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Two whistle-blowers who defied Chinese government censorship to reveal the blind spots of the coronavirus health crisis remain missing.

Citizen journalists Chen Quishi and Fang Bin disappeared in February after uploading videos showing the devastation the killer bug was having in Wuhan.

While doctor Ai Fen was unreachable from March 29, she was apparently confirmed to be working again at Wuhan Hospital this weekend.

This comes as China imposed restrictions on the publication of academic research on the origins of Covid-19, according to a central government directive and notices published by two Chinese universities, which have since been removed.

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President of Reporters Without Borders, Pierre Haski, told Le Parisian there is "very little" information about what happened to the missing pair but said "the only certainty" is they are in "the hands of the Chinese authorities".

Human rights lawyer Mr Quishi, 34, had captured disturbing footage inside a Wuhan hospital in February showing how overwhelmed staff were with the influx of patients.

He had previously uploaded other videos and in one posted on January 29, revealed he was "scared" - apparently stuck in between the virus and the authorities.

On February 6 his Weibo account was deleted and the next day his parents were told he had been put "in quarantine" despite having had no symptoms of the virus.

There has been no sign of him since.

Similarly, Mr Bin, a clothing store manager, published a five-minute video on February 1 showing what he claimed to be eight corpses of patients found in front of or inside a Wuhan hospital.

The video reached over one million views.

His laptop was subsequently confiscated by police after a lengthy interrogation.

Mr Fen's video on February 4 showed men in protective suits trying to enter his home, while no other videos have been uploaded since February 8.

“His family remains, to this day, without news," said Mr Haski.

Questions also remain around emergency director of Wuhan Hospital, Dr Fen, another virus activist - though she has been confirmed to have since been back at work.

The doctor was apparently silenced two weeks after criticizing the regime's censorship of the coronavirus epidemic in a Chinese magazine last month.

If the two other whistle-blowers were arrested, the "mystery remains" about this last case, claims Mr Haski.

He said there is again "absolutely no information" as to what happened to her or where she was, but suggested she could have simply gone to ground.

Mr Haski said it is "much more difficult for the Chinese authorities to send a doctor to prison" than the other two whistle-blowers, for example.

On Monday, a French journalist reported having spoken to Dr Fen at the hospital in Wuhan.

"She is doing very well, she is at her post," said Arnauld Miguet on Twitter, showing a photo of the director.

Mr Haski said there are two possible outcomes for the two missing civilian journalists.

One is that the authorities keep them hidden until the crisis dies down in a few weeks, but he said this remains unlikely.

The other scenario he said is that in about six months time we suddenly hear they have been indicted for subversion.

What's more, Ren Zhiqiang, a former businessman close to vice president Wang Qishan, has been missing since March 12 after publicly criticising the regime's lack of transparency in its handling of the crisis.

(Image: AFP via Getty Images)

He had apparently compared president Xi Jinping to a tyrannical "clown".

On Tuesday it was announced he was being sought by the authorities as the target of an investigation.

Mr Haski said any potential trials will likely be "held behind closed doors" with sentences dictated by the Chinese Communist Party.

In relation to the new restrictions on academic publications, all such papers will be subject to extra vetting and must be approved by the government, according to now removed posts, reports CNN.

The virus has so far claimed more than 100,000 lives and infected around 1.7 million worldwide.

One Hong Kong medical expert claimed research he was involved in did not undergo the new levels of scrutiny back in February when it was published.

Chinese researchers have been publishing studies since late January with some findings raising questions over the government's accounts of the crisis.

The Chinese authorities now appear to be clamping down on what is released into the public domain.

One anonymous researcher told CNN that this development would hamper scientific research into the virus and accused the government of trying to "control [the] narrative" and "paint it as if the outbreak did not originate in China".

The new directives were issued during a State Council task force meeting on March 25 and apparently published on Fudan University's website on Friday, before being taken down, said CNN.

The China University of Geoscience in Wuhan posted a similar notice which was also deleted, the American news network claimed.