Delta sub-lineage AY.4.2 designated as a Variant Under Investigation


The Delta variant sub-lineage known as Delta AY.4.2 was designated a Variant Under Investigation (VUI) by the UK Health Security Agency on 20 October 2021 and has been given the official name VUI-21OCT-01.


The designation was made on the basis that this sub-lineage has become increasingly common in the UK in recent months, and there is some early evidence that it may have an increased growth rate in the UK compared to Delta. More evidence is needed to know whether this is due to changes in the virus’ behaviour or to epidemiological conditions.


The genome of VUI-21OCT-01 does not have many mutations compared to Delta. However, a small change may be enough to cause a difference in the virus properties in some circumstances. UKHSA is monitoring this closely.


The original Delta variant remains overwhelmingly dominant in the UK, making up approximately 99.8% of all cases. As of 20 October, there were 15,120 cases of VUI-21OCT-01 confirmed by whole genome sequences in England since it was first detected in July. In the last week, VUI-21OCT-01 accounted for approximately 6% of all Delta cases. Cases have been confirmed through whole genome sequencing in all 9 regions of England.


While evidence is still emerging, so far it does not appear this variant causes more severe disease or renders the vaccines currently deployed any less effective. As is routine for any new variants under investigation, UKHSA is carrying out laboratory and epidemiological investigations to better understand the properties of this variant.

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