Researchers in China say preliminary research shows there are two strains of the novel coronavirus that has been declared a global pandemic.

Scientists at Peking University’s School of Life Sciences and the Institut Pasteur of Shanghai report that a more aggressive strain has infected roughly 70 percent of those tested, while a less aggressive strain was linked to the remaining 30 percent, according to Reuters.

The scientists warned that the data in the study was still very limited.

Researchers named the aggressive strain “L type,” and the less aggressive version “S type.” The L type was seen more often in Wuhan, China, the epicenter of the outbreak, but the frequency of this type of virus has since decreased from early January.

Scientists said the results show the development of new variations of the spike in COVID-19 cases was likely due to “mutations and natural selection besides recombination.”

“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),” the study said.