The number of Americans who have died from Covid-19 has surpassed 600,000 - the most of any nation - according to data from John Hopkins University.
The US also ranks highest in total number of recorded cases, with nearly 33.5 million infections since 2020.
This latest milestone comes as President Joe Biden's goal of getting 70% of US adults vaccinated by 4 July appears increasingly likely to fail.
Over 173 million people, around 52% of the US, have had at least one dose.
Roughly 43% of the US population, or about 144 million Americans, are fully vaccinated, according to the Centers for Disease Control and Prevention (CDC). The CDC says people are fully vaccinated at least two weeks after their final vaccine dose.
As of Tuesday, 600,012 people in the US have died of complications brought on by the coronavirus.
Brazil and India have reported the next highest death tolls, with Brazil at more than 488,000 deaths and India at more than 377,000.
It took around four months for the US to go from 500,000 to 600,000 deaths - about as long as it took to go from none to 100,000. It marks a significant change from the height of the pandemic when the death toll jumped from 300,000 to 400,000 in one month last winter.
But the country's vaccination rate has begun to fall to around one million doses administered per day - down from a high this April of nearly 3.4 million after it was first made available to all adults over 18.
Though a majority of adult Americans have received at least one dose, the percentages vary wildly from state to state. Vermont has fully vaccinated 71% of its residents, but only 36% of Mississippians have received both jabs, according to a New York Times database.
Health experts have warned of vaccine scepticism and misinformation. A recent Kaiser Family Foundation poll found around a third of US adults were not planning on getting vaccinated yet.
The vaccination hesitancy has led to measures like lotteries and giveaways in states eager to get people signed up for jabs.
Ohio has been selecting one vaccinated resident each week to win $1m (£708,000). On Tuesday, California will give $1.5m each to 10 residents.
Other winnable items include alcohol, cannabis, guns, food vouchers and gift cards.
The White House is also working to boost vaccinations. As part of a national tour to raise awareness, Vice-President Kamala Harris travelled to South Carolina - a state with a relatively low vaccination rate - to visit a pop-up medical clinic on Monday.
"They are safe. And they are free. And they are effective. And it is that simple," Ms Harris said during her visit.
On Monday night, the governor of California - the most populous US state - announced a "grand reopening".
The state, which ordered the first coronavirus lockdown, will now allow vaccinated people to not wear masks or socially distance in most settings. Public health measures remain for events with over 5,000 people.
Also on Monday, US drug maker Novavax announced that its vaccine was found to be 90.4% effective after testing was conducted on 30,000 participants.
The company said they plan to file for US federal approval in the third business quarter. If approved, it will be the fourth coronavirus vaccine to hit the US market.
The Biden administration has already ordered enough doses of the existing vaccines for the entire US population, US media report, so Novavax may become part of the global vaccine-sharing deal to donate doses.
The US has already pledged over half a billion doses of the Pfizer vaccine to the scheme.