The numbers of a new variant of the Delta strain of COVID-19 are rising in the UK, new data has revealed.
AY4.2 is a sub-lineage of the Delta variant and some experts fear that this “Delta plus” variant, as it has been labelled, appears to be more transmissible.
Government figures published on Friday said that AY4.2 cases account for a "slowly increasing proportion of cases in the UK".
The data shows AY4.2 accounted for 8.5% of Delta cases in the week ending 10 October. Preliminary analysis suggests this could have risen to 10.3% of all Delta cases for the following week and 11.3% the week ending 24 October.
Critically, initial analysis does not suggest the new variant causes a "significant reduction" in vaccine effectiveness. However, health chiefs are carrying out more comprehensive testing to confirm.
The Health Security Agency (HSA) moved the AY4.2 into the category of Variant Under Investigation (VUI) on 21 October, adding that it was “expanding” and “on an increasing trajectory”.
As a VUI the strain will undergo a risk assessment with the relevant expert committee, after which it could be designated Variant of Concern (VOC).
The AY4.2 strain has been detected in 33 countries across the world, but the overwhelming majority of cases are in the UK, according to cov-lineages.org, a website that documents the lineages of the virus.
Denmark, Germany and Ireland have also reported cases of the Delta plus as well as the US, Israel, Russia and India.
The HSA has said AY4.2 does not appear to be causing more severe disease or render vaccines any less effective, however, work has already begun on a modified version of the Oxford vaccine to target the Delta coronavirus variant.
Experts have said that the variant's secondary attack rate – the probability of an infection occurring in a group of people – is around 12.4 per cent for a household in the UK, while it is 11.1 per cent for the Delta variant, meaning there is a greater chance of infection spreading within a household if one person is infected with Delta plus.
The UK has experienced a marked rise in COVID cases in the past few weeks, though that increase has tapered off in recent days.
On October 23 there were 44,985 new cases recorded in one day, the highest figure since the January peak, but on Thursday the number had dropped slightly to 39,0006.
On Thursday, Sir Patrick Vallance said that waning immunity and the public's behaviour will be the main factors that determine whether the UK sees further spikes in cases of COVID.
“Nobody is really clear which direction this is going in, but they are clear about the two big variables that could change that," he told BBC Breakfast.
“One is waning immunity; so if immunity wanes faster than expected, you’ll see a bigger increase, and that’s why it’s so important to get booster shots going in the vulnerable and the elderly in particular.
“The second is the behavioural change, how quickly we return to pre-pandemic behaviours… if you aggregate the models, most are saying: ‘Actually, it looks fairly flat, don’t expect the very big peaks we’ve had in the past, it looks fairly flat, but at a very high level at the moment.’
“So, the high level remains a concern and from a high level you can go up quite quickly.”