Scientists say they've identified 2 strains of COVID-19 One strain was common in Wuhan, China, then dropped off significantly.

Scientists from China said they've identified two strains of COVID-19 linked to the recent outbreak.

Coronaviruses are a large family of RNA viruses, and when RNA viruses replicate quickly, they often mutate.

Researchers analyzed 103 sequenced genomes using strains from China, and found that 70% of strains were one type, which they called "L." The "L" strain was more aggressive than the remaining 30% of strains, which were dubbed "S."

The first strain was more common at the beginning of the outbreak, in Wuhan, China, but its frequency decreased after early January. That drop-off could be a result of the strict measures China put in place to try and stem the spread of the virus.

The new paper, published Tuesday in the National Science Review from the Chinese Academy of Sciences, is preliminary, the paper's authors cautioned.

The scientists only analyzed strains from China, so more information is needed about strains from other countries to determine whether the same viruses have spread worldwide.

Dr. Stanley Perlman, a professor of microbiology and immunology at the University of Iowa who has researched SARS and MERS, said that the new paper didn't prove that one strain was more aggressive or faster spreading than the other.

"For now, it looks like there are two strains, but we do not know exactly what this means," said Perlman, who is not connected to the new paper.