New evidence from China is affirming what epidemiologists have long suspected: The coronavirus likely began spreading unnoticed through the Wuhan area in November 2019, before exploding in multiple different locations around the city in December.
Chinese authorities have identified 174 confirmed cases of Covid-19 in the city since December 2019, World Health Organization researchers said, enough to suggest that there were many more mild, asymptomatic or undetected cases than previously known. thought previously.
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 researchers 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 possible previous cases, the team members said.
When examining 13 genetic sequences of the December virus, 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 researchers. The two sets have likely started to diverge between mid-November and early December, but could possibly indicate infections as early as September, said Marion Koopmans, a Dutch virologist on the WHO team.
This, and other evidence, suggests that the coronavirus could have jumped to humans sometime during or shortly before the second half of November, he said, sickening too few people to attract attention until it sparked an explosive outbreak in Wuhan. . By December, the virus was spreading much more, both among people who had a link to the market and among others without a link.
“There must have been many, many more cases in December that people were not aware of or did not recognize,” said Dominic Dwyer, an Australian microbiologist on the WHO team.
Thea Fischer, a Danish epidemiologist on the WHO team, said last week that the virus “has definitely circulated in the population” before the first severe cases were diagnosed, 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, unnoticed level in November, laying the groundwork for a much larger outbreak of cases in December. Other places, like New York or northern Italy, saw a similar pattern of hidden cumulus clouds burning for several weeks before bursting into flare-ups.
WHO is seeking access to blood samples from the months prior to December 2019 for antibody testing. They also want access to raw data on thousands of people who became ill in that period to try to establish how widely the virus was spreading before the first officially confirmed case, which according to Chinese authorities was on December 8.
The virus likely first spread to humans sometime between late September and late November 2019, researchers at the University of Cambridge who examined the pathogen’s mutation rate said last year.
Similarly, a study from the University of Arizona and the University of California, San Diego suggested that infections in Hubei province, of which Wuhan is the capital, may have occurred as early as mid-October 2019.
Joel Wertheim, an evolutionary biologist at UCSD and author of the study, described the WHO generalized circulation finding in December 2019 as “very much in line with our model.”
China’s Foreign Ministry and National Health Commission did not respond to requests for comment.
The WHO team found no evidence of large clusters of cases in or around Wuhan before December 2019.
“I have no doubt that there would have been some sporadic cases here and there before December,” said Peter Daszak, a member of the WHO team. “But I don’t think this is something that was widely circulated in China before December.”
The Wall Street Journal reported last week 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 performed more than a year after any possible infection disappeared.
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The team is seeking access to data used to identify potential patients for more than 70,000 cases of influenza-like illness, fever or pneumonia identified in that period. In an interview with Science magazine, the team leader, food safety scientist Peter Ben Embarek, suggested using less stringent criteria to identify around 1,000 potential cases of Covid-19.
Last week, Liang Wannian, head of a Covid-19 expert panel for the National Health Commission of China, said there was no evidence that the virus spread through Wuhan before December 2019. But he agreed. that the genetic sequences showed that “there was already some diversity of virus present in the early phase of the pandemic in Wuhan, suggesting unsampled chains of transmission beyond the Huanan market.”
The WHO findings have sparked new controversy over the origins of the pandemic, with the United States and Britain expressing concern over what they say is a lack of transparency from China.
Viruses that come from animals, such as coronavirus, are transmitted to humans in “spread” events that may initially infect only one or two people. A new virus doesn’t always spread to other humans right away; it may take a few tries. However, some viruses begin to spread gradually and cases multiply depending on the virus’s transmission capacity and the length of its incubation period.
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The first scientifically documented case of Covid-19 was symptomatic on December 1, 2019, but Chinese authorities say the first confirmed case fell ill on December 8 and the first with market ties fell ill on December 12. who treated the patient on December 1 said that he was an elderly man who had other chronic diseases and could not speak, and that the exact date of onset of the symptoms was not clear, since the relatives had estimated it.
Researchers at the University of Arizona and UCSD determined that the virus was spreading in mid-October by working backward from the first case reports and taking into account viral spread and mutation patterns.
Not surprisingly, the researchers didn’t find large clumps before December 2019, Dr. Wertheim said, with fewer than 20 people infected by the end of November, modeled after their study. But eventually, the virus began to spread, with cases doubling every four to five days in December, the scientist said. Viruses cannot circulate for long in a population before they diversify genetically, and that likely started to happen between mid-November and mid-December, he said.
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