Chinese health officials are warning that the deadly coronavirus could be much more contagious than initially thought, as infected patients can spread the flu-like illness before showing any symptoms.

China’s National Health Commissioner Minister Ma Xiaowei announced during a press conference Sunday the virus is infectious during its incubation period of one to 14 days, and its ability to spread is increasing.

The official said authorities have limited knowledge on the new virus and are unclear on the risks posed by the mutations of the virus.

A longtime adviser to the U.S. Centers for Disease Control and Prevention, Dr. William Schaffner, told CNN the new development means “the infection is much more contagious than we originally thought.”

Schaffner called it a game changer and warned current preventative methods won’t be enough to fight off the outbreak since tracking down the contacts a patient had before experience symptoms complicates the situation.

At least 81 people have died in China as a result of the coronavirus, and more than 2,700 others have been infected in more than a dozen countries, including five confirmed cases in the United States. At least 60 people were being tested in the U.S. for the disease as well. Officials expect more deaths and infections to follow.

A public health expert at London’s Imperial College estimates there could be 100,000 people already infected with the coronavirus in China’s Hubei province.

Professor Neil Ferguson told CBS News “a lot more information will become available in the next few days and weeks, and case numbers will continue to increase rapidly.”

Ferguson said the fast-multiplying figures “do not necessarily represent a huge growth rate of the epidemic; it is much more likely down to the health authorities catching up” with efforts to accurately diagnose a brand new illness.

A combined population of more than 50 million people are on lockdown in China in at least 16 cities, as Chinese authorities scramble to curb the spread of the virus. Authorities believe the virus spread into the human population from an infected animal at a market in the Chinese city of Wuhan.