Experts say the Delta variant now accounts for nearly all of the coronavirus infections globally.
Scientists have said they remained on alert although vaccines are still able to defend against serious disease and death from Delta.
The Delta variant, first detected in India in December 2020, remains the most worrisome version of the SARS-CoV-2 virus.
Delta also can cause symptoms two to three days sooner than the original coronavirus, giving the immune system less time to mount a defense.
People infected with Delta carry around 1,200 times more virus in their noses compared with the original version of the coronavirus.
The World Health Organization classifies Delta as a variant of concern, a category that means the variant is capable of increasing transmissibility, causing more severe disease or reducing the benefit of vaccines and treatments.
According to the WHO, Delta makes up 99.5 per cent of all genomic sequences reported to public databases and has “outcompeted” other variants in most countries.
A key exception is South America, where Delta has spread more gradually, and other variants previously seen as possible global threats – notably Gamma, Lambda and Mu – still contribute to a significant proportion of reported cases.