Zhang Zhan, 40, a lawyer turned citizen journalist, was detained in May 2020 after she travelled to Wuhan to report on the early days of the Covid-19 pandemic. Her videos and social media posts drew attention to the government’s stifling of information about the spread of the disease and the harsh lockdowns that were being imposed.

In December 2020, Zhang was convicted of “picking quarrels and provoking trouble” and sentenced to four years in prison. Her conviction was widely criticised by human rights organisations.

Reporters Without Borders awarded Zhang a 2021 press freedom award for courage for “her reporting from the heart of the pandemic’s initial epicentre” which “was one of the main sources of independent information about the health situation in Wuhan at the time”.

She had been imprisoned in Shanghai women’s prison and was due to be released on Monday. But it is not clear if, or under what circumstances, she has been let out. There were reports in recent weeks that her family and friends were under intense pressure from the authorities not to contact foreign media or overseas supporters about her case.

Calls to Shanghai women’s prison on Tuesday and Wednesday did not connect.

Aleksandra Bielakowska, an advocacy officer for Reporters Without Borders in Taiwan, said Zhang’s family had had limited contact with activists in recent weeks and that the organisation had received “no information” about Zhang since Monday. “We don’t know if they have been threatened, if they are under surveillance or if they’ve been taken away,” she said.

There are concerns that Zhang may have been released into limited freedom, with restrictions on her movement and communications. One of her former lawyers told the Guardian that Zhang might have been “taken away for vacation”, a euphemism for when the police in China force a person deemed troublesome to go on a chaperoned trip to keep them under surveillance. “Everyone is paying attention and trying to find out what’s going on, but there’s no news,” the lawyer said.

Maya Wang, the associate director in the Asia division at Human Rights Watch, said: “At the moment we have not had confirmation about Zhang Zhan’s release, which is especially concerning given her very poor health in prison.”

Zhang engaged in periodic hunger strikes while in prison, causing her to lose several stone in weight, and is thought to have been subjected to force-feeding. At one point she was admitted to hospital due to severe malnutrition, according to Amnesty International.

Wang said: “We fear that, upon release, Zhang will remain under tight police surveillance and restrictions on her movement, as has been the case for too many activists. It is important that the world does not forget brave activists like Zhang, and demand that she be freed without restrictions.”

Wang Wenbin, China’s foreign ministry spokesperson, reportedly declined to comment about Zhang’s case on Monday.

Bielakowska said: “The fact that there is silence is a message itself.”