Researchers in China claim they have identified more than 30 different genetic variations of the new coronavirus that has infected almost 2.7 million people since the outbreak began late last year.

The study posted to the medRxiv preprint server by Zhejiang University researchers analyzed samples taken from 11 patients admitted to hospitals in Hangzhou, China between Jan. 22 and Feb. 4. Scientists were able to identify 33 mutations of the new coronavirus, 19 of which were new, using “ultra-deep sequencing.” The study has yet to be peer reviewed.

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“Sars-CoV-2 has acquired mutations capable of substantially changing its pathogenicity,” the researchers wrote in the paper. “Drug and vaccine development, while urgent, need to take the impact of these accumulating mutations into account to avoid potential pitfalls.”

Researchers infected cells with COVID-19 strains carrying different variations. The most aggressive strains were found to generate as much as 270 times as much viral load as the weakest strains and killed human cells the fastest, according to the study.

The South China Morning Post reports the deadliest mutations found in patients in the study had also been found in a significant portion of patients across Europe, while milder strains were predominant in western parts of the U.S., such as Washington state. A separate study found strains in New York had been imported from Europe. The death rate in New York was similar to that in many European countries.

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Researchers said the findings show “the true diversity of the viral strains is still largely underappreciated.”

Mary E. Schmidt, an infectious disease specialist and president of the Schmidt and Libby Health Advisory Group, said the subtle variations are typical of RNA viruses such as the new novel coronavirus and influenza and agreed with researchers that they could cause issues in the development of an effective vaccine.

“It potentially could cause a problem,” Schmidt said. “It might make it difficult for a vaccine that’s created in a year from now...it might not be the vaccine that’s effective because it won’t be targeting the right molecule, and it’s targeting could change a little bit.”

More than 187,000 people have died from the coronavirus worldwide. The U.S. currently leads the world in the number of confirmed cases and deaths.

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