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Computer modeling of the coronavirus outbreak’s course in China, in the weeks before a travel shutdown was imposed on Jan. 23, suggest that 86% of the infections went undocumented.

Those undocumented infections were about half as contagious as the documented cases, but were the source of two-thirds of the documented cases, according to a study published online today by the journal Science.

The findings parallel other research into the role of what’s known as stealth or cryptic transmission in spreading the potentially deadly virus. They also underscore the importance of widespread testing, even if patients aren’t experiencing serious symptoms, particularly during the early phases of transmission.

“More active testing procedures would catch more cases,” Columbia University’s Jeffrey Shaman, a co-author of the study, told reporters today. “How that would be implemented is something that we can debate for quite some time. And obviously this all has to be done under the backdrop of the logistics and costs of implementing lots more tests.”

Shaman and his colleagues tracked reports of infections in China from Jan. 10 to 23, and from Jan. 24 to Feb. 8, and fed those reports along with data about population mobility into their computer model.

When the researchers started putting their data together, they suspected the model would show that reported cases were just “the tip of the iceberg” for the spread of infection, Shaman said. That suspicion was based on past research involving more benign coronaviruses — such as those that cause the common cold. Such research suggested that most infections caused symptoms so mild that the people who had the virus didn’t bother to report them.

Their suspicion turned out to be correct. “It’s the undocumented infections which are driving the spread of the outbreak,” Shaman said.

Currently, about 180,000 coronavirus cases have been documented worldwide. Based on that statistic and the study’s findings, the actual number of infections could be well more than a million.

The good news is that increased testing, in addition to limits on travel and social contact, can significantly change the equation. The model shows that during the two weeks after China imposed its restriction, 60% of infections were documented.

Public health officials have found that the effect of COVID-19 infections varies widely, depending on the presence of underlying medical conditions such as respiratory or heart problems, as well as demographic factors such as age. Patients over 60 may be hit particularly hard, while patients under 30 are more likely to experience mild symptoms.

Another factor may be how transmissible the virus is before the onset of symptoms, but it’s too early to get definitive metrics on that factor, Shaman said. It’s also too early to say definitively whether people who have been through COVID-19 build up lifelong immunity. That’s not typically the case with colds.

“We don’t know how long this virus is going to be around,” he said. “It’s a big planet, with billions of people, so it could be here for a couple of years at least. It could become endemic. So I think it’s imperative to develop a vaccine, or vaccines, as quickly as possible.”

Imperial College London’s Ruiyun Li is the principal author of the study in Science, “Substantial Undocumented Infection Facilitates the Rapid Dissemination of Novel Coronavirus (SARS-CoV2).” In addition to Shaman, the co-authors include Sen Pei, Bin Chen, Yimeng Song, Tao Zhang and Wan Yang.

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