People with mild or no symptoms of COVID-19 are the main drivers behind the spread of the coronavirus epidemic, according to an important new analysis of China’s outbreak released on Monday.

These so-called “stealth” transmissions underscore the importance of “social distancing” measures that are now being enacted in communities across the U.S. and Europe, the researchers said. Unrecognized infections can expose a far greater portion of the population to virus than would otherwise occur.

“We need to keep people apart,” said researcher Jeffrey Shaman of Columbia University Mailman School, who co-authored the study with colleague Sen Pei.

The research is published in Monday’s issue of the journal Science, the same day 6 Bay Area counties announced strict new orders telling most of the region’s 7 million residents to stay home except for essential trips such as to the supermarket. The order takes effect at 12:01 a.m. March 17 and expires on April 7 unless extended.

“This is different than what we’re used to seeing in Western democratic societies,” he conceded. “This virus thrives on people mingling and meeting each other.”

Six of every seven infections – 86% — were undetected in China before January’s strict travel restrictions, according to the team’s computer modeling. That time of unfettered travel and visiting was similar to life in bustling U.S. and European cities prior to new guidelines and restrictions.

“The majority of these infections are mild, with few symptoms at all,” but if transmitted they can kill the elderly or medically vulnerable, said Shaman. “People may not recognize it. Or they think they have a cold.”

These undocumented infections are about half as contagious as documented infections because people are not coughing and sneezing.

Yet they were found to be responsible for about two-thirds of documented infections – because they are so abundant.

People with mild symptoms can transmit a disease that creates major disease in others, they said.

These findings explain the rapid geographic spread of the disease and suggest that control could be difficult.

“It will continue to present a major challenge to the containment of this outbreak going forward,” said Shaman. His team developed a mathematical model that simulated the dynamics of infection among 375 Chinese cities to estimate the contagiousness and proportion of undocumented infections during the weeks before and after the shutdown of travel in and out of Wuhan.

If applied to the U.S. population, this means that we have far more than the 1,629 infections currently reported by the U.S. Centers for Disease Control. The real number would be closer to 13,000.

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The true number of infections will only be known with better and more routine testing, said Betz Halloran of the Fred Hutchinson Cancer Research Center in Seattle.

“It’s crucial to help slow the epidemic” because then more people can self-isolate, she said.

Added Shaman: “We’re seeing just the tip of the iceberg. How much is submerged – five times? Ten times? We can argue about what the exact number is, but if you’re not in a place where you can proactively sample and test, the majority of infections will be undetected.”

The trajectory of China’s epidemic began to flatten only after strict control measures – travel restrictions between major cities, self-quarantining, rapid testing and “social distancing” – were imposed on Jan. 23.

Meanwhile, other emerging research is revealing new insights about the disease:

The COVID-19 virus appears to be more easily transmitted than seasonal influenza. The “reproductive number” – the number of new infections generated from one infected individual – is estimated to be about 2.5 for this virus, which is higher than for the flu. In other words, each infected person spreads it to 2.5 other people. For an epidemic to vanish, transmission needs to drop below 1.

While the symptoms of flu and COVID-19 are similar, COVID-19 seems to be causing more severe disease. Of infections, 80% of infections are mild or asymptomatic, 15% are severe and require oxygen and 5% are critical, requiring ventilation. These fractions of severe and critical infection would be higher than what is observed for influenza infection.

The time between cases in a chain of transmission is, on average, four days, according to a team of scientists from the University of Texas and France, China and Hong Kong.

Almost all viruses are seasonal, peaking in the winter and fading in the summer. Will transmission start to ease up? No one knows — but without marked changes in behavior, there are suspicions it will continue to creep along. “One might guess it will go down, but not go away, in summer months,” said Ira Longini Jr. of the University of Florida’s Center for Statistics and Quantitative Infectious Diseases.

Travel restrictions work, but only up to a point. They delay the spread of infection but don’t stop it. A recent study found that the travel quarantine of Wuhan delayed the overall epidemic progression into mainland China by only three to five days.

“Closing airports will buy you time, but it’s not enough,” according to Northeastern University’s Matteo Chinazzi.

Travel restrictions must be coupled with “social distancing,” according to the team. In other words, shutting borders will not entirely stop the viral spread – but canceling concerts might.

Many very important questions must still be answered, according to an editorial in Monday’s issue of the journal Science.

After recovery, is there life-long immunity? Will a vaccine cause dangerous side effects? Can drugs thwart the virus? Do school closings help or hurt? What happens if hospitals become overwhelmed? If we discover an effective vaccine or drug, can enough be made and delivered to everyone? What are the long-term effects of this crisis on mental health, social well-being, and the economy?

And what happens when social restrictions — like those we’re facing now — are lifted?