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The Delta Plus variant of coronavirus is establishing a foothold in the UK and now makes up more than 1 in 10 of all new infections.
Government figures published on Friday show there were 9,188 confirmed Delta Plus infections in the week preceding 17 November, compared with nearly 70,000 confirmed cases of the "original" Delta variant.
Delta Plus - known as AY.4.2 - is an offshoot of the original Delta strain that has been the dominant strain for the last few months of the pandemic.
That new data means the new strain makes up around 11.6% of Delta infections. The figures also show that the Alpha/ Kent variant, which devastated country last year, is now almost extinct with just 14 new infections in the past week.
The Delta Plus strain is being examined by experts, but early indications suggest it may be more transmissible - between 10-15% more infectious than the original Delta - yet less dangerous than previous mutations.
It also does not seem to render vaccines less effective.
The mutation is more likely to result in asymptomatic infection, scientists say. It is also thought to be less severe.
Typical COVID symptoms – loss or change of smell or taste, a fever, or a new persistent cough – appeared in just 33.3% of AY.4.2 cases compared to 46.3% of AY.4 original Delta strain cases, according to the Imperial College London REACT study.
Christl Donnelly, professor of statistical epidemiology, Imperial College London, said: “It is absolutely the case that if people are waiting for symptoms to do a test and to therefore identify that they are infected, and therefore cut back their contacts, being asymptomatic may facilitate transmission for example.
“It is asymptomatic transmission that really can make the difference between what’s relatively easily containable and what needs vaccination.”
On Monday, Sajid Javid reassured the public that vaccinations are working to protect them against severe infection and complications as a result of COVID-19, but warned against too much optimism.
He said: “Although we have built up this huge protection, this is not a time for complacency.
"Earlier this month the WHO’s (World Health Organization) Europe director said that Europe was back at the epicentre of the pandemic. Just this weekend the Netherlands and Austria have put in place partial lockdowns after surges in cases.
“We also still face the risk of new variants and, just as we have seen with the emergence of the so-called Delta-plus variant.
“Although Delta-plus may be more infectious than the original Delta variant, our investigations indicate that our vaccines remain effective against it, but we do all know that there will be more variants in the future.”
Austria has since announced a full lockdown and mandatory vaccinations - the first European country to do so.
Researchers say the observational nature of survey data and the relatively small proportion of unvaccinated adults calls into question the comparability of vaccinated and unvaccinated groups.
However, they found that third vaccine doses for eligible adults and the vaccination of children aged 12 and over are associated with lower infection risk.
The researchers say they should therefore remain a high priority – with possible extension to children aged 5-12 years – and this should help reduce COVID transmission over the winter.
Watch: How the world could be better after COVID