The United States has the most coronavirus cases in the world — but in sequencing cases to check for genetic changes, it lags at an astoundingly low rate.

Of the more than 18 million cases officially reported in the United States, just 51,212, or 0.3 percent, have been genetically analyzed for variants, according to the latest data from the GISAID Initiative, which provides a global database of coronavirus genomes. Worldwide, the United States ranks 43rd in percentage of cases sequenced among countries with more than 100 reported infections.

Britain, by contrast, is No. 8, having analyzed 7.4 percent of its more than 2 million documented cases, according to the Covid-19 Genomics UK (COG-UK) Consortium. Britain’s relatively high rate probably contributed to its detection of a highly transmissible variant circulating in southern England since September.

“It makes sense that it was detected first in the U.K. because they have probably the world’s best surveillance program,” Angela Rasmussen, a virologist at the Georgetown Center for Global Health Science and Security, told The Washington Post.

Australia, which has reported more than 28,200 coronavirus cases, has sequenced 58.6 percent, the highest rate worldwide. New Zealand ranks second, with 48.6 percent of the country’s more than 2,100 cases analyzed. Taiwan, Denmark and Iceland also make the top five.

South Africa, which globally falls at 42, just ahead of the United States, announced Dec. 18 that it had detected a similar variant after a surge in cases there. South Africa is battling the worst outbreak in Africa.

The U.S. Centers for Disease Control and Prevention has not documented any cases of the British variant in the United States — but experts, even at the CDC, say it’s probably already here.

“Given the small fraction of U.S. infections that have been sequenced, the variant could already be in the United States without having been detected,” the CDC wrote on its website.