the PHE study, senior clinical lecturer at University of Exeter Dr David Strain said it "confirms what we are seeing in clinical practice".

"In addition to the Delta variant being more infectious than the original or the Alpha variants, it is also causing more severe illness, in populations that previously would have had only mild infections," said Strain.

"The modification to the spike protein – the key, as it were, to the cellular lock – makes it easier for the virus to enter the cell thus making the move from viral carrier to infected person much quicker."

How many cases of the Delta variant have been detected in the UK?

So far, there have been 799,109 cases in the UK that are known to be caused by the Delta variant, or the sub-strain Delta Plus.

In the seven days prior to 15 September 2021, there had been 64,579 new Delta cases reported. The strain with the second most occurrences in the same time period is the Alpha variant, of which there have been 126 new cases.

According to the UK government's coronavirus dashboard, there have been a total of 5,242 patients admitted to hospital with COVID-19 in the week leading up to 24 September 2021. The number of COVID-related deaths is declining, with a total of 920 deaths in the same seven day period.

A map of occurrences of the B.1.617.2 Delta variant of coronavirus, as of 17 September 2021 ©
A map of occurrences of the B.1.617.2 Delta variant of coronavirus, as of 17 September 2021 ©

What are the symptoms of the Delta variant?

At present, there is nothing to suggest that the symptoms of infection with the Delta variant are different to other coronavirus strains.

The main symptoms of COVID-19, according to the NHS, are:

  • a high temperature – this means you feel hot to touch on your chest or back (you do not need to measure your temperature)
  • a new, continuous cough – this means coughing a lot for more than an hour, or three or more coughing episodes in 24 hours (if you usually have a cough, it may be worse than usual)
  • a loss or change to your sense of smell or taste – this means you’ve noticed you cannot smell or taste anything, or things smell or taste different to normal

The NHS say that most people who have symptoms of COVID-19 will have at least one of the above.

However, Strain says that by being more infectious, the Delta variant will also have the possibility to cause disease with increased severity.

"The Delta variant produces up to a 1,000 times more copies when it is replicating. This is not just pertinent when it comes to transmission between individuals, but also when it comes to spreading the virus throughout the body of the person who is infected.

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"The combination of more viral copies and better cellular penetration makes it more likely that the cells, tissues and organs will become overwhelmed before the immune system, particularly that of an unvaccinated individual, has had chance to mount a defence.”

The new study by PHE and University of Cambridge "clearly demonstrates the pre-conception that [younger adults] do not get severe COVID is no longer true" according to Strain.

Will vaccines still work against the Delta variant?

A pre-print study by researchers at University of Oxford – which has yet to be peer-reviewed – suggests that vaccination still reduces new Delta infections.

However, the data from over 700,000 participants showed that the effectiveness of the Pfizer-BioNTech and Oxford-AstraZeneca vaccines on the Delta strain is reduced compared with Alpha.

Participants who had two doses of either vaccine were shown to have the same level of protection against the disease as those who had gained a natural immunity due to having had COVID-19, the study found.

While there was limited information available regarding the effectiveness of the Moderna vaccine, the experts say they found that a single dose had similar, or greater, effectiveness against the Delta variant as single doses of the other vaccines.

Researchers say that getting two doses of a coronavirus vaccine "remains the most effective way to ensure protection against the COVID-19 Delta variant".

What other variants of concern have been identified in the UK?

It is common for viruses to mutate when they replicate. Few of these small, genetic changes lead to a more harmful infection.

As of 24 September 2021, there are five strains considered to be ‘of concern’ by PHE, including the Delta and Alpha variants.


The PHE are monitoring a total of 16 variants.

About our expert, Dr David Strain

Dr David Strain is a senior clinical lecturer at the University of Exeter Medical School. His research focuses on older adults with diabetes, but since the start of the pandemic has been working closely with the COVID-19 team at the university. He is currently working with the Royal Devon & Exeter NHS Foundation Trust as a clinical lead for COVID services.


Amy ArthurEditorial Assistant, BBC Science Focus

Amy is the Editorial Assistant at BBC Science Focus. Her BA degree specialised in science publishing and she has been working as a journalist since graduating in 2018. In 2020, Amy was named Editorial Assistant of the Year by the British Society of Magazine Editors. She looks after all things books, culture and media. Her interests range from natural history and wildlife, to women in STEM and accessibility tech.