Researchers from the University of Cambridge have found that the strain of coronavirus currently rocking the U.S. is the original variation of the virus that emerged from China. After mapping the genetic history of the infection from December to March, the researchers discovered three distinct yet closely related variants of the deadly virus.

An analysis conducted on the strains shows that the Type A virus, which is the original virus that was transferred from bats via pangolins to humans, is not the most common strain in China. Instead, it is the Type B coronavirus, which started to circulate in Wuhan, China on Christmas Eve.

The Type A strain is the most prevalent in Australia and the U.S., which has recorded at least 400,000 cases of COVID-19 since February. Around two-thirds of coronavirus patients in the U.S. were infected with the Type A virus, most of them coming from the West Coast.

According to the researchers, the virus, which is now officially called SARS-CoV-2, is continuously mutating to disable immune system resistance in various populations. While it is odd that it was the original strain that had spread through the West Coast of the U.S. despite it not being prevalent in the ground zero of the disease, scientists say it doesn’t mean that the virus had arrived earlier in the U.S. and was only not detected.

Both Type A and Type B strains were already in existence by January, and the first 35-year-old coronavirus patient in the U.S. admitted that he had just arrived in Washington after visiting his family from Wuhan, China when he started experiencing coronavirus symptoms. This suggests that the first coronavirus case in the U.S. must have caught the Type A virus from Wuhan before it mutated into Type B within China.

Up to now, scientists remain clueless as to how the Type B virus pushed aside Type A to become more common in China. However, their study reveals that Type B is more comfortable in the immune systems of people in Wuhan than in other populations, such as Westerners.

China Coronavirus COVID-19 An elderly woman arrives in an ambulance to Wuhan Red Cross Hospital after being transferred from another hospital after recovering from the COVID-19 coronavirus in Wuhan on March 30, 2020. HECTOR RETAMAL/AFP via Getty Images