New evidence from China is affirming what epidemiologists have long suspected: The coronavirus likely began spreading unnoticed around the Wuhan area in November 2019, before it exploded in multiple different locations throughout the city in December.

Chinese authorities have identified 174 confirmed Covid-19 cases around the city from December 2019, said World Health Organization researchers, enough to suggest there were many more mild, asymptomatic or otherwise undetected cases than previously thought.

Many of the 174 cases had no known connection to the market that was initially considered the source of the outbreak, according to information gathered by WHO investigators during the four-week mission to China to examine the origins of the virus. Chinese authorities declined to give the WHO team raw data on these cases and potential earlier ones, team members said.

This, and other evidence, suggest the coronavirus might have jumped to humans sometime during or shortly before the second half of November, she said, sickening too few people to attract attention until it led to an explosive outbreak in Wuhan. By December, the virus was spreading much more widely, both among people who had a link to the market, as well as others with no tie.

“There must have been many, many more cases in December that people didn’t know or recognize,” Dominic Dwyer, an Australian microbiologist on the WHO team, said.

Thea Fischer, a Danish epidemiologist on the WHO team, said last week the virus “has definitely circulated in the population” before the first diagnosed serious cases, which she called “the tip of the iceberg.”

These assessments, offered by six WHO researchers interviewed by The Wall Street Journal, paint a common picture of how the virus took off in Wuhan: circulating at a lower level, unnoticed, in November, laying the groundwork for a much bigger flare-up of cases in December. Other places, such as New York or Northern Italy, saw a similar pattern of hidden clusters smoldering for several weeks before exploding into outbreaks.