By Betsy McKay, Drew Hinshaw & Jeremy Page

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.

In examining 13 genetic sequences of the virus from December, Chinese authorities found similar sequences among those linked to the market, but slight differences in those of people without any link to it, according to the WHO investigators. The two sets likely began to diverge between mid-November and early December, but could possibly indicate infections as far back as September, said Marion Koopmans, a Dutch virologist on the WHO team.

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, 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.

The WHO is seeking access to blood samples from the months before December 2019, to test for antibodies. They also want access to raw data on thousands of people who fell ill in that period to try to establish how widely the virus was spreading before the first officially confirmed case, which Chinese authorities say was on Dec 8.

The virus likely first spread to humans sometime between late September and late November 2019, Cambridge University researchers examining the pathogen’s rate of mutations said last year.

Similarly, a study by the University of Arizona and the University of California San Diego suggested infections in Hubei province, of which Wuhan is the capital, may have occurred as far back as mid-October 2019.

Joel Wertheim, an UCSD evolutionary biologist and an author of the study, described the WHO’s finding of widespread circulation in December 2019 as “very much in line with our model.”

China’s foreign ministry and national health commission didn’t respond to requests for comment.

The WHO team didn’t find evidence of large clusters of cases in Wuhan or its environs before December 2019.

“I have no doubt that there would have been a sporadic few cases here and there before December,” Peter Daszak, a WHO team member said. “But I don’t think this is something that was circulating widely in China prior to December.”

Last week it was reported that Chinese authorities had identified 92 hospital patients between October and December 2019, whose symptoms suggest they may have had Covid-19. None of them tested positive for antibodies, but the WHO team considers those results inconclusive because the tests were done more than a year after any possible infection cleared.

The team is seeking access to the data used to identify the potential patients from more than 70,000 cases of influenza-like illness, fever or pneumonia identified in that period. In an interview with Science magazine, the team’s lead, food-safety scientist Peter Ben Embarek, suggested using less stringent criteria to identify about 1,000 potential Covid-19 cases.

Last week, Liang Wannian, the head of a Covid-19 expert panel for China’s National Health Commission, said there was no evidence of the virus spreading around Wuhan before December 2019. But he agreed that genetic sequences showed “some diversity of viruses was already present in the early phase of the pandemic in Wuhan, suggesting unsampled chains of transmission beyond the Huanan market.”

The WHO’s findings have triggered fresh controversy over the pandemic’s origins, with the U.S. and Britain expressing concern over what they say is a lack of transparency from China.

Viruses that come from animals, like the coronavirus, jump to humans in “spillover” events that may initially infect just one or two people. A new virus doesn’t always spread to other humans right away; it can take a few tries. Gradually, though, some viruses do start to spread, and cases then multiply based on how transmissible a virus is and the length of its incubation period.

The first scientifically documented case of Covid-19 was symptomatic on Dec. 1, 2019, but Chinese authorities say the first confirmed case became ill on Dec. 8 and the first with links to the market got sick on Dec. 12. A doctor who treated the Dec. 1 patient said he was an elderly man who had other chronic illnesses and couldn’t speak, and his exact date of symptoms onset was unclear as it had been estimated by relatives.

The researchers at the University of Arizona and the UCSD determined that the virus was spreading as early as mid-October by working backward from the earliest reports of cases and factoring in patterns of viral spread and mutation.

It is unsurprising investigators haven’t found large clusters before December 2019, Dr. Wertheim said, with fewer than 20 people infected by the end of November, according to his study’s model. But eventually, the virus did start to spread, with cases doubling every four to five days in December, the scientist said. Viruses can’t circulate for long in a population before they diversify genetically, and that likely started to happen between mid-November and mid-December, he said.