The UK, South Africa and Brazil variants could be much more contagious or easy to catch.
All three have undergone changes to their spike protein - the part of the virus which attaches to human cells.
As a result, they seem to be better at infecting cells and spreading.
Experts think the UK or "Kent" strain emerged in September and may be up to 70% more transmissible or infectious. The latest research by Public Health England puts it between 30% and 50%.
The South Africa variant emerged in October, and has more potentially important changes in the spike protein. Experts have recently found a small number of cases of the UK variant that have one of these more concerning changes too.
It involves a key mutation - called E484K - that may help the virus evade parts of the immune system called antibodies.
South Africa coronavirus variant: What's the risk?
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The Brazil variant emerged in July and has this E484K mutation too.