Batches of a new coronavirus vaccine were fanning out across the US on Tuesday amid a surge in cases and fears of a faster-spreading variant of the pathogen that was recently discovered in Britain.
The launch of the second anti-Covid-19 vaccine, produced by Moderna, began this week as the US headed into the festive season having surpassed 18 million cases of the virus and 320,000 deaths.
Moncef Slaoui, chief adviser to the federal government's Operation Warp Speed vaccine scheme, said on Monday that US scientists were studying whether the coronavirus variant in the UK was more contagious than the dominant strain.
"There is no hard evidence that this virus is actually more transmissible, [but] there is clear evidence that there is more of it in the population," said Dr Slaoui, a former pharmaceutical executive.
"It may be just seeding happened in the shadows and we're now seeing a surge, or maybe it has higher transmissibility."
The National Institutes for Health began laboratory studies on the variant to determine how contagious it is and whether antibodies against the more dominant strain of Covid-19 are effective against it, Dr Slaoui said.
"What's clear is it's not more pathogenic," he said.
The discovery of the new variant prompted UK authorities on Sunday to lock down London and tighten restrictions elsewhere. Dozens of countries in Europe, Asia, South America and the Middle East have restricted travel from the UK and other areas with documented cases of the variant.
Across the US, healthcare workers and care-home residents were on Tuesday to receive the first of their double-dose inoculations against Covid-19 as the country tries to turn the page on a pandemic that has ravaged the economy.
About six million doses of the recently authorised Moderna vaccine were being shipped across the US, adding to the nearly three million doses of the Pfizer-BioNTech vaccine that started being dispatched nationwide last week.
Within days, President Donald Trump is expected to sign a $900 billion pandemic relief package approved by legislators with big majorities in both houses late on Monday after months of fraught negotiations.
The package includes a $600 stimulus payment to most Americans, $300 per week unemployment benefit top-ups and cash to prop up struggling businesses and fund the country's huge vaccination campaign.