Can the coronavirus be spread by people who don't have symptoms?

Health experts emphasize that the vast majority of cases are spread by people exhibiting symptoms, like coughing.
Image: Novel coronavirus
An electron microscope image showing the novel coronavirus (in yellow) emerging from the surface of cells (in pink and blue).NIAID-RML / via AP

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By Jacqueline Stenson

A key question on the minds of experts studying the coronavirus outbreak is whether the virus can spread asymptomatically, a development that could make it much more difficult to track and contain.

A report Wednesday in the New England Journal of Medicine adds to the case that the virus may spread before people know they are sick. The report, from researchers in China, found that an asymptomatic patient had similar levels of the virus in their upper respiratory tract as symptomatic patients had in theirs. Higher levels of virus in respiratory secretions could make a person more contagious.

Infectious disease experts say the report adds to the understanding of the virus, but doesn’t resolve the question of asymptomatic spread. And while there have been anecdotal reports from doctors in China about asymptomatic spread, experts believe hard scientific proof is still lacking. The virus has sickened more than 76,000 people worldwide and killed at least 2,200, mostly in China.

Other viruses in the coronavirus family include those that cause Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS), as well as coronaviruses that cause the common cold.

There is no documented proof that any of them is spread asymptomatically, said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins University Center for Health Security in Baltimore, and a spokesperson for the Infectious Diseases Society of America.

“We don’t have definitive evidence that that type of phenomenon is occurring" with the new virus, Adalja said. “There is some possibility that it’s occurring, but we haven’t seen that before with other viruses in the family, and so we have to kind of view any claims of that with a little bit of skepticism and they need to be properly studied.”

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It is clear, however, that other types of respiratory viruses can spread asymptomatically.

“We do know that, for example, influenza and rhinoviruses — which are one of the major causes of the common cold — can do that,” Adalja said. “But it hasn’t been seen with coronavirus, and that’s not to say that it’s not possible or that it hasn’t happened. It’s just that it’s not been definitely proven.”

A letter from January, also published in the New England Journal of Medicine, that has since been discredited reported that the coronavirus was spread asymptomatically by a Chinese woman visiting Germany on business. After the report was published, though, it was revealed that the researchers didn’t actually speak to the woman before they wrote the letter, and she actually did have mild symptoms and had been taking fever-lowering medication.

Symptoms of the coronavirus infection can include coughing, fever and sore throat, but doctors know it’s possible for some people to be infected but have either no symptoms or very mild symptoms that don’t seem concerning.

Still, anecdotal reports from China of asymptomatic spread are raising concerns.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said he suspects asymptomatic transmission can occur.

“Early case reports indicate in certain circumstances [the coronavirus] is transmissible in asymptomatic people,” Fauci said in a statement to NBC News. “We do not know the extent to which asymptomatic transmission is impacting the outbreak. However, we know that people with symptoms are usually the main driver of outbreaks of other respiratory diseases.”

We know that people with symptoms are usually the main driver of outbreaks of other respiratory diseases.

In other words, as with other respiratory illnesses, even if the coronavirus can be spread asymptomatically, the vast majority of cases are spread by symptomatic people.

Respiratory droplets are spewed into the air when someone coughs or sneezes, so people who are visibly sick are the most contagious, said Dr. Charles Chiu, a professor of laboratory medicine in the division of infectious diseases at the University of California, San Francisco.

“When somebody sneezes or coughs, the respiratory secretions are aerosolized, and if you’re near, typically within 6 feet, you may be at risk of being exposed,” Chiu said. “That’s the most common route of transmission. Patients who have minimal symptoms or no symptoms may be infectious — they may have the virus in their mucus or their secretions — but unless they’re actually coughing or sneezing, it’s unlikely that they would transmit to someone else.”

He said it’s possible that someone who is infected but not sneezing or coughing could spread the virus by touching their nose, mouth or eyes and then contaminating a surface such as a doorknob that someone else then touches, but that’s not the likeliest way the virus is spread.

“I think that probably, yes, the new coronavirus can be transmitted asymptomatically, although that is very likely to be a rare or at least uncommon route of transmission,” Chiu said.

With the coronavirus, though, roughly 80 percent of cases are mild, so people actually may be symptomatic but not realize it. Headaches and back pain, for instance, easily could be dismissed as everyday aches and pains in someone who actually is infected. As a result, patients may inaccurately report they were asymptomatic, and health officials may have a more difficult time determining which patients truly were asymptomatic and which ones had mild symptoms that were not attributed to the illness.

Understanding exactly how coronavirus is spread is important in containing the illness and keeping people healthy, said Dr. Trish Perl, chief of infectious diseases at the University of Texas Southwestern Medical Center in Dallas.

“I think the question is how quickly the new coronavirus will spread, and can we limit the spread long enough that we can bring in other interventions like vaccines,” Perl said. “We’d like to get more tools [at our disposal], should this spread more widely, so that the implications are just not quite as dramatic and devastating as they have been thus far.”

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