BA.4.6, a subvariant of the omicron COVID variant which has been quickly gaining traction in the US, is now confirmed to be spreading in the UK. BA.4.6 seems to be even better at evading our immune response than BA.5.
The latest briefing document on COVID variants from the UK Health Security Agency (UKHSA) noted that during the week beginning August 14, BA.4.6 accounted for 3.3% of samples in the UK. It has since grown to make up around 9% of sequenced cases.
Similarly, according to the Centers for Disease Control and Prevention, BA.4.6 now accounts for more than 9% of recent cases across the US. The variant has also been identified in several other countries around the world.
So what do we know about BA.4.6, and should we be worried? Let’s take a look at the information we have so far.
It is not entirely clear how BA.4.6 has emerged, but it’s possible it could be a recombinant variant. Recombination happens when two different variants of SARS-CoV-2 (the virus that causes COVID-19) infect the same person, at the same time.
While BA.4.6 will be similar to BA.4 in many ways, it carries a mutation to the spike protein, a protein on the surface of the virus which allows it to enter our cells. This mutation, R346T, has been seen in other variants and is associated with immune evasion, meaning it helps the virus to escape antibodies acquired from vaccination and prior infection.
Severity, infectiousness and immune evasion
Fortunately, omicron infections generally cause less serious illness, and we’ve seen fewer deaths with omicron than with earlier variants. We would expect this to apply to BA.4.6 too. Indeed, there have been no reports yet that this variant is causing more severe symptoms.
But we also know that omicron subvariants tend to be more transmissible than previous variants. BA.4.6 appears to be even better at evading the immune system than BA.5, the currently dominant variant. Although this information is based on a preprint (a study that is yet to be peer-reviewed), other emerging data supports this.
According to the UKHSA’s briefing, early estimates suggest BA.4.6 has a 6.55% relative fitness advantage over BA.5 in England. This indicates that BA.4.6 replicates more quickly in the early stages of infection and has a higher growth rate than BA.5.
The relative fitness advantage of BA.4.6 is considerably smaller than that of BA.5 over BA.2, which was 45% to 55%.