IN THE hunt for the origins of the virus that has touched off a global pandemic, the very first cases are extremely important. They could reveal vital information about where the virus came from. China has stated that the outbreak began in Wuhan in December 2019, but this may not be the whole story: There have been suggestions by Chinese and Western scientists that some cases arose earlier. Who were the people who fell ill, and where did they encounter the virus? China should help solve this mystery, but it so far has thrown a cloak over it.

Beijing steadfastly denies there was an inadvertent leak from the Wuhan Institute of Virology, or WIV, one of two theories being pursued about the virus origin. The other is a zoonotic spillover from an animal. President Biden announced Wednesday that the U.S. intelligence community was divided over the competing theories and said he had urged further investigation.

Identification of the earliest cases could help resolve the issue. On Jan. 15, outgoing Secretary of State Mike Pompeo said the U.S. government “has reason to believe that several researchers inside” the Wuhan Institute of Virology “became sick in autumn 2019, before the first identified case of the outbreak, with symptoms consistent with both COVID-19 and common seasonal illnesses.” But he provided no further evidence for this vague claim, and when a joint World Health Organization-China mission went to the WIV on Feb. 3, the director, Yuan Zhiming, categorically denied Mr. Pompeo’s assertion. It would help if the administration declassified the U.S. intelligence.

Here’s what’s known so far about early cases: On Dec. 31, 2019, China notified the WHO of 44 cases of pneumonia of unknown cause. By the time China agreed to a joint study with the WHO in summer 2020, the number of these cases had grown to 124. The resulting joint report in March 2021 said Chinese records had established 174 confirmed cases in December 2019, and the first onset among these was Dec. 8.

Were there others before that? In preparation for the WHO-China joint mission, Chinese officials examined 76,253 cases of fever or respiratory illness in 233 health-care institutions from Oct. 1 to Dec. 10, 2019. Out of this mass of records, they identified 92 people who might have been infected with SARS-CoV-2 in the autumn. However, on further scrutiny, all 92 cases were rejected as covid.

Chinese officials refused to let the WHO see raw data on these potentially significant 92 early cases. Moreover, the survey was too small and selective. Any investigation should reach back to much earlier, to at least the summer of 2019, and involve matched control patients and healthy controls from other populations in Hubei province and elsewhere in China. This wasn’t done, and then the 92 cases were tossed out altogether. WHO officials left with a feeling the job was unfinished.

There are other clues. The South China Morning Post, based in Hong Kong, published an article by Josephine Ma, the newspaper’s China news editor, in March 2020. She reported having seen government records, never made public, showing that the earliest onset of a case was Nov. 17, 2019, and that authorities “identified at least 266 people who were infected last year, all of whom came under medical surveillance at some point.” Nine confirmed cases were in November, she reported.

Ms. Ma also found that the records show an unusual leap in cases from 266 total on Dec. 31 to 381 on Jan. 1, 2020. Peter Ben Embarek, a WHO food-safety expert who led the delegation to Wuhan, told CNN afterward that the Chinese report of 174 cases in December was likely a vast undercount, that the disease was widely circulating in Wuhan by that month and could have hit 1,000 people, many of them asymptomatic.

Another hint came from parallel Chinese and Western studies of how the virus evolves. These reports used methods that rely on a molecular clock and simulations of viral evolution to estimate when the virus first got a foothold in humans. The Chinese researchers, led by Yezhen Zhao of Southern Medical University in Guangzhou, estimated in February 2020 that the first cases arose between Sept. 23 and Dec. 15, 2019. Using similar methods somewhat later, Jonathan Pekar of the University of California at San Diego and others found that it was probably mid-October to mid-November of 2019.

So far, no one has identified “patient zero” or the other early patients. Basic questions about where these individuals lived, worked, traveled, shopped and socialized might explain how they got infected — and from where. Were any associated with the multiple laboratories in Wuhan that worked with bat coronaviruses? If patients provided samples for testing on a visit to a hospital or clinic, the samples might be used to examine the genome of the virus at that early moment, which could provide hints about its origin.

The early cases may have been puzzling to Chinese doctors; the underlying cause might not have been known. But by late December, according to Ms. Ma’s article and other accounts, they had identified the source as a coronavirus. Those at the highest levels of the Chinese government knew by early January that the virus was spreading, yet irresponsibly continued to cover it up until late that month, while tens of thousands of travellers left Wuhan on trains and planes for destinations beyond, potentially spreading it further.

The mystery of the virus’s origins — including the early cases — needs a full investigation based on the science and the facts. Perhaps it will be proved eventually that the virus jumped from animals to humans and did not first pass through the Wuhan Institute of Virology or another laboratory. That would be extremely important to know, as would evidence of a laboratory leak. Much depends on China, but it has refused to even consider the kind of rigorous probe that is necessary. That only deepens the suspicions it has something to hide.

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