Scientific advisors to the Government have warned that there is no reason to believe that Delta will be most contagious Covid variant.

Both Alpha and Delta caused an explosion of infections in the UK when they first emerged, prompting ministers to either impose restrictions or delay plans for lifting controls. 

However, No10’s top scientists, Professor Chris Whitty, and Sir Patrick Vallance were told that it is still possible for the scenario to be repeated by an even more infectious strain.

This is despite other virologists warning that we are very close to the maximum threshold in terms of transmissibility for Covid and its variants. 

Writing in papers published by SAGE today but handed to policymakers in mid-October, experts said: ‘There is no reason to assumer the SARS-CoV-2 virus has reached its theoretical limit — other variants could arise with even higher basic reproductive ratios.’

The R0 — the basic reproduction rate which shows just how contagious every disease is — of the ancestral form of the virus which first emerged in Wuhan was thought to be around 2.4 to 2.6. However, the R0 of Delta is thought to be eight. This means that everyone infected would spread it to eight others theoretically. 

The actual R rate — which reflects how quickly an outbreak is growing or shrinking — is always much lower than R0 because it takes into account real-world data such as population immunity. 

This comes amid growing concern about an offshoot variant of the Delta virus called AY.4.2 which is responsible for almost one in ten new infections here in the UK. Scientists believe it could be up to 15% more infectious than its ancestor. 

Figures show that although the percentage of cases resulting from the mutant strain are increasing, their curve is flattening. 

Some people believe that AY.4.2s spread because of the ‘founder Effect’. This is when a strain spreads quicker because it is the one in a particular group of cases, such as a school.

This graph shows how many people someone infected with the above diseases is likely to transfer them on to. For people who catch chickenpox, scientists estimate they will pass on the infection to between 10 and 12 others. And for those who catch measles, they are thought to pass it on to 12 to 18 others. Scientists at the CDC have estimated every person who catches the Delta variant of Covid could pass it on to eight other people, far higher than the 2.5 estimate for the Wuhan ancestral version

This graph shows how likely it is for someone with the above diseases to spread their disease to others. Scientists predict that the infection will spread to between 10 and 12 people if they contract chickenpox. They are also thought to spread measles to 12 to 18 people if they catch it. Scientists from the CDC believe that anyone who catches the Delta form of Covid can pass it to eight people. This is much more than the 2.5 estimate given for the Wuhan ancestral variant.

Covid vaccines appear to be just as effective against the more transmissible Delta offshoot, early tests today showed. Graph shows: The cumulative cases for each Covid variant since the fifth case was reported including Delta (lilac) and AY.4.2 (red). While cases of the mutant strain are continuing to increase, its curve is flattening off. It is increasing more slowly than its predecessor did at this point after it was first sequenced

Early tests today have shown that Covid vaccines are just as effective against the more transmissible Delta offshoot. Graph shows: The cumulative numbers of cases for each Covid variant, including Delta (lilac), and AY.4.2(red) since the fifth case was reported. The curve of the mutant strain is flattening, even though its cases are increasing. It is growing slower than its predecessor at this point in the sequence.

The variant was most prevalent in people aged 10 to 19 as of October 25, with 5,473 people in the age group having been infected with the strain. They were followed by 40- to 49-year-olds (2,433), 30- to 39-year-olds (2,015) and 20- to 29-year-olds (1,900)

The strain was most prevalent among people aged 10 to 19, with 5,473 people having been infected as of October 25. They were followed by 40 to 49-year olds (2.433), 30- and 39-years-olds (2.015), and 20- to 29 year-olds (1.900).


The above chart showed AY.4.2 accounted for a slightly higher proportion of cases in the latest week — one in ten — compared to two weeks ago — one in 13. Scientists said the slow rise was still compatible with a 10 per cent transmission advantage over Delta

The above chart showed AY.4.2 accounted for a slightly higher proportion of cases in the latest week — one in ten — compared to two weeks ago — one in 13. Scientists believe the slow rise is still compatible with a 10% transmission advantage over Delta.

The above map shows the 12 areas AY.4.2 was not detected in (white) over the two weeks to October 16, the latest available. It has spread to almost every area of England

The map above shows the 12 areas where AY.4.2 was not found in (white) during the two weeks leading up to October 16, when the latest information became available. It has spread to almost all areas of England

The document that the warning came from was called “Control Options to Mitigating a Rapid Increase in Infections”. Juniper, a group composed of experts from universities such as Oxford, Warwick, and Exeter, wrote it. 

It ruled that the UK would likely face the worst scenario if a variant could be developed that could prevent vaccine-triggered immunity. This would reverse the UK’s progress in its inoculation drive.

Experts warn that allowing viruses to “run hot” increases the chance of new variants, as it gives the virus more opportunities to evolve naturally and find ways to escape the immune system.

Some have suggested that the virus may need to be made less susceptible to vaccines in order to spread more widely between people.

AY.4.2. Everything you need to Know 

Where did AY.4.2 come form?

According to UK-based tracking, this sub-variant was first identified in the UK on June 26.

Scientists believe that AY.4.2 developed in the UK because of its higher number of cases than other countries.

It is possible, however, that the variant was imported from overseas and spread throughout the country.   

It is only available in a handful of countries.

AY.4.2 has been spotted in over 40 countries, including the UK, Germany and Denmark.

It might not have been detected in other locations due to lack Covid surveillance. This could lead to new subvariants not being identified.

The slow spread may be due to travel restrictions, which make it less likely that the virus can be passed between countries.

How infectious is the subvariant?

Experts estimate that AY.4.2 may be around 10% more infectious than the Delta version.

They believe this will lead to a slightly higher number of cases but not a spike comparable to the one seen when Delta arrived in Britain. 

Do I have to be concerned about AY.4.2 

Scientists agree that there is no reason for concern about AY.4.2.

There is no evidence suggesting that vaccines are less effective against the subvariant or that it increases the likelihood of hospitalisation and even death.

However, laboratory tests are being conducted in the UK and Denmark to confirm this. 

Professor Lawrence Young, Warwick University, stated that there is no reason for vaccines to not be as effective.

Professor Anders Fomsgaard, from Denmark’s Covid surveillance center, said that he was not concerned about this. There is nothing that suggests it is more contagious or resistant to infection. 

Professor Wendy Barclay told the BBC in June — when the Delta variant was dominant — that there was ‘still space for it to move higher’.

She referred to the R0 numbers of measles (18 and 12) to show that it could still be infectious.

Dr Aris Katzourakis, an Oxford University virologist, stated that the virus had shown a “phenomenal” ability to change after two new Covid versions emerged in 18 months.

He added, “Ultimately there are limitations and there’s no super-ultimate virus with every bad combination of mutations.

“It is possible that the virus could be made more resistant to vaccines, which could compromise its ability to transmit.

Professor Lawrence Young of Warwick University is a virologist and says that it is unlikely that a more transmissible Covid version than Delta will emerge.

MailOnline reported that he said last month that while we may have reached peak infectiousness for the Delta variant, what we haven’t reached is peak immune avoidance.

The World Health Organization named the Mu Covid variant. The virologist stated that he would be “very, very surprised” if it was more transmissible to humans than Delta.

Dr CathGreen, who developed the AstraZeneca vaccination at Oxford University, also stated that the virus will never escape vaccine-triggered immunity.

In Vaxxed, she wrote: “The good news is that it is unlikely that virus can mutate in such a way that it keeps it functioning but renders our vaccine useless.”

‘That’s because a change in the spike protein – which allows the coronavirus to enter and infect human cells – that is radical enough to make our vaccine completely ineffective would also, almost certainly, be so extreme as to make the virus non-functional. 

“So, even though it feels like Groundhog Day and we are all exhausted and wondering when it will ever stop, we are now tackling the new variant situation with our teeth set and the fortifying knowledge of what we are doing and where it is going.

It was revealed in a report published today by UK Health Security Agency (UKHSA), replacing Public Health England. The report showed that vaccines work as well against AY.4.2, as they did against its ancestor. 

It was found that jabs could prevent AY.4.2-infected people from developing symptoms. Two doses of the original strain can block approximately 83% of those who become ill. 

UKHSA admitted that preliminary results did not show a significant drop in vaccine effectiveness for AY.4.2 as compared to Delta, but that it could have been due to chance.  

According to the agency: ‘After adjustment of potential confounding variables, there was no evidence AY4.2 showed any significant difference to non-AY4.2 Delta cases, both symptomatic as well as asymptomatic, across all three vaccines in circulation. 

It comes after the World Health Organization (WHO), this week, admitted that it was now monitoring the variant.  

Experts are beginning to question whether the new strain is actually more transmissible than previously thought.

Scientists from Northumbria University are involved in variant surveillance. They say it is still unclear if AY.4.2 is more transmissible, as too little information is available about its mutations.

As an alternative explanation, they suggested the ‘founder effect’, which is when a strain spreads quickly because it is the only one in a particular cluster of cases like a school. 

Professor Francois Balloux at University College London, a geneticist, and Covid commentator, raised concerns about the variant last Wednesday. He said that the slower rise was’still compatible with’ a 10% transmission advantage.

Professor Jeffrey Barrett, the head of Covid surveillance for the Sanger Institute, stated that data was consistent with a small but real growth advantage vs. other Deltas.

According to UK-based tracking, this sub-variant was first identified in the UK on June 26.

Scientists believe that AY.4.2 developed in the UK due to its higher case numbers than other nations.

However, it is possible the variant could have been imported from abroad due to the fact that other countries have worse surveillance than the UK.

It contains two key mutations: A222V, and Y145H. These both slightly alter the form of spike protein, which the virus uses in order to infect cells.

Scientists claim A222V was seen previously on another variant (B.1.177), first spotted in Spain and then spreading to other parts of the world. 

However, studies show that the strain was not more transmissible and that it was spread only by holidaymakers returning to their homeland.

Regionally, the South West had the highest number of AY.4.2 cases in the week ending October 18 — reflecting overall infection numbers — with 426 sequenced during the week

Regionally, the South West had the highest number of AY.4.2 cases in the week ending October 18 — reflecting overall infection numbers — with 426 sequenced during the week

The new Covid variant AY.4.2 has been found in 42 countries, but has been most prevalent in the UK, US, Denmark, Poland and Germany. This graph shows the percentage of AY.4.2 cases as a proportion of the country's total Covid cases. The UK has led case growth since the variant was fist identified in July, but in recent weeks Poland has eclipsed this, and there are signs Germany is also catching up

The new Covid variant AY.4.2 can be found in 42 countries. However, it is most common in the UK and US, Denmark as well as Poland and Germany. This graph shows the proportion of AY.4.2 cases to the country’s total Covid cases. Since July was the first time that the variant was identified, the UK has led in case growth. But, in recent weeks, Poland has eclipsed this figure and there are indications that Germany is also catching-up.

The mutation Y145H is a concern. It slightly alters how antibodies bind to the site, making it harder to stop an infection. 

Scientists believe this mutation is due to mutations in Delta and could make the subtype more resistant than its parent to vaccines.

AY.4.2 has been reported in around 40 countries, but the UK is currently the only country that has seen a sustained outbreak, outside of Poland.

In Denmark, it was at one in 50 Covid cases by September. However, it has fallen to one in 100 now. Experts in the country aren’t concerned about AY.4.2.

Officials from the UK declared it a ‘variant under examination’ last week. This category is reserved to variants that are spreading in Britain and may be more transmissible than other mutant strains or better able of evading vaccines. It is one step below ‘variant-of-concern’, which includes Alpha.