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Coronavirus

Japan scientists urge use of PCR tests to detect COVID superspreaders

Genetic material value reveals infectiousness of patients, researchers say

A traveler takes a PCR test for coronavirus at Narita Airport.   © Kyodo

TOKYO -- As Japan contends with surging COVID-19 infections just three months before the Tokyo Olympics, a team of researchers has a suggestion for the government; Use PCR tests to identify highly contagious people.

Polymerase chain reaction tests detect a tiny amount of genetic materials from the virus in samples in a process called amplification. In tests administered by local governments in Japan, this copying cycle is repeated 40 times.

The cycle threshold (Ct) value is the number of cycles at which point a reaction in PCR tests is detected. Ct values are inversely correlated to viral loads: the lower the Ct number, the higher the virus level.

"With infected people with an exceedingly small level of viral load, there is almost no concern that they would pass on the virus even if they test positive," said Hiroshi Nishihara, a specialist on genomic medicine at Keio University. Nishihara worked with officials from Setagaya Ward in Tokyo.

Earlier research overseas showed that people with a Ct value exceeding 35 were less infectious, according to the Japanese team.

In their study, Nishihara and Setagaya officials examined 78 positive samples and found 27 patients with Ct values of 25 or below. Of these, about 80% of them were 65 and older.

They believe people with a Ct level of 30 or less are more contagious.

"If test results are required to include Ct values" and not just whether a person is positive, "we can narrow down infected patients who need to be quarantined and close contacts for contact tracing," Nishihara said.

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