A mobile Covid-19 vaccination center in front of Bolton City Hall in Bolton, where the number of cases of the Delta variant identified for the first time in India was relatively high.

Peter Byrne | PA pictures | Getty Images

The variant of Delta Covid, first discovered in India, has now spread to more than 80 countries and continues to mutate as it spreads around the world, World Health Organization officials said on Wednesday.

The variant now accounts for 10% of all new cases in the US, up from 6% last week. Studies have shown that the variant is even more transferable than other variants. WHO officials said some reports found it also caused more severe symptoms, but more research is needed to confirm these conclusions.

WHO is also following recent reports of a “Delta Plus” variant. “I think this means that an additional mutation has been identified,” said Maria Van Kerkhove, WHO technical director for Covid-19. “In some of the Delta variants, we saw one less mutation or one deletion instead of an additional one, so let’s look at everything.”

In the UK, the Delta variant recently became the dominant strain there, outperforming its native alpha variant, which was first discovered in the country last fall. The Delta variant now accounts for more than 60% of new cases in the UK

Dr. Anthony Fauci, chief medical adviser to the president, said last week that “we cannot allow this to happen in the United States” when he urged more people, especially young adults, to be vaccinated.

The Centers for Disease Control and Prevention classified the Delta variant as a variant of concern in the United States on Tuesday. The WHO classified the Delta variant as a worrying variant in early May.

The WHO also added another Covid mutation, the lambda variant, to its list of interesting variants on Tuesday. The agency is monitoring more than 50 different variants of Covid, but not all of them become public health threats to get on the official WHO watch list. The lambda variant has several mutations in the spike protein that could affect its transmissibility, but more study is needed to fully understand the mutations, Van Kerkhove said.

The lambda variant was discovered by scientists in South America, including Chile, Peru, Ecuador and Argentina, thanks to increased genome surveillance.