Large infection risk found in 30% of asymptomatic virus patients

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Potentially big spreaders of the novel coronavirus can be the people least suspected of carrying the contagion, a recent study showed.

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In a “social testing” project that started in October 2020, Tokyo’s Setagaya Ward office tested residents and workers at care facilities and related businesses for elderly and disabled people.

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All 21,710 people tested as of April 27 showed no signs of COVID-19 symptoms, but 117 were in fact infected with the novel coronavirus.

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Samples are analyzed under an amplification process to detect the virus. Larger quantities of the virus, which increase the risk of infecting others, require less amplification for detection.

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Hiroshi Nishihara, a professor at Keio University Hospital Clinical and Translational Research Center, analyzed the data under commission from the ward office.

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He said he focused on the Ct value, which indicates how many times amplification was repeated to confirm the presence of the virus.

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Nishihara said the amplification value was small in 27 of 78 positive cases detected over a four-month period. That means 30 percent of the asymptomatic patients carried large quantities of the virus and a greater danger of infecting others.

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“It is urgent to find people with certainty who are at high risk of spreading the infection,” Nishihara said.

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The ward mayor, Nobuto Hosaka, released the data from the study on April 28.

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“It is important to know not only the result—positive or negative—but also the strength of infection,” Hosaka said. “The time has come to focus on the Ct value and think about effective measures.”

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The ward office also started “pool testing” project on April 19, in which five specimens are tested together, and the ones with a “positive” result are then individually tested.

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The project is intended to save time, reagent volume and costs, the ward said.

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“The biggest infection risk is via saliva when people talk and eat without wearing a mask,” Nishihara said. “A pool polymerase chain reaction (PCR) test that uses saliva is effective.”

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The ward office plans to continue its study until September.

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Based on the results, the ward office plans to submit a proposal to the central government on ways to contain the virus while maintaining socio-economic activities.

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