The World Health Organisation designated the new B.11.529 Covid-19 mutation as a “variant of concern” following an emergency meeting as governments ban incoming flights from South Africa, where at least 77 cases have been detected.
The agency named the new variant Omicron.
“This variant has a large number of mutations, some of which are concerning. Preliminary evidence suggests an increased risk of reinfection with this variant, as compared to other [variants of concern]," the WHO said in a statement on Friday.
The variant, which was first detected in South Africa, has led to flight bans and a scramble to determine vaccine efficacy.
The EU, US and Canada all followed Britain's move to impose travel restrictions on visitors from southern Africa.
Omicron is the latest coronavirus strain to emerge, following the currently dominant Delta variant, which was first detected in India in October 2020.
The Delta variant has overtaken other strains of the virus in most countries and has accounted for 99.8 per cent of sequenced cases over the past 60 days, the latest weekly report from the WHO shows.
The Gamma variant, which accounts for 0.1 per cent of sequenced cases, followed Alpha and Beta, which each make up less than 0.1 per cent.
Lesser-known variants like Mu and Lambda are rarely sequenced.
However, the WHO says that at a regional and national level, there are variations in which strains dominate, especially in certain South American countries where Delta has been slower to spread.
The emergence of variants is a natural process that happens as the virus mutates to ensure its survival over time.
Video: WHO declares B.1.1.529 a variant of concern
Most mutations have little or no impact on the virus's ability to infect or cause severe disease.
But certain mutations can affect how easily the virus can spread, the degree of severity of the disease they cause and how well a vaccine can respond to them.
WHO experts following the evolution of Covid-19 gathered on Friday to determine whether the latest variant should be classified as one “of concern” or “of interest".
They announced that it would take “several weeks” to understand how contagious B. 1.1.529 is and the severity of the illness it can cause.
Work is now underway to look at tweaking vaccines against new coronavirus mutations.
Novavax said it has "already initiated development of a new recombinant spike protein based on the known genetic sequence of B.1.1.529 and will have it ready to begin testing and manufacturing within the next few weeks".
Moderna said: "Since early 2021, Moderna has advanced a comprehensive strategy to anticipate new variants of concern.
"This strategy includes three levels of response should the currently authorised 50 microgram booster dose of mRNA-1273 prove insufficient to boost waning immunity against the Omicron variant."
In a statement reported by the Guardian, Pfizer and BioNTech said they took action "months ago to be able to adapt the mRNA vaccine within six weeks and ship initial batches within 100 days in the event of an escape variant"
Previously, the WHO classified the Alpha, Beta, Gamma and Delta variants as “of concern".
They are considered worrying because they are highly contagious or cause more severe illness.
Mu and Lambda are considered “variants of interest” because they show genetic changes that mean they have the potential to become more contagious, harder to detect or cause more severe illness.
All variants are classified by “lineage” and have a specific spot in the family tree that originates with the first Sars-CoV-2 virus.
The Delta variant is about twice as contagious as previous variants and vaccines are about 40 per cent less effective at preventing infection from Delta than from other variants.