In Shanghai, a surge in cases raises fears of another lockdown.

Barely six weeks after Shanghai fully lifted a prolonged and harshly enforced lockdown, China’s biggest city is again grappling with a surge of coronavirus cases. Residents wary of being suddenly confined in their homes have been alarmed by mixed messages from official sources circulating on social media, including advice that they stock up on food and medicine.

Since early this month, Shanghai has recorded over 400 infections across the city, many of which have been traced to a cluster at a karaoke bar. The authorities in the city have imposed lockdowns on residential buildings where cases and close contacts have been identified, while a dozen of the city’s 16 districts have ordered residents to do two P.C.R. tests within three days this week.

Residents expressed concern that the surge could trigger another lockdown of the city like the two-month confinement in May and June that shuttered businesses and schools and brought life to a standstill. Public anxiety grew amid reports, including in the Health Times, a Communist Party newspaper, that two residential committees in Shanghai had sent “friendly reminders” to residents to stock up food and medicine for 14 days, just in case apartment buildings were suddenly confined.

The city government moved to assure the public that there was no plan to impose a citywide lockdown. But some residents remained skeptical.

“The government has lost the trust of the public,” said Norah Liu, a tech industry worker in Shanghai. “Whatever they will do, I have enough staple food for one month of survival at home anyway.”

City officials said at a news conference on Monday that more than 300,000 people have been screened or placed in centralized isolation facilities. China’s stringent approach to eliminating coronavirus infections relies heavily on quarantines and isolation: anyone who tests positive or is deemed a close contact is likely to be confined in a facility or at home.

To cope with the rising number of people deemed to be at risk of spreading the virus, Shanghai has reopened at least one isolation facility for Covid patients that had been used during the spring outbreak, according to Caixin, an influential newsmagazine.

The city’s measures are already drawing criticism for being excessive. Some residents complained about being categorized by health workers as “secondary close contacts” simply because they had been in the same mobile messaging chat group as people who tested positive.

The local health agency admitted that in trying to conduct contact tracing around the karaoke bar cluster, workers cast a wide net to try to stop the spread of infections, and apologized for the inconvenience.

Elsewhere in China, the Omicron variant and its BA.5 subvariant, are slipping through the country’s many defenses, posing a challenge to the leadership’s insistence on eradicating infections. The city of Lanzhou in northwestern China imposed a one-week lockdown on its population of about 4 million starting Wednesday after recording 122 cases in the past week.

At the same time, local officials are under pressure to help revive the economy, which has been dragged down by Covid controls. In a sign that Beijing is concerned about managing the costs of Covid testing, China’s health authority said that local governments were no longer required to test certain imported goods. Officials had previously blamed contaminated packaging of imported products for spreading the virus, although studies show that transmission of the virus from packaging is extremely rare.