On Friday, the director of the Centres for Disease Control and Prevention (CDC), Rochelle Walensky, forecast the variant to become the dominant form of Covid in the US in the coming weeks – thanks to its transmissibility.
“It is more transmissible than the alpha variant, or the UK (Alpha) variant that we have here,” Ms Walensky told ABC News.
“We saw that quickly become the dominant strain in a period of one or two months, and I anticipate that is going to be what happens with the Delta strain here.”
Despite Covid cases flatlining, Delta has been doubling nearly every week in the US, accounting for as few as 2.7 per cent of Covid infections on 22 May, and 0.6 per cent in April.
By Friday, Delta was believed to be 10 per cent of all Covid infections, with the CDC classifying it as a “variant of concern” – along with Gamma (first identified in Brazil), and Alpha (first identified in the UK).
The designation is an acknowledgement of Delta’s increased transmissibility, and concerns that Delta (first identified in India) could be resistant to vaccines.
Initial findings from the UK, where the Delta variant has delayed reopening from Covid restrictions by four weeks, indicate that two doses of the Pfizer vaccine are highly effective – at 96 per cent – at preventing hospitalisation from Delta.
In the US, disparities between north and south, and between Democrats and Republicans, in rates of vaccinations are worrying both officials and analysts about the dangers of Delta – which is found in 41 states, according to the Washington Post.
“This is the most dangerous phase of the pandemic coming up for people who are unvaccinated," said Dr Gregory Poland, director of the Mayo Clinic's Vaccine Research Group in Rochester, Minnesota, in comments to NBC News.
US president Joe Biden, in a plea for Americans to get both doses of a Covid vaccine on Friday, also warned of the risks of variants with restrictions across the US disappearing.
His administration is expected to fall shy of a 70 per cent vaccination target by 4th July.