The existence of a substantial but unknown number of asymptomatic carriers of coronavirus in China has raised concerns among the public that people could still be spreading Covid-19 without knowing they are sick.

As the virus continues to wreak havoc across the world, China is close to declaring victory and is already easing travel restrictions. The border of Hubei province, epicenter of the virus, opened on Wednesday after a two-month shutdown.

But there are concerns that the end of the lockdown will release thousands of infectious people back into circulation.

Asymptomatic cases present a huge challenge in the control of infectious disease, making it harder to detect and stop transmission.

In China, the number of known asymptomatic cases is classified, and it is not included in the official data, though the South China Morning Post, citing unpublished official documents, recently said it was more than 40,000.

China had reported 81,218 coronavirus cases, and 3,281 deaths by the end of Tuesday.

Asymptomatic cases are currently found through “contact tracing”. China identifies people exposed to someone with a confirmed diagnosis, and if they test positive, they are quarantined whether or not they manifest symptoms.

“Asymptomatic patients have all been discovered during our contact tracing,” said Wu Zunyou of the China Center for Disease Control and Prevention at a briefing on Tuesday. “So will they be able to create transmission? They won’t.”

Still, the failure to include them in the official data has raised concerns about Beijing’s commitment to transparency, and some experts say it could also create a misleading picture about how the epidemic spreads and whether or not it is under control.

Despite recording zero new infections from March 18 to March 22, the Covid-19 hotspot city of Wuhan disclosed on March 20 that one newly diagnosed case was not included in the official data because the patient, a 62-year old man surnamed Zhang had shown no symptoms.

Citing hospital sources, the news magazine Caixin also reported that a new case in Wuhan on Tuesday was a doctor who had been infected by an asymptomatic patient.

China says asymptomatic patients will be added to the list of confirmed patients if they show symptoms at a later stage.

But it remains unclear how many asymptomatic cases remain undiagnosed and therefore unquarantined.

Some experts warn that undetected, asymptomatic patients could create fresh transmission routes once lockdowns are eased.

“It is especially concerning given that many countries have yet to implement sufficient levels of widespread community testing,” said Adam Kamradt-Scott, a public health specialist at the University of Sydney.

It is also unclear how much they might infect others.

“We know that that is possible, but we do not believe that that’s a major driver of transmission,” said Maria Van Kerkhove of the World Health Organization at a briefing early in March.

New studies show that asymptomatic carriers could pose risks. One analysis of the Diamond Princess cruise ship outbreak showed that 33 of the 104 infected passengers remained asymptomatic even after an average of 10 days of observation at the Self-Defense Forces Central Hospital in Japan.

While many appeared healthy throughout, a few other initially asymptomatic passengers quickly became seriously ill.

Another study published on March 23, looking at cases in the southwestern Chinese city of Chongqing, said 18% of patients were asymptomatic. Another even found that people are more likely to transmit the disease when symptoms are at their mildest.

The Yale School of Public Health said the existence of presymptomatic (asymptomatic) patients meant that screening procedures at airports and other points of entry were insufficient to prevent the coronavirus passing from China to other countries.

“The real picture will only come to light when we have a serological test to find out who has been infected,” said Ian Henderson, Director of the Institute for Molecular Bioscience at Queensland University.