Dr. Tedros Adhanom Ghebreyesus, the organization’s director general, said in a news conference in Geneva that Covid-19 was deadlier than the seasonal flu but did not transmit as easily. “Globally, about 3.4 percent of reported Covid-19 cases have died,” Dr. Tedros said. “By comparison, seasonal flu generally kills far fewer than 1 percent of those infected.”

The estimate takes into account the growing number of infections being recorded outside China, mostly in Iran, Italy and South Korea.

Dr. Margaret Harris, a W.H.O. spokeswoman, said the figure was a “crudely calculated” snapshot of the disease’s death rate globally, and is expected to “change over time, and vary from place to place.”

It also does not include mild cases that do not require medical attention and is skewed by Wuhan, where the death rate is several times higher than elsewhere in China.