A new Covid variant which has sparked a surge in cases in the US has been dubbed the most contagious yet. 

The XBB.1.5 mutation of the virus is projected to be the cause of 40 per cent of infections in America and has now become the dominant strain since it emerged at the end of the year. 

The World Health Organisation (WHO) has called the variant the “most transmissible yet”, based on how quickly it has spread – but it is not thought to cause people to become more ill than other forms of Covid.  

XBB.1.5 itself evolved from XBB, which began circulating in the UK in September 2022, but which has not been classified as a so-called "variant of concern" by health authorities. Around 4% of cases in this country are thought to stem from the new strain.  

Maria Van Kerkhove, the WHO’s Covid-19 technical lead, said global health officials are worried about how quickly the subvariant is spreading in the northeastern U.S.

“It is the most transmissible subvariant that has been detected yet,” Van Kerkhove told reporters during a press conference in Geneva on Wednesday. “The reason for this are the mutations that are within this subvariant of omicron allowing this virus to adhere to the cell and replicate easily.” 

The Herald:

Covid has not gone away

Scientists from the WHO said that XBB.1.5 has a "growth advantage" above all other sub-variants seen so far. 

But they said there was no indication it was more serious or harmful than previous Omicron variants. 

The WHO said it would keep a close watch on lab studies, hospital data and infection rates to find out more about its impact on patients.

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US scientist Eric Topol said it is now out-competing all variants and "we've not seen such rapid growth of a variant" since the original Omicron emerged a year ago. 

In response, Tim Spector, professor of genetic epidemiology at King's College London and the founder of the COVID Zoe app, tweeted: "XBB could be the new variant to watch out for .. in 2023." 

XBB.1.5 is a mutated version of Omicron XBB, which was first detected in India in August. 

XBB has been found in at least 70 countries, according to the World Health Organisation, causing surges of infection in some parts of Asia, including India and Singapore, in October. 

Studies have found that the strain is capable of evading antibodies from previous COVID infections or vaccinations. 

The Herald:

Vaccination 'remains the best defence' 

Dr Meera Chand, director of clinical and emerging infection at the UKHSA, said: "It is not unexpected to see new variants of SARS-CoV-2 emerge. UKHSA is monitoring the situation closely, as always. 

"Vaccination remains our best defence against future COVID-19 waves, so it is still as important as ever that people come take up all the doses for which they are eligible as soon as possible." 

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But Professor Lawrence Young, a virologist at Warwick University, said the new strain is a "wake-up call" and could exacerbate the NHS crisis. 

He told Mail Online: "We don't know how this variant is going to behave in the UK in a population that has been previously exposed to other Omicron variants and where many of the over 50s have had booster shots with a bivalent vaccine. 

"Nevertheless, this is a wake-up call - a sharp reminder that we can't be complacent about COVID. 

"The threat of XBB.1.5 and other COVID variants further exacerbating the current NHS crisis stresses the need for us to remain vigilant."