In the opening scene of 76 Days, the extraordinary inside story of how Wuhan’s hospitals coped with the initial wave of Covid, a sobbing daughter in full PPE begs to see her dying father. The staff refuse and restrain her. Soon after, she pursues her father’s body bag on to the street, only to watch as it is driven away. The woman crumples in the road, distraught and bereft.

Such harrowing partings have since been replicated hundreds of thousands of times the world over. Yet 76 Days derives incredible strength from being a chronicle of the first outbreak. It is a journey into the unknown. The intensive care unit is full of people infected with an unidentified plague. Overwhelmed nurses bolt the doors to a ward as scores of older patients shuffle in the cold on the landing, begging to be admitted.

The sense of chaos is all the more powerful because we are also offered a glimpse inside the secretive Chinese state. As China approaches the anniversary of the start of its first lockdown, on 23 January, the precise source of the outbreak and the degree of subsequent cover-up remain contested. The country is determined to celebrate its resilience and relative success at returning to something approaching normal life.

But 76 Days is not an overtly political film. While the co-director Hao Wu has trenchant views on why Chinese officials lied about the spread of the pandemic, this is not a film that takes its audience backstage on decision-making or fallout management.

Such a film could not be made, says Wu. Instead, 76 Days is a memory of a trauma. It is stripped of music, commentary, news clips, talking heads and almost all footage outside the hospital. “I want to take the viewers to the eye of the storm and let them experience it, rather than manipulate their feelings,” he says. “The reality is horrific enough.”

The footage was taken by two local cameramen: one, a photojournalist who remains unidentified for his own safety; the other, Weixi Chen, a video reporter for Esquire China. Wu, who is based in New York, has made two revelatory films about China, focusing on its soulless consumerism. He chose his colleagues for their craftsmanship, their ability to capture emotion by letting the camera linger.

The virus had been spreading in Wuhan since November and was openly discussed on social media. But it wasn’t until 20 January that one of the country’s most prominent infectious disease experts, Zhong Nanshan, acknowledged person-to-person transmission on TV. Three days later, the 11 million residents of Wuhan went into complete lockdown for 76 days.

Wu says the start of the lockdown was a rare moment when a lot of hard-hitting Chinese investigative journalism was allowed, leading to key political and health leaders in the chaotic provincial and Wuhan governments being removed by Beijing. Wu’s fellow film-makers were among those who capitalised on the moment of openness. Two of the worst-affected Wuhan hospitals kept their doors closed to the media in February, but Wu’s colleagues gained unrestricted access to four ICUs, filming for more than 200 hours. He thinks the hospital directors gave access in part to persuade the Chinese government to ship in more equipment.

By mid-March, when Donald Trump’s attacks on the “China virus” began and the pandemic started to spread through Europe, the government’s censors took back control of the narrative. Filming was banned. Wu’s China-based collaborators got cold feet; only when he sent them an initial cut, which focused on the human struggle, did they give him permission to continue.

Wu remains mystified by China’s handling of the crisis. “When I started researching this film, one of the key questions I could not answer was: ‘Why was it covered up?’ Because China really suffered from Sars and lied about it for six months. Why would they lie again? They should have known about the effect of lying.

“You cannot wish these outbreaks away; not even Trump could do that. They had this elaborate test and tracking system. I spoke to an ex-senior CCP [Chinese Communist party] official and she could not understand it. The first conclusion was that Sars had been 10 years previously and people had stopped being vigilant.

“The other issue was the level at which the decision was made not to broadcast what was happening. At the time, the finger was pointed at the provincial and municipal governments. Wuhan is trying to promote itself as one of the top-tier cities, on the level of Beijing and Shanghai; managing the city’s image was of the first importance. It was also the Chinese new year and if people were sent into a panic it might disrupt the economy. The People’s Congress was meeting in January at municipal level, so there was a need for stability. But we don’t know if the buck stops there, or if central government knew what was going on in December. The way Chinese politics operates, it is almost impossible to know, so I did not not want to point fingers.”

Wu hopes his film may be especially helpful in countries where Covid deniers still abound and where previous pandemics are less familiar.

He tried unsuccessfully to shoot in New York hospitals, he says. “Because of privacy laws and liability issues, there has been very little raw, harrowing footage of what has been happening. There have been interviews with the families of the dead, but we need to see more of it, so people realise it is real.”

Although the film was first shown in the US in September, word about it is only now reaching China. Wu has avoided any interviews with the Chinese press. “Although our film is nonpolitical – our goal is to provide a historical record so no one ever forgets the trauma and the pain – we were not sure whether it was too negative at the beginning, showing too much chaos, or the ending was not triumphant enough. There was a risk that the increasingly nationalistic and patriotic internet users would be spun. That is why my co-director wanted to remain anonymous.”

But this week on Weibo, China’s equivalent of Twitter, an influencer with 19 million followers who focuses on film reported that 76 Days was being spoken about as a possible Oscar winner and posted the English-language trailer. Within 24 hours, it had 160,000 likes. “Then, every newspaper started referring to it very positively,” says Wu, “and now my Chinese friends have been in touch saying: ‘We did not know you were even making this film.’ Only five people knew about it, because we did not want to get our co-directors in trouble.

“I suppose, in normal circumstances, I should be really happy, but it is ironic China is now trying to claim this film. I do not know if the board of censors have seen the whole thing.

“Most Chinese people I know, including me, my family and friends, were so angry and confused when Wuhan was put under lockdown. It was when western countries started to fall, one by one, and America really screwed up its response, that people started thinking about China’s early response. Now thousands are dying every day and it is the new normal.”

76 Days will be available in the UK on Dogwoof On Demand and other digital platforms from 22 January

• This article was amended on 15 January 2021. An earlier version incorrectly stated that the mother of a woman in the opening scene of 76 Days was dying from Covid-19. In fact it was her father.