(AP) — It took six months for the world to reach 10 million confirmed cases of the coronavirus. It took just over six weeks for that number to double.

The worldwide count of known COVID-19 infections climbed past 20 million on Monday, with more than half of them from just three countries: the U.S., India and Brazil, according to the tally kept by Johns Hopkins University.

The average number of new cases per day in the U.S. has declined in recent weeks but is still running high at over 54,000, versus almost 59,000 in India and nearly 44,000 in Brazil.

In other developments Tuesday:

o Russia became the first country to approve a vaccine against the virus. While a proven coronavirus vaccine would be an epic medical breakthrough, the move raised alarms among scientists because the shots have not been subjected to large-scale testing in humans. They have only been studied in dozens of people, not the thousands typically involved.

The severe and sustained crisis in the U.S. — over 5 million cases and 163,000 deaths, easily the highest totals of any country — has dismayed and surprised many around the world, given the nation’s vaunted scientific ingenuity and the head start it had over Europe and Asia to prepare.

South Africa, Colombia, Mexico, Peru, Argentina, Russia and the Philippines round out the list of the top 10 countries contributing the most new cases to the global tally since July 22, according to an Associated Press analysis of Johns Hopkins data through Monday.

The real number of people infected by the virus around the world is believed to be much higher — perhaps 10 times higher in the U.S., according to the Centers for Disease Control and Prevention — given testing limitations and the many mild cases that have gone unreported or unrecognized.

The U.S., with about 4% of the world’s population, accounts for about 25% of the known coronavirus infections and 22% of the deaths.

Mexico has reported nearly 500,000 cases and more than 50,300 deaths, but the president’s point man on the epidemic, Assistant Health Secretary Hugo Lopez-Gatell, said a full lockdown would prove too costly for people with little savings and tenuous daily incomes.

“We do not want a solution that would, in social terms, be more costly than the disease itself,” he said.