It is very common for viruses to mutate, and Covid-19 has changed thousands of times since the start of the pandemic.
However, several of these variants have caused concern, including the new strains found in the UK, South Africa and Brazil.
Experts think all three of these variants are more transmissible - and there are fears that vaccines might not work as well against them.
So, how do these three variants differ to the original version of the virus and from each other?
Here is everything you need to know.
What are the identified strains?
So far, there are three different strains that are causing alarm in the UK.
The UK strain, first found in London and Kent in September, was the first to prompt fears when it seemed to be linked with an exponential rise in positive cases in the south east of England.
It has the E484K mutation which could help the virus to evade the immune system.
The South African variant, discovered back in October and announced in December, also carries the E484K mutation.
Meanwhile, the Brazil variant was found in the country as early as July.
This third variant is thought to be similar to the South African one, and it also has the E484K mutation.
Are there different symptoms?
While it is not thought that people will experience different symptoms if they catch the South African variant, the symptoms for the UK strain may differ to those caused by the original version.
ONS Research carried out in England reported that a cough, fatigue, muscle pain and sore throat may be more common in people who test positive for the Kent strain.
People in the study also reported that loss of taste and smell was a less common symptom of the variant.
Research is still being conducted on all three variants to determine more information about their symptoms.
How many cases of the new strains are there?
The UK variant has spread rapidly since it was discovered, and it now accounts for as many as 40 per cent of new cases in the country.
It was one of the reasons fresh lockdowns were introduced in Scotland and England in January.
It is thought that there are about one hundred cases of the South African variant in the UK, but this number could increase in the coming weeks.
One of two variants that first emerged in Brazil was identified in the UK in January, according to a leading virologist, but it was confirmed that this was not the one causing alarm.
To limit the spread of the Brazil variant, travel bans were introduced between South American countries and the UK.
Are the strains more transmissible?
All three of these mutated versions of the virus have had a change on the spike protein called N501Y, which could mean they are more able to attach to the body and spread.
Both the new South African and UK variants do appear to be more contagious.
The UK strain is thought to be up to 70 per cent more transmissible than regular coronavirus, and the South African one could be up to 60 per cent more infectious.
Scientists in South Africa think that the variant may have contributed to a surge of infections and hospitalisations there, similar to the effects of the UK strain here.
The Brazil variant also has “some of the features” of the UK and South African variants which could make it more transmissible, according to the UK Government’s Chief Scientific Adviser Sir Patrick Vallance.
Are the variants more deadly?
Boris Johnson suggested for the first time in January that the UK strain could be more lethal than the original virus.
Early studies have suggested that this variant could be around 30 percent more fatal.
The old variant has a fatality rate of around 10 in 1000, while the new variant has a slightly more deadly rate of 13 or 14 in 1,000.
There is currently no evidence to suggest that the South African or Brazilian variants cause more serious illness, according to experts.
But because they could be more transmissible, this could lead to a huge spike in cases which would result in increased hospital admissions and deaths.
Will the vaccines work against the strains?
Three vaccines have so far been approved for use in the UK - Pfizer-BioNTech, Oxford-AstraZeneca and Moderna.
Boris Johnson has confirmed that all these jabs should work to immunise against the UK variant.
Pfizer and BioNTech also released their own study, which is not yet peer-reviewed, that suggests their vaccine works against the South African variant, but that it is slightly less effective.
The Moderna vaccine has also shown to be effective against the South African variant.
Meanwhile, scientists at the University of Oxford are looking at the impact its vaccine has on the other new variants and hope to have more data soon.
Studies are still ongoing to determine the effectiveness of the vaccines on the Brazil variant, but there is no strong evidence that suggests they will not work against it.
However, scientists have said it is possible that this variant can evade antibodies, which could impact the effectiveness of a vaccine.