A Delta subvariant of Covid that is more infectious than its ancestor strain is now is behind one in six cases in England and is on its way to becoming dominant in months.
The AY.4.2 variant is 10 to 15 per cent more infectious than the already highly-virulent original Delta virus and is currently growing at a rate of about two per cent a week.
According to the Sanger Institute, the largest variant surveillance centre in the country, AY.4.2 was responsible for 16 per cent of new cases in England in the fortnight up to November 13.
But its rate of growth is speeding up and experts predict it could be dominant in England as soon as January, before outpacing Delta in the rest of the UK shortly after.
A Government-funded study last week found the new strain is slightly less likely to cause illness, meaning the UK could be dealing with a more manageable and mild form of Covid next year.
Around two thirds of people (66.7 per cent) who catch AY.4.2 suffer symptoms compared to three-quarters (76.4 per cent) from regular Delta.
It is believed to have originated in London or the South East and has two very slight changes to its spike protein, which the virus uses to enter cells.
Scientists are still unsure if the subvariant is biologically more infectious than its predecessor strain or if it is better at infecting vaccinated people, therefore giving it an evolutionary edge over the original Delta strain.
Professor Jeffrey Barrett, who heads up sequencing at the Sanger Institute, said he expects the subvariant to become dominant in January.
The above graph shows the number of cases of each variant that have been identified since the start of this year. In May the Indian ‘Delta’ variant replaced the Kent ‘Alpha’ variant to become the dominant strain
The above graph shows the proportion of infections sparked by different strains in England. The Indian ‘Delta’ variant is green, AY.4.2 is maroon, and the Kent ‘Alpha’ variant is purple. The dark green and pink areas represent the old virus
The above maps show the proportion of cases that were triggered by AY.4.2 in the fortnights to Novemebr 13 (left) and November 6 (right). A darker colour means the subvariant was behind a higher proportion of cases
AY.4.2: Everything you need to know
Where did AY.4.2 come from?
This sub-variant of Delta was first detected in the UK on June 26, according to UK-based tracking.
Scientists say it is likely that AY.4.2 evolved here because the UK has much higher case numbers than other countries.
But it is possible that the variant was imported from abroad and then started to spread in the country.
How infectious is the sub-variant?
Experts estimate that AY.4.2 is around 10 per cent more infectious than the Delta variant.
They say this may lead to a marginally higher number of cases, but that it will not trigger a spike similar to that seen when Delta arrived in the UK.
Should I be concerned about AY.4.2?
Scientists say there is no reason to be too concerned about AY.4.2.
There is no evidence to suggest that vaccines are less effective against the sub-variant, or that it increases the risk of hospitalisation and death.
But laboratory tests are underway at labs in the UK and Denmark to assess this.
Professor Lawrence Young from Warwick University said: ‘There is no reason to suggest vaccines won’t be as effective.’
And Professor Anders Fomsgaard from Denmark’s Covid surveillance centre said: ‘We are not concerned by this. We see nothing in this point of time that indicates it is more contagious, resistant or pathogenic.’
AY.4.2 was first detected in the UK in June, and has very gradually spread across the whole country.
Some 44,812 cases have been detected in the country to date, including 5,329 in Scotland and 5,782 in Wales.
Experts believe it first emerged in London or the South East, but there is no clear proof of its origin yet.
It carries two key mutations, A222V and Y145H, which both only slightly alter the shape of the spike protein which the virus uses to invade cells.
Scientists claim A222V was previously seen on another variant (B.1.177) first spotted in Spain before spreading to other countries.
But studies suggest it did not make the strain more transmissible, and that it was only spread by holidaymakers returning home.
There is more concern about the mutation Y145H, which slightly changes the shape of the site antibodies bind to making it harder for them to stop an infection from happening.
Scientists say this builds on mutations in Delta, and could make the subtype even more resistant to vaccines than its parent.
AY.4.2 has been recorded in more than 40 countries to date, and there have been some 45,000 cases globally.
The Sanger’s weekly surveillance figures also highlighted another Delta off-shoot — AY.4.2.1 — which is gradually increasing in frequency.
It was behind 2.7 per cent of cases over the latest fortnight, from 2.1 per cent previously.
There are more than a hundred different AY lineages — offshoots of the Delta variant — and the vast majority are not concerning.
Because Delta is so virulent and dominant, it will acquire lots of different mutations as it spreads through the population, most of which will not amount to any significant change.
Some, however, develop an evolutionary edge like being more transmissible or resistant to vaccines.
It comes after the REACT study — which measures the spread of the virus in England based on more than 100,000 swab tests — found the subvariant is ‘less likely to be associated with symptoms’.
Imperial College London researchers behind the study said just two-thirds of people who tested positive for AY.4.2 reported coronavirus symptoms, such as a loss or change to smell or taste, a fever or persistent cough.
Meanwhile, three-quarters of people who caught an older version of Delta — called AY.4 — suffered the tell-tale virus symptoms. And experts said the milder strain will slowly become dominant in the UK.
Separate data from the UK Health Security Agency (UKHSA), which replaced the now defunct Public Health England, showed the subvariant’s weekly growth was between one and two per cent.
Scientists previously predicted Covid would eventually morph into a flu-like virus that continues to spread but barely causes any deaths or severe illness.
Meaghan Kall, an epidemiologist at the UKHSA said AY.4.2’s ‘advantage in infectiousness means it will become the dominant strain’.
She said the subvariant ‘does not appear to differ’ from the original Delta strain in any way that is a cause for concern.
But Ms Kall said it is a ‘slow burner’, increasing in prevalence at a rate of one to two per cent each week.
If its weekly growth continues at its current rate, it could become dominant by March.
Paul Hunter, an infectious diseases expert at the University of East Anglia, said the coronavirus will likely reach a stable point over the next few years, where it would continue to spread but not cause severe disease.
And because the virus will be endemic, meaning it will never be eradicated, people will gradually build-up natural immunity and symptoms will eventually ‘resemble that of a common cold’, he said.
‘The virus and ourselves will find an equilibrium and that equilibrium within a very few years will not include many severe cases or deaths,’ he added.