TOKYO/HONG KONG -- The deadly new coronavirus from Wuhan was spreading from person to person as early as mid-December, weeks before China officially confirmed such transmission, government-funded Chinese researchers report in a top American medical journal.

The paper in the New England Journal of Medicine analyzes data on the first 425 confirmed cases in the central Chinese city of Wuhan, where the outbreak originated.

"On the basis of this information, there is evidence that human-to-human transmission has occurred among close contacts since the middle of December 2019," it reads. The paper notes that seven health care workers contracted the virus between Jan. 1 and Jan. 11 -- significant evidence of human-to-human transmission.

The findings sharply contrast with the accounts of Wuhan health authorities who had maintained until mid-January that there was "no clear evidence" that the virus could be passed among humans. Officials also claimed Jan. 11 that no health care workers had been infected.

Public acknowledgment only came Jan. 20 via an interview with an expert commissioned by the National Health Commission on state broadcaster CCTV.

The paper also found that only 55% of the 47 infected patients before Jan. 1 were linked to the seafood and live-animal market thought to be ground zero. This means that other patients either got infected elsewhere or from other people.

The new findings have sparked a massive backlash on Chinese social media, with users questioning why the information did not come out earlier.

Many of the paper's authors work for the Chinese Center for Disease Control and Prevention and local counterparts, and many were among the first batch of medical experts to have entered Wuhan for on-the-ground investigations. They developed "a tailored surveillance protocol to identify potential cases" on Jan. 3.

"They all knew," a user on the Weibo microblogging platform said. "They just didn't say, but lied to us."

"If only they could have told people earlier, we could have taken better preventive measures, and the virus would not have spread this fast," another wrote.

In a country where academic performance is often linked to promotions, many social media users have also raised doubts as to whether the authors prioritized their own careers over the urgency of controlling the epidemic.

"I'm demanding an explanation from the paper's authors," wrote one Weibo user. "You clearly knew about people-to-people transmission three weeks earlier than the public, but did you do what you were supposed to do?"

The Ministry of Science and Technology issued a statement on Thursday urging researchers in the country to "not devote their efforts to writing essays before completing the task of combating the novel coronavirus."

Some social media users in China pointed to another possibility.

"Perhaps the researchers did not have a way to share their findings, and publishing it in a scientific journal was their last resort to warn the public," one WeChat user wrote on a group discussion.

In December, eight people were detained for "spreading false rumors" as they discussed the spread of a then-unknown coronavirus on the internet. The eight people were later identified as doctors working in Wuhan, which sparked a public outcry over government censorship.

As the postings about the issue increased, the Chinese Center for Disease Control issued a statement on Friday, claiming the details of the 425 cases analyzed in the study were already made public before the paper was published.

It said it learned about the infection of health care workers only after the National Health Commission expert revealed it on Jan. 20, and researchers conducted a "retrospective analysis" to draw the conclusion that human-to-human transmission was happening in December.

The public anger emerged as Beijing was trying to restore public trust. A health care official in Huanggang, a city near Wuhan, was fired on Thursday shortly after she failed to answer questions pertaining to the number of confirmed cases. In recent weeks, at least two government officials from Tianjin and Hebei Province have been removed from their posts due to dereliction of duty.

Health authorities in China on Friday said the death toll relating to the coronavirus has risen to 213.