Beijing and Shanghai resumed mass testing for COVID-19 while a planned reopening of schools in the capital was delayed as virus outbreaks emerge just days after the two cities eased social curbs that had been in place for months.

Beijing restarted screening for the virus in several neighborhoods of the Chaoyang district, where many company headquarters and embassies are based, after a flareup in a bar ended a five-day streak of zero community spread. Shanghai briefly locked down most of the city Saturday to undertake mass testing as authorities continue to report COVID cases outside of quarantine areas.

The quick escalation adds to concerns that China’s COVID Zero strategy may send cities into repeated lockdowns and re-openings given the high transmissibility of the omicron variant, threatening a sustainable economic recovery. The new testing comes after Shanghai lifted its two-month lockdown on June 1 and Beijing rolled back some of its curbs from June 6.

Beijing meanwhile delayed a planned reopening for most schools after it reported 46 new COVID cases for Saturday. Students in primary and middle schools, except for graduating classes in junior high, won’t return to class as scheduled on June 13. A new reopening date hasn’t been set, municipal government spokesperson Xu Hejian told reporters Saturday.

The latest bar cluster has led to 115 infections since Thursday, with more cases expected, officials said. Beijing has reported 1,946 COVID cases in the latest outbreak since April 22, including 472 in central Chaoyang district.

In Shanghai, four new community cases, including one asymptomatic, were reported for Saturday as of 5 p.m., according to a local press briefing. That added to the 16 cases for Friday, six of which were found outside quarantined areas. The financial hub said it would lock down 15 of its 16 districts over the weekend for mass testing. The news triggered a renewed run on groceries for fear that the temporary lockdown may be extended.

Beijing’s daily COVID case count jumped to 61 for Friday from just eight the previous day, data from the Beijing Municipal Health Commission showed. The infections involved 36 that showed symptoms and 25 that were asymptomatic. At least 13 districts in Beijing have so far reported positive cases linked to the cluster at a popular bar in the Sanlitun area of Chaoyang.

The city announced a halt to operations at entertainment venues in Chaoyang from Thursday and sent more than 4,400 people who were in close contact with the infected cases into government-mandated quarantine facilities.

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Three of the newly-found cases reside in the staff apartment complex at the Universal Beijing Resort, according to the statement from the health commission. The theme park announced late Friday that it would delay its scheduled reopening to cooperate with the government’s COVID-prevention program.

The return of restrictions and mass testing is once again clouding the outlook of the world’s second-largest economy. The median estimate in a Bloomberg survey of economists shows the China’s gross domestic product is expected to grow 4.5% in 2022 from a year earlier, well below the government’s target of “around 5.5%.”

Economic activity showed some signs of improvement in May on easing virus curbs, but the fragile recovery could quickly lose steam if the measures are expanded and extended.

Meanwhile, Hong Kong recorded a total of 851 new daily COVID cases, the highest number in almost two months, with authorities reporting two more clusters of infections at bars in the city’s Central district.