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

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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 the asymptomatic spread, experts believe the 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.”