(NewsNation Now) — The global total of confirmed coronavirus cases topped more than 100 million Tuesday as the effort to end the pandemic has turned to a race between the vaccinating the public and virus variants.

The milestone was reached just over a year after the coronavirus was first detected in Wuhan, China and less than two weeks after the global death toll exceeded two million.

The United States has the highest total number of confirmed coronavirus cases for a single country at 25.3 million according to data compiled by Johns Hopkins University. Close behind, India has more than 10.6 million confirmed cases and Brazil has more 8.8 million.

More than 19.2 million Americans have received their first shot of the vaccine, according to the Centers for Disease Control and Prevention. That is well short of the hundreds of millions who experts say will need to be inoculated to vanquish the outbreak. The CDC reports that more than 41.4 million doses have been distributed across the country.

Health experts have warned that the more contagious and possibly more deadly variant sweeping through Britain will probably become the dominant source of infection in the U.S. by March. It has been reported in over 20 states so far. Another mutant version is circulating in South Africa.

The more the virus spreads, the more opportunities it has to mutate.

To guard against the new variants, President Joe Biden Monday added South Africa to the list of more than two dozen countries whose residents are subject to coronavirus-related limits on entering the U.S.

Moderna, the maker of one of the two vaccines approved in the U.S., announced on Monday that it is beginning to test a possible booster dose against the South African variant. Moderna CEO Stephane Bancel said the move was out of “an abundance of caution” after preliminary lab tests suggested its shot produced a weaker immune response to that variant.

Globally more than 2.1 million people have died from the virus.

The Associated Press contributed to this report.