When the novel coronavirus was first discovered in China last winter, the country responded aggressively, placing tens of millions of people into strict lockdown. As Covid-19 spread from Wuhan to the rest of the world, the Chinese government was just as forceful in controlling how the health crisis was portrayed and discussed among its own people.

Politically sensitive material, like references to the 1989 Tiananmen Square protests, have long been forbidden on China’s highly censored internet, but researchers at the University of Toronto’s Citizen Lab say these efforts reached a new level during the pandemic. “The blunt range of censored content goes beyond what we expected, including general health information such as the fact [that] the virus spreads from human contact,” says Masashi Crete-Nishihata, the associate director of Citizen Lab, a research group that focuses on technology and human rights.

Citizen Lab’s latest report, published earlier this week, finds that between January and May this year, more than 2,000 keywords related to the pandemic were suppressed on the Chinese messaging platform WeChat, which has more than one billion users in the country. Many of the censored terms referenced events and organizations in the United States.

Unlike in the US, internet platforms in China are responsible for carrying out the government’s censorship orders and can be held liable for what their users post. Tencent, which owns WeChat, did not comment in time for publication. WeChat blocks content via a remote server, meaning it’s not possible for research groups like Citizen Lab to study censorship on the app by looking at its code. “We can send messages through the server and see if they are received or not, but we can’t see inside of it, so the exact censorship rules are a bit of a mystery,” Crete-Nishihata says.

For its latest report, Citizen Lab sent text copied from Chinese-language news articles to a group chat it created on WeChat with three dummy accounts, one registered to a mainland Chinese phone number and two registered to Canadian phone numbers. They used articles from a range of outlets, including some based in Hong Kong and Taiwan as well as Chinese state-controlled publications. If a message was blocked, the researchers performed further tests to identify which words triggered the censorship. Some of the blocked messages had originally been published by Chinese state media. In other words, while a person or topic may be freely discussed in the government-controlled press, it’s still banned on WeChat.

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The Citizen Lab report demonstrates the extent to which the Chinese government tried to control the narrative from the beginning. As residents in Wuhan remained in lockdown, WeChat blocked phrases about Li Wenliang, a local doctor who warned colleagues about a new infectious disease before it was disclosed by the government, and who became a popular hero for free speech after he died of Covid-19 in February. WeChat also blocked its users from discussing an announcement by Chinese officials that they had informed the US government about the pandemic for the first time on January 3, almost three weeks before they said anything to their own citizens. And it censored mentions of the US Centers for Disease Control and Prevention, when the organization was coupled with the word “coronavirus.”

By March, Covid-19 had become a global pandemic, and WeChat began blocking some mentions of international groups like the World Health Organization and the Red Cross. It also censored references to outbreaks in other countries like Saudi Arabia, Turkey, Russia, and the United Kingdom. Citizen Lab found that the majority of blocked words related to international relations were about the United States, the subject of the third portion of the report.