When people’s chances of becoming infected vary, an outbreak is more likely to be eventually contained (by tracing contacts and isolating cases); it might reach a cumulative 550,000 cases in Wuhan, Allard and his colleagues concluded. If everyone has the same chance, as with flu (absent vaccination), the probability of containment is significantly lower and could reach 4.4 million there. Or as the researchers warn, “the outbreak almost certainly cannot be contained and we must prepare for a pandemic ….”

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In the current outbreak, researchers are building models not only to peek into the future but also to reality-check the present. Working backwards from confirmed infections in countries other than mainland China, researchers at Imperial College London who advise the World Health Organization estimated that Wuhan had 1,000 to 9,700 symptomatic cases as of Jan. 18. Three days later, all of mainland China had officially reported 440 cases, supporting the concerns of global health officials that China was undercounting.

In a more recent model run, Jonathan Read of England’s University of Lancaster and his colleagues estimated “that only about 1 in 20 infections were being detected” in late January, Read said: There were probably 11,090 to 33,490 infections in Wuhan as of Jan. 22, when China reported 547 cases. “It highlights how difficult it is to track down and identify this virus,” Read said, especially with residents of quarantined Wuhan being turned away from overwhelmed hospitals and clinics without being tested for the virus. Using a similar approach, modelers led by Dr. Wai-Kit Ming of Jinan University in Guangzhou estimated that through Jan. 31, China probably had 88,000 cases, not the 11,200 reported.