The World Health Organization (WHO) Monday said that it is tracking a few dozen cases of two new sub-variants - BA.4 and BA.5 – of Omicron.
The global health body said it is assessing whether the two new variants are more infectious or dangerous.
The WHO is already tracking BA.1 and BA.2 - now globally dominant - as well as BA.1.1 and BA.3.
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BA.4 and BA.5
Both BA.4 and BA.5 are sister variants of the original BA.1 Omicron variant.
Only a few dozen cases of BA.4 and BA.5 have been reported to the global GISAID database, according to WHO.
The WHO said it had begun tracking them because of their "additional mutations that need to be further studied to understand their impact on immune escape potential".
Viruses mutate all the time but only some mutations affect their ability to spread or evade prior immunity from vaccination or infection, or the severity of disease they cause.
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Meanwhile, BA.2 variant of Omicron now represents nearly 94% of all sequenced cases and is more transmissible than its siblings. However, the evidence so far suggests it is no more likely to cause severe disease.
The UK's Health Security Agency said last week BA.4 had been found in South Africa, Denmark, Botswana, Scotland and England from January 10 to March 30.
All the BA.5 cases were in South Africa as of last week, but on Monday Botswana's health ministry said it had identified four cases of BA.4 and BA.5, all among people aged 30 to 50 who were fully vaccinated and experiencing mild symptoms.
(With Reuters inputs)
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