News Delta is the ‘most serious’ COVID-19 variant, scientists say. How will it affect the US? June 16, 2021 Karen Weintraub, USA TODAY Comments Off on Delta is the ‘most serious’ COVID-19 variant, scientists say. How will it affect the US? The Delta variant of COVID-19 accounts for 6% of US cases. The danger, if any, will be to those are are not vaccinated, experts say.