Today, health chiefs raised the concern from the more transmissible Delta offshoot amid fears that it may have infected 150,000 Britons.

The UK Health Security Agency (UKHSA) has labelled AY.4.2 a variant under investigation’. 

This category is reserved only for variants that are spreading in Britain and may be more transmissible than other mutant strains or better able of evading vaccines.

Other variants in this category are Mu, the most recent variant, and Kappa, a relative of Delta. 

It is just one step above ‘variant or concern’, the label that Alpha and Delta have been given. This label is reserved for mutant strains which could alter the course the pandemic.

Scientists believe that the threat level has increased due to rising case numbers and early evidence that it is more transmissible then Delta. 

It is possible that it could spread up to 15% faster, according to some estimates. However, this has yet to be confirmed.

Health chiefs claimed that 15120 cases have been detected to date. However this may only be 10% of the total, as labs only sweep a fraction Covid cases for variants. This suggests that the true number could be closer to 150,000. 

It has been responsible for almost one in ten cases of infection in the country, more than twice its share in the past month. Adur, West Sussex, is the hotspot.

There have been suggestions that the variant may be elevated to 'Variant under Investigation'. If this is the case the World Health Organization is likely to give it the name 'Nu', which is the next letter in the Greek alphabet

There have been suggestions that this variant could be elevated to ‘Variant Under Investigation’. If this is true, the World Health Organization will likely give the variant the name ‘Nu’. This is the next letter in Greek alphabet.

AY.4.2 has been adopted in all of England, except for two dozen.

Official figures suggest that less than two dozen regions in England have yet to record one case of the new Delta offshoot.

One of the largest Covid surveillance programs by the Government was found in AY.4.2. This is 93% of all 315 local authorities, the most recent statistics are available for.

But there were 21 areas — scattered in pockets mostly across the North East and Midlands — that still have not recorded a single case of the subtype. 

It comes amid fears that AY.4.2 could be turbo-charging outbreaks in parts South West, where there have been record highs of cases in the wake a lab testing error. 

More than one-fifth of all infections are now caused by the strain in Somerset, Wiltshire, and Devon. These areas have some of the highest growth rates for infection in the country. 

It is responsible for more than 10% of all infections in England. This figure has doubled in three weeks. 

Scientists are worried about AY.4.2. It moved quickly from London and South East to other parts of the country in just a few weeks. 

On Wednesday, they tried to calm fears by saying that it had taken four months for it to spread and is only 10-15 per cent more transmissible then Delta. Data shows that it has not caused the same dramatic change as Alpha’s replacement this year. 

AY.4.2 first became known in the UK in June, in London and the South East. It then spread to the rest of the country. Only two-dozen areas in England have yet to experience a single case.

It carries just two mutations — Y145H and A222V — which have previously been spotted on other mutant strains.

It is not known if it is more likely than the Delta strain to cause severe illness or render vaccines less effective.

Although the World Health Organization has yet to raise the threat level for the variant, scientists believe it will likely name it a “variant under investigation” in the coming weeks.

It is likely that it will be called ‘Nu’ in order to be the next letter of the Greek alphabet. 

The UKHSA claimed that it was conducting lab experiments to determine if it can evade vaccine triggered immunity.

Dr Jenny Harries is the agency’s chief executive. She stated that virus mutates often and at random. It is therefore not surprising that new variants of viruses will continue to emerge as the pandemic progresses, especially since the case rate is still high. 

“It is testament both to the scientific expertise of my colleagues at UKHSA and the genomic sequencing capability developed through the pandemic that this new variant was identified and analysed so quickly. 

“But, it should serve to be objective evidence that this pandemic has not ended.”

She advised people to get their boosters and vaccines if they are eligible. She also advised them to wear face masks in crowded areas again.

Although AY.4.2 has been found in many countries, it is only being detected in the UK or Denmark.

The head of lab research at Denmark’s Statens Serum Institute, who monitors variants in the nation, stated that they were not concerned about AY.4.2, but would be performing lab tests to see whether it was better able avoid vaccines.

MailOnline was informed by Professor Anders Fomsgaard that they are not concerned about the situation. It’s a small amount of cases that we have been having for some time now, and it’s remained low.

“We don’t see anything that suggests it is more contagious or resistant, but we do take every suspicion seriously.”  

One of the largest Covid surveillance programs by the Government was found in AY.4.2. This is 93% of all 315 local authorities, the most recent statistics are available for.

But there were 21 areas — scattered in pockets mostly across the North East and Midlands — that still have not recorded a single case of the subtype.

This map shows the proportion of cases that were AY.4.2 in the fortnight to September 25

This map shows the proportion of cases that were down to AY.4.2 in the fortnight to October 9, the latest available

The above maps show how many cases were AY.4.2 within the fortnight ending September 25, (left) and October 9, (right). This sub-variant was responsible for a higher percentage of infections. 

The above graph shows the proportion of cases down to different variants in England over time. It reveals AY.4.2 (yellow) is beginning to make up a higher proportion of cases. But Delta (light green) remains the dominant Covid strain. Alpha (purple) was dominant last winter, after it replaced the old virus (pink and dark green)

The graph above shows the percentage of cases that have been affected by different variants in England over time. It shows that AY.4.2 (yellow), is starting to make up a greater proportion of cases. Delta (light-green) is still the dominant Covid strain. After the replacement of the old virus (pink-dark green), Alpha (purple), was the dominant Covid strain last winter.

There was concern that the mutant strain could have caused epidemics in South West areas, where there have been record-breaking cases in the wake a lab testing error.

MailOnline was told by scientists that the surge is not caused by AY.4.2.

Dr Jonathan Stoye is the head of a virus laboratory at Francis Crick Institute. He said that more research was needed in order to determine if AY.4.2 triggers spikes in cases. 

MailOnline was informed by him that AY.4.2 changes most likely result in minimal changes to transmissibility. Therefore, I believe that the increase in cases is not due to the introduction of this variant.

“However any changes in transmissibility could potentially have serious consequences, and we must continue to monitor this variant’s spread.”

Britain’s last lockdown occurred in the wake of the Alpha virus, which was more transmittable than the original virus.