China is imposing another lockdown on more than 100 million citizens as Beijing tries to head off another outbreak while the country’s top immunologist issues a dire warning: a lack of herd immunity could be a mass killer in the communist nation.

Cities in the Chinese province of Jilin cut off trains, shut down schools and quarantined tens of thousands of people as officials worry a cluster of 34 infections to could balloon. The lockdowns come as Chinese officials boast about the country’s successful response to coronavirus, which originated in Wuhan, China, and killed more than 300,000 people worldwide.

Chinese health officials suspect the patients may have come into contact with infected people returning from Russia. “Imported cases from overseas and clustered infections domestically have created dual pressure on us in containing the virus,” Wang Bin, a National Health Commission official, said during a briefing on Sunday.

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China’s top immunologist Zhong Nanshan is warning that China’s lack of herd immunity puts citizens at serious risk.

“The majority of … Chinese at the moment are still susceptible of the Covid-19 infection, because (of) a lack of immunity,” Zhong said during a CNN interview that aired Sunday. “We are facing (a) big challenge, it’s not better than the foreign countries I think at the moment.”

The restrictions are harsh. The Shulan government, a city located inJilin, said Monday on one of China’s social media platforms  that it would close off residential compounds containing suspected COVID-19 cases, allowing only one person from each family to leave for two hours at a time to collect groceries or other essentials.

Citizens are noticing the changes.

“You can feel that control is stricter,” Shen Jia, a salesman at a life science company in the city, told Bloomberg. People “have been more careful and reduced outdoor activities.”

China has so far reported more than 80,000 coronavirus cases and a death toll of at least 4,633, according to data from the country’s National Health Commission (NHC). United States officials and world leaders believe China is not being transparent about the country’s number of COVID-19 deaths or the number of cases it dealt with during the December outbreak in Wuhan.

The intelligence community reportedly sent a report to the Trump administration in April explaining how Beijing consistently underreports the number of coronavirus cases. Coronavirus task force member Dr. Deborah Birx, for her part, criticized China for not providing the rest of the world with timely data.