(CNN) — 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.
“As AY.4.2 is still at fairly low frequency, a 10% increase its transmissibility could have caused only a small number of additional cases. As such it hasn’t been driving the recent increase in case numbers in the UK,” Balloux, Professor of Computational Systems Biology and Director at the UCL Genetics Institute, told the SMC.
While new variants have repeatedly overtaken one another to become the dominant strain globally in the past year, experts say it is too soon to know whether AY.4.2 will become significant. In the UK, “Delta very rapidly in a matter of weeks” outpaced the Alpha variant by the summer, Deepti Gurdasani, a senior epidemiology lecturer at Queen Mary University of London, told CNN. “That’s not what we’re seeing here, we’re seeing sort of a slow increase in proportion that suggests that it’s not hugely more transmissible, it might be slightly more transmissible.
Balloux agreed, telling SMC that “this [is] not a situation comparable to the emergence of Alpha and Delta that were far more transmissible (50% or more) than any strain in circulation at the time. Here we are dealing with a potential small increase in transmissibility that would not have a comparable impact on the pandemic.”
AY.4.2 has caught the attention of public health experts across the pond. In a series of tweets Sunday, former US Food and Drug Administration Commissioner Dr. Scott Gottlieb called for “urgent research” into this Delta offshoot and said it was a “reminder that we need robust systems to identify, characterize new variants.”
The emergence of AY.4.2 in Britain, however, points to what scientists have warned throughout the pandemic: soaring transmission can create new variants. The UK has had the highest rate of daily Covid-19 cases and deaths per million people in Western Europe since most pandemic restrictions were dropped in the summer. On Tuesday, it reported 223 Covid-19 deaths, the highest daily figure since early March, and health leaders are urging the government to reintroduce measures such as mask mandates in enclosed spaces to help ease the pressure on the health system.
The “whole problem with this approach to living with the virus and allowing between 30,000 to 50,000 cases a day — which has been the UK’s case rate since [the summer] — is the [virus’s] evolution will continue … we need to suppress cases and suppress the virus,” Gurdasani said.
