A medical worker produces traditional Chinese medicine to treat patients infected by the COVID-19 coronavirus at a hospital in Wuhan in China's central Hubei province on March 2, 2020.

Researchers in China have found that two different types of the new coronavirus could be causing infections worldwide.

In a preliminary study published Tuesday, scientists at Peking University's School of Life Sciences and the Institut Pasteur of Shanghai found that a more aggressive type of the new coronavirus had accounted for roughly 70% of analyzed strains, while 30% had been linked to a less aggressive type.

The more aggressive type of virus was found to be prevalent in the early stages of the outbreak in Wuhan — the Chinese city where COVID-19 was first detected late last year.

But the frequency of this type of virus has since decreased from early January.

The researchers said their results indicate the development of new variations of the spike in COVID-19 cases was "likely caused by 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)," they said.

Researchers cautioned that data examined in the study was still "very limited," emphasizing that follow-up studies of a larger set of data would be needed to gain a "better understanding" of the evolution and epidemiology of COVID-19.

The findings were published in the National Science Review, the journal of the Chinese Academy of Sciences.