Scientists worldwide are closely tracking a descendent of the highly infectious Delta variant that is spreading in the UK. England's public-health authority said in a report on Friday that it was monitoring a subtype of the Delta variant called AY.4.2, which had infected more and more people recently. Francois Balloux, director at the University College London Genetics Institute, said on Twitter on Saturday that data about AY.4.2 suggested it could be 10 percent more transmissible than the most common Delta variant in the UK, called AY.4. "As such, it feels worthwhile keeping an eye on it," he said. As of 27 September, 6 percent of UK sequenced tests were AY.4.2, Public Health England (PHE) said in its report on Friday, adding that estimates could be imprecise because it was difficult to sequence the variant's mutations. Dr. Scott Gottlieb, the former Food and Drug Administration commissioner, said that the new variant wasn't an "immediate cause for concern," but called for "urgent research" to work out if it was more infectious or able to avoid the body's immune response. "We should work to more quickly characterize these and other new variants. We have the tools," he said on Twitter on Sunday, adding that a coordinated, global response was required. Dr Jeffrey Barrett, medical genomics group leader at the Wellcome Trust Sanger Institute, said on Twitter on Tuesday that AY.4.2 was the only Delta descendant that was steadily increasing, suggesting a "consistent advantage" over Delta. Barrett cautioned that AY.4.2 was replacing Delta at a much slower rate than Delta had replaced the formerly-dominant Alpha variant. The Delta variant is estimated to be about 60 percent more infectious than Alpha. The same pattern for AY.4.2 hasn't yet been seen in other countries. Balloux said in a statement on Tuesday that the variant was "rare" outside of the UK, with only three cases detected in the US so far. "In Denmark, the other country that besides the UK has excellent genomic surveillance in place, it reached a 2 percent frequency but has gone down since," he said. New mutations The virus that causes COVID-19 gets about two new mutations per month, and there are now 56 Delta descendants, according to Scripps University's Outbreak.info, which includes data from the Centers for Disease Control and Prevention. Before AY.4.2, PHE had tended to group Delta and its descendants together. AY.4.2 has two new mutations in the part of the virus that attaches to human cells, which is called the spike protein. It's not yet clear how these mutations will affect the virus' behavior. Source Balloux said neither of these mutations had been found in other variants of concern.