As the 2019 novel coronavirus outbreak continues to spread in China, researchers have found that people carrying the virus but not showing symptoms may be able to infect others. 

If infected people can spread 2019-nCoV while asymptomatic, it could be harder to trace contacts and contain the epidemic, which is already a global health emergency (SN: 1/30/20).

An unnamed Shanghai woman passed the virus to business colleagues in Germany before she showed signs of the illness, doctors report January 30 in the New England Journal of Medicine. The woman had attended a business meeting at the headquarters of the auto supplier Webasto in Stockdorf on January 20 and flew back to China on January 22. She became ill with mild symptoms on the flight back to China and tested positive for the virus.

Meanwhile, one of her German colleagues fell ill on January 24 with a fever, sore throat, chills and muscle aches. His illness was brief, and he returned to work on January 27, the same day that the woman informed the company she carried the virus. Nasal swabs and sputum, or phlegm, samples from the man contained high levels of the novel coronavirus even though his symptoms had passed.

Three other employees of the company also tested positive for the virus. Tracing their contacts, doctors conclude that the first man and another person caught the virus from their Chinese colleague.

What’s also concerning is that the first man apparently passed the virus to the other two coworkers, who both had contact with him before he developed symptoms. All cases of the illness have been mild.

These cases suggest that people shed the virus before they show symptoms and after recovery from the illness, say Camilla Rothe, a tropical medicine and infectious disease specialist at the University Hospital of Ludwig-Maximilians-Universität in Munich, and her colleagues.

Asymptomatic spread, though common for influenza viruses for example, would be a new trick for coronaviruses. The coronaviruses that cause severe acute respiratory syndrome, or SARS, and Middle East respiratory syndrome, or MERS, are not contagious before people show symptoms (SN: 1/28/20).

Another coworker of the firm was confirmed to have the virus on January 30, and a child of one of the infected workers has also contracted the virus, bringing the case count to six, health officials in the German state of Bavaria said January 31. The company has closed its headquarters near Munich until February 2 and began testing contacts of the ill employees on January 29.