Coronavirus: Undetected cases drive virus spread in the community

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Eighty-six percent of people in China infected with novel coronavirus (SARS-CoV-2) but undetected before travel restrictions came into effect were the source of infection for 79% of documented cases, a paper in Science says.

Eighty-six percent of people in China who were infected with novel coronavirus (SARS-CoV-2) were not tested in the two week period before travel restrictions came into force on January 23 in Wuhan and other cities. And these undetected cases contributed to the majority of virus spread in the community, says a paper published in the journal Science. The paper says that undetected cases can expose a large population to the virus than would otherwise occur. The study found undetected cases were the source of infection for 79% of documented cases.

The researchers used a computer model that draws on observations of reported infection and spread within China in conjunction with mobility data from January 10-23 and January 24-February 8.

People with mild symptoms are 55% infectious

People who experienced mild, limited or no symptoms were not detected but spread the virus anyway. “Per person, these undocumented infections were 55% as contagious as the documented infections. About half as infectious per person as a documented case who has more severe symptoms and maybe shedding more,” Prof. Jeffrey Shaman from Columbia University, New York and one of the corresponding authors of the paper said during a press briefing.

Even though the undetected cases are only 55% contagious compared with those who exhibit severe symptoms and are detected, due to their greater numbers, facilitated the rapid spread of the virus throughout mainland China.

“There are many more of these undocumented cases and it’s the undocumented infections that are driving the spread and growth of the outbreak,” Prof. Shaman said. “The undocumented infections which tend to be milder are distributing the virus broadly. They’re contributing essentially to what we call self transmission of the virus because it’s really undetected and it’s flying below the radar.”

The modelling study suggests that a “radical increase” in identifying and isolating people who have not been tested yet would be needed to fully control the spread of the virus.

Social distancing, preventive measures help

The paper says that steps taken by many individuals and governments to restrict travel, shut down schools, prevent large gatherings, isolating suspected cases, and use of face mask and regular hand washing could have helped slow down the spread of the virus. And some countries have also started community testing.

The authors say that while travel restrictions and other control measures could have reduced the spread of the virus in the community, the study does not provide evidence that such control measures would be sufficient to end the spread locally and prevent a rebound once travel and other restrictions are removed or relaxed. “Similar control measures and travel restrictions would have to be implemented outside China to prevent reintroduction of the virus,” they write.

The paper concludes by saying that there are already four coronavirus strains that are circulating in the human population. And if the novel coronavirus follows the pattern of H1N1, then it would spread globally and become the fifth coronavirus to become endemic in the human population.

Published in The Hindu on March 17, 2020