The spread of highly contagious coronavirus variants is threatening to fuel a "potential fourth surge of cases" in the US, a top health official has warned.The head of the US Centers for Disease Control and Prevention (CDC) said she was concerned by recent Covid-19 data.Dr Rochelle Walensky said about 70,000 new cases a day had been recorded last week - "a very high number".There were nearly 2,000 deaths a day in the same period, she added."Please hear me clearly: at this level of cases, with variants spreading, we stand to completely lose the hard-earned ground we have gained," Dr Walensky said. "These variants are a very real threat to our people and our progress."There are many different versions, or variants, of Covid-19 circulating, but health experts are particularly concerned about a few which appear to be more contagious, including those first detected in the UK, South Africa and Brazil.The CDC has predicted the highly contagious B.1.1.7 variant first found in the UK will become the dominant strain in the US this month.IMAGE COPYRIGHTGETTY IMAGESimage captionThree vaccines against the coronavirus are being rolled out across the USGiven this, Dr Walensky said she was "really worried" about reports of US states "rolling back the exact public health measures we have recommended to protect people from Covid-19"."We have the ability to stop a potential fourth surge of cases in this country. Please stay strong in your conviction," she said.Imagining what 500,000 lost lives look like