US President Joe Biden announced on Tuesday a new goal of administering at least one dose of Covid-19 vaccine to 70 per cent of the adult population over the next two months amid debate about reaching herd immunity.
Mr Biden also is aiming for 170 million adults in the US to be fully vaccinated by July 4, the nation's independence day.
"As we've anticipated, the pace of the vaccinations is slowing now that the majority of American adults have already gotten their first shot," he said.
"Soon, we will have reached the adults most eager to be vaccinated and this effort will shift to a new phase."
About 56.3 per cent of US adults have been immunised with at least one dose, according to the Centres for Disease Control and Prevention.
About 105 million adults have been fully vaccinated.
The CDC defines fully vaccinated people as those who are two weeks past their final Covid-19 vaccine dose.
"The light at the end of the tunnel is actually growing brighter and brighter," Mr Biden said.
His new goal is a step towards a promise made during a prime-time speech in March, when he spoke of his hope for a country "closer to normal" by July 4.
"We will begin to mark our independence from this virus," Mr Biden said. "July 4 with your loved ones is the goal."
The White House says the Biden administration will accomplish the goal by having pharmacies agree to accept walk-in vaccinations, sending doses to rural health organisations and increasing promotion.
Mr Biden also announced that he will ask the Federal Emergency Management Agency to support smaller pop-up vaccination sites and mobile drives.
"Now we're going to have to bring the vaccine to people who are less eager," he said.
"So we also know that there are millions of Americans who just need a little bit of encouragement to get the shot."
Mr Biden spoke of efforts to develop a Covid-19 vaccine for children aged 12 to 15 years old, which the Food and Drug Administration may approve for emergency use.
Pfizer has applied for emergency use authorisation for a vaccine for teens, and media reports say the FDA may approve it as early as next week.
The Pfizer vaccine is the only approved shot for people aged 16 to 18 years.
The Moderna and Johnson & Johnson shots are only approved for people 18 years or older.
"If that announcement comes, we are ready to move immediately," Mr Biden said.
Scientists and public health researchers have been discussing how the US can reach herd immunity, a level where widespread vaccination can fend off the threat of severe outbreaks.
The country's top infectious diseases expert, Dr Anthony Fauci, said the focus should be placed on vaccinations.
"I always say there's this elusive, somewhat mystical terminology of 'herd immunity' and 'herd immunity threshold'," Dr Fauci told a radio station in Boston on Monday.
He has estimated that herd immunity would be achieved if between 70 and 85 per cent of the population is vaccinated.
Other experts say it may be higher if a vaccine-resistant variant becomes prevalent.
"I'd like to get it to 100 per cent but I think, realistically, we can get to that place by July 4," Mr Biden said.
More than 577,000 people in the US have died from Covid-19, the highest toll in the world, and the country leads in total cases.
Mr Biden also announced that he will share 10 per cent of the US vaccine supply with the rest of the world by July 4.