A new UK variant of coronavirus was identified shortly before Christmas and is thought to be behind the huge surge in cases, particularly in London and the south east of England.
The alarming spread is one of the reasons that Boris Johnson imposed a third national lockdown in January as daily cases rose to more than 58,000 and increase of nearly 50 per cent in just a week.
Deaths and admissions to hospital have also risen by 20 per cent since Christmas week and are likely to continue to rise in the coming weeks until the severe restrictions begin to lower transmission.
At a Downing Street press conference on Jan 5, Professor Chris Whitty said that data from the two weeks ending Dec 30 shows that cases in the UK have increased by 70 per cent.
The most recent data from the Office of National Statistics (ONS) shows that across the country as a whole, roughly 1 in 50 people have got the virus, which is higher in some parts and lower in others.
That is an estimated 1.1 million people in private households in England had Covid-19 between Dec 27 and Jan 2 - equivalent to around 2.06% of the population.
In England there has been a "relative increase" of this new variant in every area, said Professor Whitty, that is "spreading around the country".
The fastest increases were in the East, South East and London, but it is "now taking off in other areas as well", he said.
More than 50 countries stopped flights to or from Britain to prevent onward transmission of the virus, although it has already been picked up in several countries.
Although Britain now has two vaccines against coronavirus, there are fears that the mutated virus may be able to evade the immune system, even after vaccination or previous infection. Here is everything we know about the UK variant:
What is the mutation and how dangerous is it?
The variant - called ‘VUI – 202012/01’ - carries a mutation to the gene which forms spike proteins. These are little grippy rods on the outside of the virus cell which attach to human cells.
Vaccines work by instructing the body's own cells to produce these spike proteins, which the immune system then spots as a foreign object and develops antibodies which attach to virus and clears it away. Once that has happened the body has a memory of coronavirus and so those antibodies are ready if it sees the virus again.
However a change in the shape of the spike protein makes it more difficult for the immune system to spot, because it has been trained on a different model. It also makes it harder for antibodies to bind to the virus so it can be eradicated from the body.
Scientists are currently studying whether the current vaccines will work against the new strain.
There is currently no evidence to suggest it is more likely to lead to serious illness. However if it can bind more easily to human cells, it may spread quicker meaning more people will become infected and need hospital treatment.
Professor Lawrence Young, a molecular oncology expert, said the new variant does two things which make it more transmissible.
Prof Young, of the Warwick Medical School, said: "One is it's getting into the body more efficiently and it looks like that's because this change (mutation) which has occurred in the spike protein increases the strength of the interaction of the virus with cells in our bodies - it increases the stickiness, if you like.
"There's also data reported last week from Nervtag (The New and Emerging Respiratory Virus Threats Advisory Group) and it looks like where you do see this virus infection individuals are making more of it as well - there are higher virus loads in the throat."
Scientists have said the mutated coronavirus strain could more easily infect children and Prof Young added that preliminary research suggests this is also due to its "stickiness".
He said children have less of the receptors which picked up the older coronavirus variant, meaning they were less likely to catch it, but the new variant "might compensate for lower levels of that receptor or that door to the virus in children by being stickier".
However there is currently no evidence that this variant - or any other studied to date - has any impact on disease severity.
Prof Neil Ferguson, speaking at a Q&A with experts from the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said there is strong evidence the new mutant strain is 50 per cent more transmissible than the previous virus.
While Professor Calum Semple, a member of the Sage scientific advisory group, told Sky News that the new variant of Sars-Cov-2 could become the dominant global strain as it has an "evolutionary advantage in transmitting more quickly".
What are the symptoms of the new strain of coronavirus?
At the moment, it seems the symptoms are the same as the more familiar strain. The variant was spotted through polymerase chain reaction (PCR) tests which are usually only given to people with traditional symptoms of the virus, so we can assume the symptoms are identical.
How was it picked up?
Government scientists have been carrying out random genetic analysis of around 10 per cent of PCR tests and spotted the new mutations. The Covid-19 Genomics UK (Cog-UK) Consortium tracks new genetic variants as they spread and investigates if these changes lead to detectable changes in the behaviour of the virus or the severity of Covid-19 infections.
How many cases have been found and where are they?
Matt Hancock said the highly transmissible coronavirus variant makes up the majority of new cases in the UK.
A random testing sample of people in the South East found that the new, more transmissible variant accounted for 71 per cent of new cases, rising to 72 per cent in London.
Data from the Office for National Statistics also found the new strain was responsible for 67 per cent of the cases in the East of England and 53 per cent across the whole of England.
The figures indicate that the new variant has come to dominate the latest cases, detected in the week ending Dec 18, as it sweeps like wildfire through communities despite the continuing tiered restriction system.
Professor Peter Horby, chairman of the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) has stated that the variant started from one person in Kent and could have been caused by 'random errors' when the virus copies.
The variant likely originated in this patient, as they may have a weakened immune system which was struggling to beat the virus. Therefore, instead of destroying the virus, they became a breeding ground for the virus to mutate.
Professor Lawrence Young, Virologist and Professor of Molecular Oncology at the University of Warwick, said: “While levels of the variant virus were low in mid-December in many parts of England, rapid rises were beginning to appear in certain regions (e.g. Oxfordshire, Buckinghamshire, Birmingham and Solihull) consistent with the conclusion that this new virus variant which is fuelling infections in the UK is more transmissible (infectious).
“The report also highlights that this variant is responsible for the increased levels of infection observed in individuals under 20 years old.
"This suggests that the spread of this virus variant during a period of lockdown may have been driven by schools remaining open and by spread of the variant over the Christmas period."
The virus is now spreading in at least 30 countries. French health officials confirmed the variant was found in a patient, who displayed no symptoms and has now entered self-isolation, on the morning of Dec 26.
Northern Ireland recorded its first case on Dec 23, and German officials revealed the variant had reached their nation on Christmas Day. Japan and South Korea also also confirmed the variant is now in the country.
Travellers from the UK are banned from entering the United States, in an attempt to keep the new variant under control.
Is this the first mutation found?
No. Coronaviruses mutate frequently and many thousands of mutations have already arisen in the SARS-CoV-2 genome since the virus emerged in 2019.
As early as March, scientists had already discovered the virus has evolved into two major lineages (dubbed ‘L’ and ‘S’ types).
The older ‘S-type’ appeared to be milder and less infectious, while the ‘L-type’ which emerged later, spreads quickly and by the Spring accounted for around 70 per cent of cases.
Another variant (D614G) has also been detected in Western Europe, and North America which, although it spreads more easily, does not cause greater illness.
A worrying mutation, dubbed ‘Cluster 5’ was found in mink farms in Denmark leading to a cull of 17 million animals.
An even more worrying variant, called the South Africa variant, was found in Britain at the end of December. This mutated virus could be even more infectious because it carries even more mutations to the spike protein.
Could it hamper a vaccine?
Possibly. Most coronavirus vaccines are targeting the Spike Protein which the virus uses to latch on to human cells. The vaccines prime the body to be able to spot the spike protein so the immune system can spot the virus.
However, if the spike protein mutates the body will no longer be able to recognise the virus and vaccines may prove ineffective.
Professor Calum Semple of Liverpool University said it is "the million-dollar question" whether vaccines will be effective against the new variant of coronavirus but he thinks they will.
The member of Sage told the BBC's Breakfast: "Some of the mutations are occurring in the key that the virus uses to unlock the cells. And we see this with flu each year and that's why the flu vaccine has to change year on year."
He added: "I would expect the vaccine still to be reasonably effective because it's currently 95 per cent effective. Even if we dropped a few percentage points, it's still going to be good enough, and much better than many other vaccines on the market.
Furthermore, the current figures follow concerns from scientific advisors who have warned that one million Covid-19 vaccinations a week will not be enough to bring the pandemic under control.
The director of the Wellcome Trust, Sir Jeremy Farrar, a member of Sage has said: "We're not going to be free of this pandemic by February; this is now a human endemic infection.
"If we do manage to hit the target of a million [vaccinated] a week, frankly I don't think that's enough to speed that up if we wanted to get the country covered."
"And the next bit of good news is that the new vaccines are essentially like emails that we send to the immune system, and they're very easy to tweak.
"So if we know that the lock has changed very slightly, we just have to edit that email, change a word or two and then the vaccine that will be ready in six to eight weeks' time after that, will be competent and better targeted to the new strain.
"So this is not a disaster. This isn't a breakdown in all our plans. This is just what we expect with a new virus, and it's what the scientists and the doctors have come to understand, and we will adapt."
The chief executive of BioNTech said he was confident that its coronavirus vaccine works against the UK variant, but further studies are need to be completely sure.
Ugur Sahin said on Dec 22 that "we don't know at the moment if our vaccine is also able to provide protection against this new variant," but because the proteins on the variant are 99 per cent the same as the prevailing strains, BioNTech has "scientific confidence" in the vaccine."
Mr Sahin said BioNTech is currently conducting further studies and hopes to have certainty within the coming weeks.
"The likelihood that our vaccine works ... is relatively high." But if needed, "we could be able to provide a new vaccine technically within six weeks," he added.
Prof Van Tam stated on Dec 30 that it would take up to two weeks for scientists to confirm the AstraZeneca and Pfizer vaccines were effective against the new strains of Covid-19.
Moderna, which manufactures a different coronavirus vaccine, is also testing its jab against the faster-spreading version of the disease.
Are the experts worried?
Most scientists are downplaying this development as most mutations found so far have not proved more deadly. Some variants actually prove to be less aggressive and many die out.
In November, scientists at UCL published research showing that there had not been a mutation so far that has increased transmissibility.
Prof Tom Solomon, the Director of the NIHR Health Protection Research Unit in Emerging and Zoonotic Infections, at the University of Liverpool, said: “SARS-CoV-2, the virus which cause Covid-19 is evolving and mutating all the time, as do all similar viruses. Such changes are completely to be expected.
“In the UK we are doing very detailed genetic assessment of many of the virus strains detected. From what Matt Hancock has announced it sounds as though a particular variant is being detected especially across the South of England.
"Just because there has been a small change in the virus’ genetic make-up this does not mean it is any more virulent, nor that vaccines won’t be effective. Our experience from previous similar viruses suggests that the vaccines will be effective despite small genetic changes.”
Dr Zania Stamataki, Viral Immunologist, of the University of Birmingham, added: “The emergence of different coronavirus strains a year after SARS-CoV-2 first jumped to humans is neither cause for panic nor unexpected. Mutations will accumulate and lead to new virus variants, pushed by our own immune system to change or perish.
“This virus doesn’t mutate as fast as influenza and, although we need to keep it under surveillance, it will not be a major undertaking to update the new vaccines when necessary in the future. 2020 saw significant advances take place, to build the infrastructure for us to keep up with this coronavirus.”
However Prof Nick Loman of Uni of Birmingham says this new variant is "concerning", as there's "a strong association" with areas of high growth & there appear to be changes in the spike protein which feel "quite likely" to influence the virus' behaviour.