The United States surpassed 400,000 COVID-19 deaths Tuesday, as the rate of casualties has accelerated in recent weeks.

The latest grim milestone in the pandemic comes just 36 days after the country hit the 300,000 mark. By comparison, it took nearly four months for the U.S. death toll to jump from 100,000 to 200,000, and almost three months to climb from 200,000 to 300,000.

The nation accounts for nearly one-fifth of all coronavirus-related deaths in the world, according to data compiled by Johns Hopkins University. And with more than 24 million confirmed cases during the pandemic, it accounts for a quarter of all infections.

The Institute for Health Metrics and Evaluation at the University of Washington’s School of Medicine projects the number of COVID-19 deaths in the U.S. will hit 566,720 by May 1.

Americans are being vaccinated, but at a much slower pace than government officials had promised. As of Friday, the last day when data are available, nearly 10.6 million people had received at least one dose of a vaccine, according to the Centers for Disease Control and Prevention. The Trump administration had projected 20 million people would be vaccinated by the end of December and 50 million by the end of January. 

President-elect Joe Biden, who will be sworn into office Wednesday, has set a goal of inoculating 100 million Americans in his first 100 days in office by invoking the Defense Production Act to boost supply and launching mobile vaccination clinics across the country.

In one encouraging sign, the number of COVID-19 hospitalizations has fallen for six straight days, now down to 123,848, according to The COVID Tracking Project. 

However, the CDC warned last week that a more highly contagious variant of the virus, first detected in England, could become the predominant strain in the United States by March. The mutation is not believed to be more likely to cause severe illness or death than the more common strain found in the U.S., but another spike in cases would inevitably lead to more hospitalizations and casualties, the CDC said.

Dr. Anthony Fauci, the federal government’s top infectious disease expert, told NBC News that health officials are also keeping a close eye on “ominous” strains seen in Brazil and South Africa.