China has ordered an unprecedented quarantine of more than 50 million people. It has closed schools and shut down live animal markets. Airports across the globe are screening passengers coming from the world’s most populous country.

But three weeks after the new coronavirus emerged as a health crisis, experts can’t yet say whether these efforts will succeed at containing an infection that now threatens at least 15 countries.

Some early signs are discouraging: Six countries, including China, have confirmed human-to-human transmission of the infection. Those include four cases in Germany connected to a single person — a worrisome sign for containment of the disease. Cases in China continue to multiply, and 5 million residents of Wuhan, where the virus originated, have left the city, some of them surely carrying the disease.

But so far, the mortality rate is less than the rate of other severe respiratory coronaviruses. In China, where 5,974 people are infected, 132 have died through Tuesday. That is a high rate, but far less than the fatality rate of SARS and MERS. And countries like the United States that quickly began screening travelers, isolating sick people and tracing their contacts have just a handful of cases. There have been no fatalities outside China.

Public health officials said Tuesday that they are grappling with a long list of unknowns that will determine how successful they are in limiting the toll of the widening outbreak. Those questions include how lethal the virus may be, how contagious it is, whether it is transmitted by people who are infected but show no symptoms, and whether it can be largely contained in its country of origin.