Two major Chinese social media platforms have been censoring coronavirus content since the beginning of the outbreak, according to a report from Citizen Lab published Tuesday.

The report said that terms related to the coronavirus were censored on WeChat, China's most popular messaging app, starting on January 1. They were also censored on livestreaming site YY starting on December 31, the same day authorities in Wuhan, the virus' city of origin, made its first public announcement about the disease and China's government notified the World Health Organization.

Citizen Lab said it found 500 different keyword combinations that had been censored between January 1 and February 15.

"Our results show that at least one Chinese social media platform began blocking COVID-19 content three weeks before this official announcement, which strongly suggests that social media companies came under government pressure to censor information at early stages of the outbreak," Citizen Lab said.

YY censored terms like "SARS outbreak in Wuhan," "Unknown Wuhan Pneumonia," and "Wuhan Seafood Market." WeChat censored terms like "Xi Jinping Epidemic spread" and "Wuhan Pneumonia Virus Li Keqiang." Li Keqiang was the official put in charge of the outbreak.

Searches for "Li Wenliang" were also censored. Li was an ophthalmologist who first sounded the alarm about the virus on December 30 in his medical school alumni group. He was later required to sign a letter saying he had lied about the virus.

China announced that the virus could transfer between people on January 20.