Patients in the northeast also appear to be taking longer than the one to two weeks observed in Wuhan to develop symptoms after infection, and this delayed onset is making it harder for authorities to catch cases before they spread, said Qiu, who is now in the northern region treating patients.

“The longer period during which infected patients show no symptoms has created clusters of family infections,” said Qiu, who was earlier sent to Wuhan to help in the original outbreak. Some 46 cases have been reported over the past two weeks spread across three cities—Shulan, Jilin city and Shengyang—in two provinces, a resurgence of infection that sparked renewed lockdown measures over a region of 100 million people.

Scientists still do not fully understand if the virus is changing in significant ways and the differences Chinese doctors are seeing could be due to the fact that they’re able to observe patients more thoroughly and from an earlier stage than in Wuhan. When the outbreak first exploded in the central Chinese city, the local health-care system was so overwhelmed that only the most serious cases were being treated. The northeast cluster is also far smaller than Hubei’s outbreak, which ultimately sickened over 68,000 people.

Still, the findings suggest that the remaining uncertainty over how the virus manifests will hinder governments’ efforts to curb its spread and re-open their battered economies. China has one of the most comprehensive virus detection and testing regimes globally and yet is still struggling to contain its new cluster.

Researchers worldwide are trying to ascertain if the virus is mutating in a significant way to become more contagious as it races through the human population, but early research suggesting this possibility has been criticized for being overblown.

“In theory, some changes in the genetic structure can lead to changes in the virus structure or how the virus behaves,” said Keiji Fukuda, director and clinical professor at the University of Hong Kong’s School of Public Health. “However, many mutations lead to no discernible changes at all.”

It’s likely that the observations in China don’t have a simple correlation with a mutation and “very clear evidence” is needed before concluding that the virus is mutating, he said.

Northeast differences

Qiu said that doctors have also noticed patients in the northeast cluster seem to have damage mostly in their lungs, whereas patients in Wuhan suffered multi-organ damage across the heart, kidney and gut.

Officials now believe that the new cluster stemmed from contact with infected arrivals from Russia, which has one of the worst outbreaks in Europe. Genetic sequencing has showed a match between the northeast cases and Russian-linked ones, said Qiu.

Among the northeast cluster, only 10% have turned critical and 26 are hospitalized.

China is moving aggressively to stem the spread of the new cluster ahead of its annual political gathering in Beijing scheduled to start this week. As thousands of delegates stream into the capital to endorse the government’s agenda, China’s central leadership is determined to project stability and control.

The northeast provinces have ordered a return of lockdown measures, halting train services, closing schools and sealing off residential compounds, dismaying residents who had thought the worst was over.

“People should not assume the peak has passed or let down their guard,” Wu Anhua, a senior infectious disease doctor, said on state television on Tuesday. “It’s totally possible that the epidemic will last for a long time.”