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 reported 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 developed a fever and chest pain about eight hours after arriving back in Shanghai.

“This is the first moment I recognized getting sick,” the woman told researchers during a telephone call from her hospital room in China on February 5, according to a detailed timeline published online February 6 in the New England Journal of Medicine. She tested positive for the virus on January 26.

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. More testing is needed to determine whether virus particles left after recovery from the illness are infectious, the researchers noted.

Sign Up For the Latest from Science News Headlines and summaries of the latest Science News articles, delivered to your inbox Client key* E-mail* Go

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, the researchers say. 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).

Quarantine procedures and travel restrictions were put in place shortly after the announcement of this cluster of cases, largely because of fears that asymptomatic infections could overwhelm efforts to screen infected people (SN: 1/31/20). Researchers have since raised questions about whether the virus really did spread asymptomatically in this case, although other instances of people passing on the new coronavirus while they were pre-symptomatic or asymptomatic have also been reported in China. “There seems to be some mounting evidence that may occur with this virus,” says epidemiologist Aubree Gordon of the University of Michigan School of Public Health in Ann Arbor. “We don’t know how common that is.”

In a follow-up to the original NEJM report on the German case, Bavarian health authorities and researchers at the Robert Kock Institute in Berlin interviewed the Chinese woman by phone and discovered that she may have had some mild non-specific symptoms, including a backache, while in Germany. She had also taken medication that lowers fevers, she told these researchers. The case is being investigated to determine whether she was, in fact, symptomatic, a spokesperson for the Bavarian Health & Food Safety Authority said in an e-mail to Science News on February 6. Those findings haven’t yet been published in a scientific journal.

But according to the detailed timeline published February 6 in NEJM by Rothe and her colleagues, the woman felt a little warm one evening but did not have a fever. She took a medicine containing acetaminophen on January 20 as a preventative measure to be ready for business meetings the next day. She also reported having mild pain in the muscles and bones of her chest and being tired in the afternoon, but that was her usual bedtime in China.

Rothe and colleagues say that while they stand by their description of the woman as asymptomatic, they published the timeline to give other doctors the opportunity to decide for themselves whether the woman had symptoms. “According to the personal opinion of the authors, the message of our initial letter still stands,” Rothe and coauthor Michael Hoelscher said in a statement to Science News.

Whether feeling a little warm, achy and tired were symptoms of the coronavirus infection or just normal consequences of jetlag may be difficult to determine.

Concerns over the original NEJM report led Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases in Bethesda, Md., to call a public health scientist in China, Fauci said February 6 during a recorded briefing with Howard Bauchner, editor in chief of the Journal of the American Medical Association.

Fauci’s contact in China confirmed that people “absolutely” may have asymptomatic infections and that those asymptomatically infected people could pass the virus to others. Such transmissions of 2019-nCoV from people who don’t have symptoms is not the primary way the virus spreads, and probably won’t affect the course of the epidemic much, Fauci said. More research is needed to understand exactly when during an illness a person can pass the virus to others, he said.

Gordon said she also has questions about the timeline for the possibly asymptomatic infection among the other coworkers in the German outbreak. For instance, the first patient supposedly only had contact with one of his coworkers on January 20 and 21 — the same time period in which that first man was exposed to the virus by his Chinese coworker. That’s probably too short of an incubation period, based on what’s known so far for the new coronavirus. She suspects there may have been forgotten contacts between the coworkers closer to the time symptoms developed, or the virus could have spread from using “the same bathroom, the same conference room or coffeepot.” It’s unclear how long this coronavirus lingers on surfaces, but most last only a few hours. (Droplets on surfaces are one way other coronaviruses may spread, she says.)

Incubation periods — the time between exposure to an infectious organism and the development of symptoms — for 2019-nCoV range between two and 14 days, according to the U.S. Centers for Disease Control and Prevention. Fauci said in the briefing that discussions with Chinese colleagues put it at more like five to six days. Usually people infected with coronaviruses aren’t infectious during the incubation period, Gordon says.

Latent periods — the time between getting infected with the virus and being infectious to others — often overlap with the incubation period. But some other viruses, such as polio, are often spread by people who have no symptoms or very mild symptoms. That’s because the virus’s latent period is shorter than the incubation period. Something similar may be happening with 2019-nCoV in which people may be able to give off infectious virus before they feel sick.

Another concern: Early symptoms of 2019-nCoV resemble colds or influenza. “It may not be that easy to detect people really early in the illness even if they do have symptoms,” Gordon says. That combined with the estimates of how infectious the virus is “really leads us to worry. It’s going to be very hard to control this virus, if that’s even possible.”