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Prior to widespread travel restrictions in China, more than 85% of COVID-19 coronavirus infections went unnoticed, a modeling study suggested.

Only 14% of COVID-19 infections were documented in the two weeks prior to implementation of these widespread travel restrictions, meaning that the majority were people with mild symptoms or who were asymptomatic and who never sought care, reported Jeffrey Shaman, PhD, of Columbia University in New York City, and colleagues.

Moreover, the of documented infections during this time, the authors wrote Monday in Science.

These data were from mid-January before the Chinese government instituted strict containment and mitigation measures, like those now in place only in "pockets" of the U.S., the researchers pointed out on a conference call with reporters.

Shaman described the data as the virus "in its natural state," moving through a developed society unimpeded by restrictions. He characterized people with undocumented infections as "a major driver" of the outbreak, facilitating the spread.

"If someone's experiencing mild symptoms, we're still going to go about our day -- still go to work, send the kids to school, go out, go shopping," he said, all while potentially shedding virus. "We suspected there may be substantial undocumented infections and those undocumented infections may be underreported."

Researchers built a mathematical model that combined data on people's movements with infection rates, and modeled infections among 375 Chinese cities from Jan. 10 to Jan. 23, 2020, prior to travel restrictions in Hubei province going into effect.

Moreover, in a model without transmission by undocumented cases, reported infections would be reduced by 79% in China and 66% in Wuhan during this time period.

"These people [undetected infected persons] were a major driver of it, they were the ones that facilitated the spread," Shaman said.

Co-author Sen Pei, PhD, also of Columbia University, added there were initially low documentation rates in China partially due to lack of testing assays.

However, from Jan. 24 to Feb. 8, after the implementation of these restrictions and increased awareness, aggressive testing, and contact tracing, as well as increased personal protective behavior, 65% of infections were documented.

A second study recently published March 6 in Science also noted the , and co-author Alessandro Vespignani, PhD, of Northeastern University in Boston, noted they had "quite a modest effect" on the spread of the virus in China, though they did have a "more marked effect" on an international scale.

Vespignani estimated they resulted in a "delay of a few weeks" in spreading of disease to other countries. When asked how this might apply to the U.S., he noted the epidemic is "already seeded" here, because we're seeing ongoing transmission in many states.

"Closing travel might delay progression of the epidemic in some places and provide some time for preparation," but he added "we need to have strong transmissibility reduction in [communities] where it's already seeded."

Reducing transmissibility includes early detection, isolation of cases, and community measures, such as social distancing and behavior changes, said co-author Elizabeth Halloran, MD, DSc, of Fred Hutchinson Cancer Research Center in Seattle.

And since undocumented cases, by definition, aren't included in official tallies, the number of cases in any given area could be exponentially higher than published counts.

Another co-author on the March 6 paper, Ira Longini, PhD, of the University of Florida, gave the example of 150,000 confirmed cases worldwide, but "generally, we're looking at an order of magnitude" higher for the number of actual infections -- thus meaning a worldwide total of around one million.

Shaman said this suggests there needs to be more proactive sampling of the population, but acknowledged the "backdrop of logistics and the cost of implementing lots more tests."

"If a society is more proactive in testing procedures, that's going to capture more infections," he said.

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