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March 20, 2020

How 'stealth transmission' helped fuel the coronavirus pandemic

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    For every documented case of the new coronavirus, there are between five and 10 "undocumented" cases—and these "stealth" infections, while generally less contagious than the documented ones, are responsible for nearly 80% of new infections, according to a study published Monday in Science.

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    The researchers cautioned that their findings are preliminary and have not yet been reviewed widely by fellow scientists. They also noted that changes to current care-seeking and diagnostic practices as well as quickly changing travel restrictions and containment efforts could make predictions difficult.

    About the global pandemic

    Reports of COVID-19, the disease caused by the virus, first surfaced in early December 2019 in Wuhan, China. While the number of new cases of COVID-19 reported in China has been dropping, newly reported cases of the disease have spiked in other countries, with COVID-19 reaching global pandemic status.

    As of Friday morning, officials reported more than 244,100 cases of COVID-19 globally. Officials said as of Friday morning there had been at least 10,184 deaths linked to the new coronavirus.

    The United States saw its first COVID-19 case in late January. Since then, the country's number of reported cases of COVID-19 have been on the rise, and have particularly surged over the past two weeks as more diagnostic tests for the disease became available. As of Friday morning, state and federal officials had reported 12,392 cases of COVID-19—up from 5,881 reported cases on Wednesday. Officials as of Friday morning also had reported 195 deaths linked to the new coronavirus in the United States, up from 107 on Wednesday.

    Study details

    For the study, researchers led by Jeffrey Shaman, an infectious disease researcher at Columbia University, simulated a landscape of 375 Chinese cities to trace the spread of the new coronavirus, using known data about how the virus behaves to generate assumptions about how it is spread among people.

    According to the researchers, the simulation was focused on at least 173 billion "travel events" that happened between Jan. 10 and Jan. 23—the time period during which people were gaining awareness about the virus, but before the Chinese government shut down all travel from Hubei, the province where the pandemic originated. After setting their hypothetical travelers in motion via the simulation, the researchers then compared the resulting infection rates against documented totals.

    The selected time period is comparable to the current situation in the United States and other countries in the West, as COVID-19 diagnosis tests are not widely available, the New York Times reports. 

    How 'stealth transmissions' drove the pandemic

    The researchers estimated that before Jan. 23, about 86% of all coronavirus infections were "undocumented," meaning the person didn't feel poorly enough to have the infection confirmed. And based of the researchers' model, nearly 90% of infections were transmitted by an infected person who had yet to show symptoms.

    On the one hand, the researchers said these undocumented cases were about 52% as contagious as confirmed cases. However, because there were substantially more undocumented carriers than documented ones, as a whole, the undocumented cases "were the infection sources for 79% of documented cases." Moreover, according to the researchers, while these undocumented cases of infection were often milder, they did not necessarily lead to milder cases when they infected others with the virus.

    The researchers said while these findings "indicate containment of this virus will be particularly challenging," they did find that social distancing and quarantines slowed the spread of the virus. When they ran simulations occurring between Jan. 24 and Feb. 8, as diagnostic efforts increased, they found that stealth transmission dropped by a factor of five and transmission rates among documented patients dropped by at least 50%.

    What it would take to contain the spread

    According to Shaman, "[s]tealth transmission" is a "major driver" of the new coronavirus pandemic and its contribution "is substantially undetected, and it's flying below the radar."

    Further, Shaman and his team said health officials should consider that "a substantial proportion of transmission" may be happening outside of official efforts to track the spread of the virus when they are developing public health strategies. Overall, the researchers noted that the overall number of undocumented cases for each confirmed case could vary twofold depending on the country, with Shaman noting, "If we have 3,500 confirmed cases in the U.S., you might be looking at 35,000 in reality."

    Shaman and colleagues also said, to better contain the spread of the new coronavirus, "a radical increase in the identification and isolation of currently undocumented infections" would be required. Based  on their findings, preventive measures such as screening for illness or fever at public events or national boarders would not have a significant effect on slowing the spread of the virus. Instead, measures such as travel restrictions and restrictions on social interactions on both healthy and sick people would be more successful, the researchers said.

    However, Shaman concluded that while "[h]eightened awareness of the outbreak, increased use of personal protective measures, and travel restriction have helped reduce the overall force of infection … it is unclear whether this reduction will be sufficient to fully stem the virus spread." Overall, after these widespread restrictions were put in place, the virus' spread was "either at or just barely above the level required for an epidemic to sustain its momentum," the Los Angeles Times reports.  (Healy, Los Angeles Times, 3/17; Carey, New York Times, 3/16; Columbia Mailman School of Public Health, release, 3/16).

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