A new variant of COVID-19 in the UK that is believed to be behind the faster spread of infections in southeast England has been described as "a real cause for concern".
But it is not the first time the virus had mutated since the start of the pandemic and it may not even be the first time a mutation - or a change in the virus' genetic material - has altered how infectious it is.
So, should we be worried?
Mutations - although scarily named - are not necessarily a bad thing.
Every virus mutates because, when it makes contact with a host, it makes new copies of itself that can infect other cells.
And initially, scientists were fairly relaxed about the discovery of the new COVID mutation.
However, as he announced new Tier 4 restrictions for millions of people in England, Prime Minister Boris Johnson said the new variant strain may be up to 70% more transmissible and could increase the R value by 0.4.
Professor Chris Whitty, England's chief medical officer, also slightly ramped up the rhetoric - and the head of an influential research charity went further, saying it was "a real cause for concern".
Professor Whitty said he had alerted the World Health Organisation - and would be concentrating on analysing data related to the mutation's spread.
He advised people not to travel outside London and the South East, as there was a "significant risk" of spreading the new strain of the virus.
He said: "As a result of the rapid spread of the new variant, preliminary modelling data and rapidly rising incidence rates in the South East, the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) now consider that the new strain can spread more quickly."
But he assured the public there was "no current evidence" to suggest the new strain caused a higher mortality rate or that it affected vaccines and treatments.
He said "urgent work" was under way to confirm this and added: "Given this latest development, it is now more vital than ever that the public continue to take action in their area to reduce transmission."
Jeremy Farrar, director of the Wellcome Trust, a medical research charity, went further than Professor Whitty, describing the strain, in a tweet, as "worrying and a real cause for concern".
He added: "Research is ongoing to understand more, but acting urgently now is critical. There is no part of the UK and globally that should not be concerned. As in many countries, the situation is fragile."
RNA viruses such as coronavirus are more prone to slight changes happening as the copies are made.
In some cases, a mutation may even make the virus weaker. But in others, they could make the virus more infectious or cause more serious illness.
COVID-19 has been mutating every week or so, with many of the mutations having no impact on the virus.
Sky's science correspondent Thomas Moore has said the new mutation is "not wholly unusual" but "it is something that they will be keeping a very close watch on".
What are the different strains?
So far, there have been at least seven major groups, or strains, of COVID-19 as it adapts to its human hosts.
The original strain, discovered in the Chinese city of Wuhan in December last year, is known as the L strain.
It then mutated into the S strain at the beginning of 2020, before being followed by the V and G strains.
Strain G has been most commonly found in Europe and North America - but because these continents were slow to restrict movement, it allowed the virus to spread faster and therefore mutate further into strains GR, GH and GV.
Meanwhile, the original L strain persisted for longer in Asia because several countries - including China - were quick to shut their borders and stop movement.
Several other less frequent mutations are grouped together as strain O.
In Denmark, authorities became worried about a strain of the virus found in 12 people related to mink farming.
They feared the mutation could hamper the effectiveness of a vaccine because it had happened in the spike protein, and as a result the government ordered a mass cull of up to 17 million animals and a month-long lockdown for people living in the northwest of the country.
What are the most common strains around the world?
G strains are now dominant around the world, particularly in Italy and Europe, coinciding with spikes in outbreaks.
A specific mutation, D614G, is the most common variant. Some experts say this variation has made the virus more infectious, but other studies have contradicted this.
Meanwhile, earlier strains such as the original L strain and the V strain are gradually disappearing.
Analysis by the Reuters news agency shows that Australia's quick reaction to the pandemic and effective social distancing measures have eliminated transmission of the earlier L and S strains in the country, and that new infections are the result of G strains brought in from overseas.
In Asia, the strains G, GH and GR have been increasing since the beginning of March, more than a month after they started spreading in Europe.
Will mutations affect the vaccine?
So far, experts have not found any variants that could make a vaccine less effective, and the virus has been slow to mutate.
Professor Whitty said it would be "surprising" if it had an effect on the vaccine, although added there should be more hard data relatively soon.
Federico Giorgi, a researcher at the University of Bologna who co-ordinated a study into strains of COVID-19, told Science Daily: "The SARS-CoV-2 coronavirus is presumably already optimised to affect human beings, and this explains its low evolutionary change.
"This means that the treatments we are developing, including a vaccine, might be effective against all the virus strains."
A group of scientists from several institutions including the University of Sheffield and Harvard University have also suggested G strains could make a better target for a vaccine because these strains have more spike proteins on their surface.
However, University College of London Genetics Institute researcher Lucy van Dorp said we should still remain "vigilant" and continue to monitor any new mutations.
The best way of ensuring the virus does not evade a vaccine is to stop infections spreading and reducing the chances of it mutating.
Catherine Bennett, epidemiology chair in the faculty of health at Melbourne's Deakin University, said: "If the virus changes substantially, particularly the spike proteins, then it might escape a vaccine. We want to slow transmission globally to slow the clock.
"That reduces the chances of a one in a squillion change that's awful news for us."
