Beijing is closing all city schools in a further tightening of COVID-19 restrictions, as China's capital seeks to prevent a wider outbreak.
The city of 21 million has already ordered three rounds of mass testing this week, with the third coming Friday.
On Thursday, the city's Education Bureau ordered all schools to end classes from Friday and said it hadn't determined when they would resume.
It also wasn't clear whether schools would be able to offer classes online or allow students facing crucial exams to return to class.
Beijing announced 50 new cases on Thursday, two of them asymptomatic, bringing its total in the latest wave of infections to around 150.
Students make up more than 30% of total cases, with clusters linked to six schools and two kindergartens in Chaoyang.
Also Thursday, residents of two housing compounds in Beijing's Chaoyang district were ordered to stay inside and some clinics and businesses shut down.
Beijing has moved more swiftly than many Chinese cities to impose restrictions while case numbers remain low and the scale of the outbreak is still manageable.
The goal is to avoid the sort of sweeping measures imposed on Shanghai, where the highly transmissible omicron variant has torn through the city of 25 million. Restrictions confining many Shanghai residents to their homes are now in their fourth week and all schools have been online since last month.
The strict measures have spurred anger and frustration over shortages of food and basic supplies, the inability of hospitals to deal with other health emergencies and poor conditions at centralized quarantine sites where anyone who tests positive — or even has contact with a positive case — is required to be sent.
The National Health Commission on Thursday reported 11,285 new cases across mainland China, most of them asymptomatic and the vast majority in Shanghai, where an additional 47 deaths were reported.
Shanghai city authorities said Wednesday they will analyze the results of new rounds of testing to determine which neighborhoods can safely expand freedom of movement for residents.
Shanghai is seeking to achieve "societal zero COVID" whereby new cases are found only in people who are already under surveillance, such as in centralized quarantine, or among those considered to be close contacts. That would indicate chains of transmission in the open community have been severed, reducing the risk of new clusters forming from previously undetected sources.
While China's overall vaccination rate stands at around 90%, just 62% of people over 60 have been vaccinated in Shanghai, the country's largest and wealthiest city. Health workers have been visiting elderly residents at home to administer vaccines in a bid to boost that figure, the city's Health Commission said Thursday.
The pandemic and stringent lockdown measures have taken a toll on the economy, especially in Shanghai, which is home to the world's busiest port and China's main stock market, along with a large international business community.
A full month's shutdown of the city will subtract 2% from China's annual economic growth, according to an analysis from ING bank earlier this month. Lockdowns could also affect spring planting, driving up food prices, while transport has also been badly hit.
Baiyun Airport, in the southern manufacturing hub of Guangzhou, saw 80% of flights canceled Thursday after "abnormal results" were found while testing airport staff, according to online state media source The Paper.
Travel, particularly between provinces and cities, is expected to fall during next week's May Day holiday. China's international borders have largely remained closed since the COVID-19 outbreak was first discovered in the central city of Wuhan.
Despite Beijing's promises to reduce the human and economic cost of its strict "zero-COVID" strategy, leaders from President Xi Jinping down have ruled out joining the United States and other governments that are dropping restrictions and trying to live with the virus.
All but 13 of China's 100 biggest cities by economic output were under some form of restrictions earlier this month, according to Gavekal Dragonomics, a research firm.