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In China, where the number of locally transmitted cases exploded in a short period of time, their effect was much greater than previously realized. In the earliest days of the outbreak in mainland China — Jan. 10 and Jan. 23 — people who were not hospitalized were the most likely carriers of the virus.

The Columbia University infectious diseases researcher and study lead for the new report, Jeffrey Shaman, called this phenomenon “stealth transmission” and said it was a “major driver” of the spread of COVID-19.

He added that this mode of transmission has remained “substantially undetected, and it’s flying below the radar.”

Shaman’s study simulated a transmission landscape that represented 375 cities in China and used available Chinese social media data to map the effects of at least 173 billion “travel events” in the early days of the virus.

Photo by Eugene Hoshiko/AP

As the researchers collected data on the virus’ behaviour, they created a simulation of how it spread between travellers, starting on Jan. 10 and through to Jan 23. They found that prior to China’s lockdown on Jan. 23, an estimated 86 per cent of all COVID-19 infections were classified as undocumented.

The model continued to prove that those who caught it but failed to show symptoms for a few weeks were quietly spreading the virus at an accelerated rate.

“They’re the ones who facilitated the spread,” Shaman said.

In another study posted to medRxiv, epidemiologists who examined 91 infections in Singapore found that 48 per cent of cases were transmitted by people who had mild or no symptoms. Although they said their findings were just preliminary, this seemingly invisible transmission is something that was not as prevalent in past epidemics, such as SARS or MERS.