The United States reported a 25% drop in new cases of Covid-19 to about 825,000 during the week ended Feb. 7, the biggest fall since the pandemic started, although health officials said they were worried new variants of the virus could slow or reverse this progress.

Moderna started development on a booster to the company’s Covid-19 vaccine, which is hoped to work against the recently discovered (and more transmissible) SARS-CoV-2 variant from South Africa.

The Covid-19 vaccine developed by Pfizer and BioNTech is likely to protect against a more infectious variant of the virus discovered in Britain which has spread around the world, according to results of further lab tests.

A new, highly transmissible variant of the coronavirus first discovered in Britain could become the dominant variant in the United States by March, the U.S. Centers for Disease Control and Prevention warned.