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

Beijing restarted screening in several neighbourhoods 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 recent outbreak that has been linked to bars in the capital is “explosive” in nature, widespread in scope and complex, a Beijing government spokesman said in a briefing on Saturday.

Of a total of 1,946 local Covid cases reported since April 22, a total of 115 were connected to the bar cluster, Liu said.

The quick escalation adds to concerns that China’s zero-Covid strategy may send cities into repeated lockdowns and reopenings given the high transmissibility of the Omicron variant, threatening a sustainable economic recovery.

The new testing round comes after Shanghai lifted its two-month lockdown on June 1 and Beijing rolled back some of its curbs from June 6.

Beijing has 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 Monday. A new reopening date hasn’t been set, municipal government spokesperson Xu Hejian said.

Most primary and middle school students will “continue to study online at home” from Monday, Xu said.

In Shanghai, four new community cases, including one asymptomatic, were reported for Saturday. 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.

In Beijing, 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.

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 Covid-prevention programme.

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.