As healthcare officials fight to stem the outbreak of coronavirus, researchers in China say preliminary research shows that COVID-19 is mutating and there are at least two types of the virus now.

One of those, the more aggressive strain, has infected 70% of the people tested, while a less aggressive strain was linked to the rest, according to doctors at Peking University’s School of Life Sciences and the Institut Pasteur of Shanghai.

Scientists dubbed the aggressive strain “L type” and the less prevalent version “S type.” The L type strain was seen more frequently in Wuhan, where the outbreak began, but researchers note that the frequency of infection has dropped off. They also cautioned that the data they had available for study was “very limited” and follow-ups were necessary to understand the evolution of the COVID-19.

“Whereas the L type was more prevalent in the early stages of the outbreak in Wuhan, the frequency of the L type decreased after early January 2020,” they wrote. “Human intervention may have placed more severe selective pressure on the L type, which might be more aggressive and spread more quickly.”

Researchers say the different strains were likely caused through a mutation of the ancestral version of the virus. And that could mean more are coming. They urged the scientific community to come together to battle and contain the outbreak.

“These findings strongly support an urgent need for further immediate, comprehensive studies that combine genomic data, epidemiological data, and chart records of the clinical symptoms of patients with coronavirus disease 2019 (COVID-19),” they wrote.

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