While countries shut borders to travellers from southern Africa fearing the spread of a new strain of the virus that causes Covid-19, vaccine makers were racing to get their hands on it to test if their products will still work.

The World Health Organization formally raised the alarm on Thursday about the B.1.1.529 variant — now known as “Omicron” — which has a worrying array of mutations. But Moderna said its team had already been working “nonstop” on it for the past few days, while researchers at BioNTech are using a pseudovirus — engineered to look like the new strain — with the aim of discovering if their vaccines will be less effective against the strain within the next two weeks.

Johnson & Johnson said it was already testing its vaccine against the strain, while University of Oxford scientists are expecting a delivery of the virus imminently, according to a person familiar with the matter. AstraZeneca said it was already conducting research in Botswana and Eswatini, where the variant is present, to assess how its vaccine stood up to the new variant.