The United States has reached another devastating milestone: COVID-19 has killed more than 350,000 people in the country, according to research from Johns Hopkins University. The gloomy number comes when a new variant of the coronavirus spreads to dozens of countries.

The coronavirus variant was first spotted last month in the UK and has now spread to dozens of countries, probably transmitted by infected people who have traveled around the world and unknowingly brought microscopic attackers with them.

The variant is now found in dozens of countries, including the United States, where it has infected people in Colorado, California and Florida.

Health workers are preparing for a particularly deadly January, after the U.S. recorded a record number of infections in December. President-elect Joe Biden warned this week that “the next few weeks and months will be a very difficult, very difficult period for our nation – perhaps the most difficult during this entire pandemic.”

Researchers say the new variant – called B.1.1.7 – probably originated in the south-east of England in September, before it was discovered there in November. According to a new report from Imperial College London, the British lock in November did little to prevent its spread, which was most prevalent among young people under the age of 20. The World Health Organization says the new variant is responsible for more than half of new infections in the UK

Europe is full of variants, which has been recorded in Belgium, France, Germany, Ireland, Italy, the Netherlands, Norway, Spain, Sweden and Switzerland. It has also been discovered in Asia, Australia, the Middle East and South America.

And the new version of the virus is already mutating – 17 mutations have been spotted, said NPR Global Health correspondent Michaeleen Doucleff Weekend edition. “Virus mutations occur all the time, when the virus grows in humans – especially when it reproduces and makes a bunch of copies of itself,” Doucleff said. Mutations occur due to random errors because the virus is copied.

“In the vast majority of cases, these errors are harmless or even weaken the virus,” Doucleff said. “But in rare cases, mutations can help the virus – they can give it a small boost or advantage over other versions.”

The good news is that the new variant doesn’t seem to be more deadly. But it’s much more contagious – researchers are still trying to determine exactly how much more, but many have estimated that it could be 50% more portable than the original strain. This may be because it leads to an increased viral load inside a person’s nose or respiratory tract – and so it is easier to disperse when people talk or cough. Another theory is that the new variant binds more easily to human cells.

This variant is helping to trigger the current increase in the number of cases in the UK, which has seen a huge jump in recent weeks. According to the Johns Hopkins Coronavirus Resource Center, December set a record in new cases there, with more than 862,000 added that month.

The UK variant is just one of multiple mutations that scientists have discovered. According to the Centers for Disease Control and Prevention, the variant that appeared in South Africa in October has some of the same mutations as the UK variant. Another mutation was found in Nigeria. It is considered that none of the variants is more serious.