Biden says US COVID vaccinations 'weeks ahead of schedule'

President Joe Biden on Thursday celebrated 50 million Covid-19 vaccines administered since he took office, saying the rollout is "weeks ahead of schedule," but warning Americans to keep masking up.

Biden came into office promising 100 million shots administered in his first 100 days, something critics said was an insufficiently ambitious target, but the White House says it is now on track easily to exceed.

"We're moving in the right direction despite the mess we inherited," Biden said, referring to the situation under his predecessor Donald Trump.

The United States is the world's hardest hit country, with coronavirus deaths crossing the 500,000 mark earlier this week.

But the Democratic president noted that the pace of the rollout had "doubled" during his six weeks in power and said that distribution of vaccines to authorities across the states had risen 70 percent.

Promising that the government is "laser focused on the greatest operational challenge this country's ever undertaken," Biden said that there would be "enough supply for all adult Americans by the end of July."

Despite falling Covid-19 cases and the improvement in the vaccinations program, "this is not a time to relax," he warned.

"We must keep washing our hands, stay socially distanced and for God's sake, for God's sake, wear a mask," Biden said. "The vaccines are safe and effective."

Biden won election against Trump in November partly on his pledge to take the pandemic seriously and ramp up the vaccination program.

According to some health experts, the United States should be targeting 2.85 million vaccines a day in order to reach herd immunity by mid-July.

Critics say Biden set the bar too low, but the White House says he didn't want to over-promise. When he entered the White House, only 16.5 million Americans had received the vaccine.

"America's administered the most shots of any country in the world, any country in the world, with among the highest percentage of population fully vaccinated. That's the progress we promised," he said.

The White House has given differing estimates of when the pandemic will be under control. Most recently, Biden has suggested Christmas will see people in a "very different circumstance."