The United States has hit a key milestone in the battle against the COVID-19 pandemic: 75% of U.S. adults have received at least one dose of a coronavirus vaccine, according to a White House official.

White House COVID-19 Data Director Cyrus Shahpar confirmed the news on Twitter, announcing that from Sunday to Tuesday, the country reported 1.51 million doses over Saturday's total, "including 681K newly vaccinated and 105K additional doses."

The news came the same day that the U.S. topped the grim total of 40 million confirmed COVID-19 cases since the start of the pandemic.

The unfortunate milestone comes as the country is grappling with a fourth wave of infections, driven by the highly contagious delta variant and sluggish vaccination rates. 

Despite making up 4.25% the world’s population, the U.S. accounts for about 18% of all known COVID-19 cases. The number of cases in the U.S. amounts to around 12% of the country’s population, although some people have been infected more than once.

The virus also has claimed nearly 650,000 lives in the U.S.

As of Sept. 4, prior to the Labor Day holiday weekend, nearly 176 million Americans — 53% of the population — have been fully vaccinated, according to the Centers for Disease Control and Prevention. Sixty-four percent of U.S. adults, or more than 165 million Americans, have been fully vaccinated.

Spectrum News' Ryan Chatelain contributed to this report.