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.

Britain’s medical regulator on Jan. 8 approved Moderna’s Covid-19 vaccine for use, the health ministry said, adding that it agreed to purchase an additional 10 million doses with plans for a spring rollout of the shot.

Britain became the first country to vaccinate its population with Oxford University and AstraZeneca’s Covid-19 shot on Jan. 4, but Prime Minister Boris Johnson was set to tighten restrictions in England in a bid to slow the spread of cases.

China approved a Covid-19 vaccine developed by an affiliate of state-backed pharmaceutical giant Sinopharm on Dec. 31, the country’s first approved shot for general public use as it braces for increased transmission risks over winter.

The Covid-19 vaccine developed by British drugmaker AstraZeneca Plc and Oxford University will likely be authorized for emergency use in the United States in April 2021, the chief adviser for the U.S. Covid-19 vaccine program said.

The overall efficacy of AstraZeneca and Oxford University’s Covid-19 vaccine approved in Britain on Dec. 30 in preventing symptomatic infections was 70.4%, compared with 95% efficacy for the Pfizer/BioNTech shot, the first novel coronavirus vaccine approved in Britain on Dec. 2.

Britain on Dec. 30 became the first country in the world to approve the coronavirus vaccine developed by Oxford University and AstraZeneca, hoping that rapid action will help it stem a record surge of infections driven by a highly contagious form of the virus.

The European Medicines Authority (EMA) will most likely not be able to approve the Covid-19 vaccine developed by drug maker AstraZeneca and the University of Oxford in January 2021, the watchdog’s Deputy Executive Director Noel Wathion said.

The World Health Organization cautioned against major alarm over a new, highly infectious variant of the coronavirus that has emerged in Britain, saying this was a normal part of a pandemic’s evolution.

Countries across the globe shut their borders to Britain on Dec. 21 due to fears about a highly infectious new coronavirus strain, causing travel chaos and raising the prospect of food shortages days before Britain is set to leave the European Union.