Chinese doctors are seeing the coronavirus manifest differently among patients in its new cluster of cases in the northeast region compared to the original outbreak in Wuhan, suggesting that the pathogen may be changing in unknown ways and complicating efforts to stamp it out.

Patients found in the northern provinces of Jilin and Heilongjiang appear to carry the virus for a longer period of time and take longer to test negative, Qiu Haibo, one of China’s top critical care doctors, told state television on Tuesday.

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.