A new Delta descendant is rising in the UK. Here's what to know

British and international authorities are closely monitoring a subtype of the Delta variant that is causing a growing number of infections in the United Kingdom.  

This descendant of the Delta variant, known as AY.4.2, accounted for an estimated 6% of cases in the week of September 27 -- the last week with complete sequencing data -- and is "on an increasing trajectory," a report by the UK Health Security Agency said.

Little is known about AY.4.2. Some experts have suggested it could be slightly more transmissible than the original Delta variant, though that has not yet been confirmed. While it accounts for a growing number of infections, it is not yet classified in the UK as a "variant of concern." It currently remains rare beyond Britain, with a small number of cases being recorded in Denmark and the US, expert Francois Balloux told the Science Media Center (SMC) on Tuesday.