Only one sub-lineage of the Delta variant is now classified as "of concern" by the World Health Organisation (WHO), with the other two downgraded.
The B.1.617 virus, which first emerged in India, had been divided into three lineages but WHO said in its latest weekly update released on Tuesday that "it has become evident that greater public health risks are currently associated with B.1.617.2, while lower rates of transmission of other lineages have been observed."
B.1.617.2 is thus the only one to remain categorised as a variant of concern.
The WHO said that it continues to "observe significantly increased transmissibility and a growing number of countries reporting outbreaks associated with this variant."
Previously known as the Indian variant, it was renamed earlier this week as the Delta variant.
The B.1.617.1 lineage was downgraded to a variant "of interest" and labelled variant Kappa. The WHO said that it also appears to be more transmissible than the original strain of the virus but that global prevalence is declining.
Meanwhile, the B.1.617.3 lineage is no longer of concern or of interest as "relatively few reports of this variant have been submitted to date," the WHO explained.
More than 3.5 million new cases of COVID-19 were recorded globally last week as well as 78,000 new deaths. These were 15 per cent and 7 per cent lower than the the previous week.
The European and South-East Asia regions reported the largest decline in new cases and deaths.
The highest number of new infections were reported from India, Brazil, Argentina, the US and Colombia. The former three registered week-on-week decreases but the latter saw a per cent jump.