A subvariant of the Delta Covid strain is less likely to cause symptoms, a mass surveillance study has found, and experts expect it to become dominant in the coming months.
Data from the REACT study — which measures the spread of the virus in England based on more than 100,000 swab tests — found the AY.4.2 subvariant made up 12 per cent of positive samples between October 19 and November 5.
The most recent findings, released on Thursday, revealed 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.
Data from the Wellcome Sanger Institute shows the AY.4.2 subvariant (shown in dark pink) accounted for 16.4 per cent of infections in the two weeks to November 13, up from 14.3 per cent in the 14 days to November 6
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.’
The REACT study recorded 841 positive Covid tests during its most recent round of testing, 99 of which were the AY.4.2 sub-lineage (11.8 per cent).
Its predecessor AY.4 continues to be the dominant Delta subvariant, accounting for 57.6 per cent of all cases.
Some 76.4 per cent of people who tested positive AY.4 reported having Covid symptoms.
But just 66.7 per cent of people who tested positive with AY.4.2 suffered symptoms.
AY.4.2 was first detected in the UK on June 26, according to UK-based tracking.
Scientists have said the subvariant likely evolved in the UK due to high infection numbers, but noted it could have been imported from another country.
Experts estimate that AY.4.2 is around 10 per cent more infectious than the original Delta variant, but do not expect it to trigger a spike in hospitalisations and deaths, because data suggests the vaccines are just as effective against it.
The variant’s prevalence has been creeping upwards in recent weeks, accounting for 11.2 per cent of cases in the week up to October 23, rising to 13 per cent one week later and 14.7 per cent by November 6.
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.