More than four million vaccinations against Covid-19 were administered in the US in a single day at the weekend, showing an acceleration in the country's vaccine distribution programme amid an ongoing rise in coronavirus cases.
Andy Slavitt, the White House senior adviser on the Covid-19 response, said on Monday that 4.1 million immunisations were given on Saturday, April 3, a record in the US vaccination programme.
The US is now averaging three million immunisations a day, a pace that is about five times faster than the global average, CNN reported.
"We will administer more shots in March than any country on Earth," President Joe Biden said in a speech last Monday. "Look at what we have done in the last 10 weeks. No other country has come close."
More than 18 per cent of the US population, or more than 61 million people, has been fully vaccinated, according to the Centres for Disease Control and Prevention.
About 32 per cent, or more than 106 million people, have had at least one dose four months into the programme.
"The war against Covid-19 is far from over, far from won," Mr Slavitt said on Monday.
"The worst thing we could do right now would be to mistake progress for victory. If we let our guard down now, we will see more of our fellow Americans get sick and die unnecessarily. Each of us can act to prevent this."
He recommended that everyone continue following coronavirus mitigation measures such as masking, distancing and being vaccinated when possible.
The White House Covid-19 Task Force is issuing caution amid this progress owing to the continuous rise in coronavirus cases being seen in the US.
Agency director Dr Rochelle Walensky said this is the fourth week of increasing coronavirus cases.
The US health agency has recorded a seven-day average of more than 65,000 daily infections.
This is still dramatically lower that the 200,000 daily cases seen in the worst surge in the winter months.
“Trends in data have been indicating cases are increasing nationally and we are seeing this occur predominantly in younger adults," she said, referring to new research that found cases were rising among young people participating in extra-curricular activities.
"This is why you’ve heard me so clearly share my concern."
The worry is that higher cases over 50,000 daily can make it easier for a surge or spike to occur and then lead to a rise in hospital admissions and possible deaths.
Reuters reports that Covid-related hospital admissions rose 4 per cent last week, ending 11 weeks of decline.
Through vaccinating the elderly and high-risk populations, a fourth surge of cases may not be as deadly.
If the pandemic continues, however, scientists say the additional time allows for the virus to mutate and create more threatening variants.
The US is host to all three "variants of concern", so labelled by the World Health Organisation, from Brazil, South Africa and the UK.
The agency additionally labelled two strains from California as "variants of concern" and said it has its eye on home-grown "variants of interest" in the New York City area, which has stagnated at a high level of infections.
"We know that these increases are due in part to more highly transmissible variants, which we are very closely monitoring," Dr Walensky said.
"And as more schools are reopening, it is even more important to make sure they do so safely with strict adherence to [agency] guidance and for all of us to roll up our sleeves for a vaccine as soon as we can.”
The nation's top infectious diseases expert, Dr Anthony Fauci, debunked hopes that warmer weather will help lower cases and fend off a fourth surge of infections.
“You might remember a year ago or a little bit more than a year ago when we were looking for the summer to rescue us from surges, it was in fact the opposite," he said.
"We saw some substantial surges in the summer."
Dr Fauci said medical experts have not seen seasonal patterns with Covid, similar to how the flu operates in the country.
"I don’t think we should even think about relying on the weather to bail us out of anything we’re in right now.”
The US has recorded more than 555,000 Covid-19 deaths, according to Johns Hopkins University. The country also leads the world in coronavirus cases, with more than 30 million known infections.