Delta sub-variant expected to be dominant in UK by January

An offshoot of the Delta coronavirus variant which is slowly spreading throughout the UK is expected to be dominant within a matter of months, experts believe.

Known as AY.4.2, the sub-variant is thought to be at least 10 cent more transmissible than its predecessor, with analysis underway to determine what accounts for its increased infectiousness.

The UK Health Security Agency (UKHSA) has said that early tests do not suggest that AY.4.2, which has been labelled a ‘Variant Under Investigation’, has acquired the ability to evade immunity generated by infection or injection.

Nor are the sub-variant’s new mutations associated with significantly improving the virus’ ability to bind and gain entry to human cells, experts have said.

Nonetheless, AY.4.2 is continuing to account for a growing proportion of cases with each passing week. Between 18 and 24 October, it made up 11.3 per cent of all sequenced infections in the UK. Two weeks before this figure stood at 8.5 per cent.

Dr Jeffrey Barrett, director of the Covid-19 Genomics Initiative at the Wellcome Sanger Institute, said each week of additional data “increases the confidence that AY.4.2 has an inherent growth advantage” over the original Delta variant which took hold in the UK.

Given its current trajectory, AY.4.2 will eventually displace its Delta predecessor and become dominant, possibly in January, Dr Barrett said. “I think it will happen,” he added. Separate analysis has suggested the sub-variant could take hold in the UK as early as December.

Ravi Gupta, a professor of clinical microbiology at the University of Cambridge, said these estimates depend on the methodology used for modelling, adding he was unconvinced that AY.4.2 will come to dominate during the winter months.

Fears have been raised over the emergence of AY.4.2 and its potential impact on the direction of the UK’s epidemic. However, Dr Barrett suggested the variant would likely not prove to be disastrous for Britain.

“All else being equal, it will increase cases, and hospitalisations by a modest amount, but it is a much smaller relative change than Delta had compared to Alpha, so won’t fundamentally change things in the same way,” he said.

Emma Thomson, a professor in infectious diseases at Glasgow University’s Centre for Virus Research, said it still remains unclear why AY.4.2 is increasing.

“It doesn’t have classical mutations that would lead us to think it is more transmissible or more able to evade immunity, although we’re currently investigating that because of the increase in numbers,” she said.

“We’ve been taken by surprise before and so until the experiments are done, I wouldn’t be 100 per cent confident that there isn’t something giving this variant a slight advantage.”

AY.4.2 is the second notable Covid variant to emerge within the UK, after Alpha, which took hold in the population last Christmas.

Scientists have said the current high rate of infections across Britain make it “inevitable” that other variants will emerge in the future.

“AY.4.2 isn’t the underlying problem,” said Professor Sam Wilson, a virologist at the Centre for Virus Research at the University of Glasgow. “The problem is that allowing the virus to spread in a relatively uncontrolled fashion increases the probability that more troublesome variants arise.”

Prof Gupta said: “The emphasis again is on our lack of interventions. Variants with high transmissibility are inevitably going to come along.”

The emergence of a seemingly more transmissible Covid variant has also sparked some concern that the virus is still evolving to better adapt to the human population and has yet to reach its peak fitness.

“It’s a bit of a frustration because I think I’m not the only one who had hoped that with Delta we’d seen essentially the most transmissible variant,” said Professor Francois Balloux, director of University College London’s Genetics Institute. “I think it will be a slight setback, but not so much.

“I’m not so much worried about the variant itself, but let’s say I’m a bit worried that actually we haven’t reached a maximum transmissibility yet.”

Dr Barrett said there was no indication of “how much further it [the virus] has to go, only that it hasn’t reached an optimum yet”.