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What are the odds COVID-19 patients with no symptoms could develop them later on?

Quite slim, according to a recent study led by a group of researchers in Aichi Prefecture, who have discovered that asymptomatic coronavirus patients tend to recover within nine days of having their infections confirmed via polymerase chain reaction (PCR) tests.

The findings, involving a study on how the disease developed in 90 people without symptoms who contracted the disease aboard the Diamond Princess cruise ship, were published Friday in the New England Journal of Medicine and showed that only 11 people without symptoms would go on to develop them later.

“In this cohort, the majority of asymptomatic infected persons remained asymptomatic throughout the course of their infection,” a team led by Yohei Doi, a doctor and professor of the department of infectious diseases at Fujita Health University, wrote in the report.

The results showed that nearly half of the patients were cleared of the virus within nine days after first testing positive for COVID-19, while 90 percent of people recovered within 15 days. The findings also showed that most patients were unlikely to recover within five days after the initial positive test.

Doi estimates that in an environment where the coronavirus is easily transmissible, the number of people without any symptoms may be equal to the number of people who have developed symptoms.

The team hopes the findings will lead to efficiency in testing.

“Our findings show that repeating PCR tests in patients with positive test results is low yield soon after the diagnosis, as the results will remain the same and as a result the resources won’t be used effectively,” Doi said in an email.

Based on the findings, the health ministry on Friday revised its guidelines for hospitalization of COVID-19 patients. It is now advising that patients who test negative twice in a row after six days after the initial positive test can be discharged.

“Initially, viral particles may appear in tests after six days, but it’s likely (asymptomatic) patients will be cleared of the virus after eight to 10 days,” Doi added.

Doi explained that such developments could lead to the conclusion that asymptomatic patients could be released 10 days after the diagnosis without being tested. In the revised guidelines, the health ministry also shortened the time of quarantine for coronavirus patients from 14 to 10 days.

“I see it as an adjustment in line with global standards based on the consensus that the amount of virus decreases significantly within that period, which makes the odds of transmission after long-course infection very low, and our study supports that,” he said.

Fujita Health University’s Okazaki Medical Center took in 128 people from the virus-hit Diamond Princess — 96 passengers who tested positive for the coronavirus but had no symptoms and 32 of their cabinmates who tested negative. All were between the ages of 9 and 77. In total, data on 90 patients without symptoms were available for analysis.

Fujita Health University Okazaki Medical Center in Okazaki, Aichi Prefecture, studied asymptomatic coronavirus patients who were aboard the Diamond Princess cruise ship. | KYODO
Fujita Health University Okazaki Medical Center in Okazaki, Aichi Prefecture, studied asymptomatic coronavirus patients who were aboard the Diamond Princess cruise ship. | KYODO

The cruise ship, which became a coronavirus breeding ground and was put under quarantine off Yokohama with some 3,700 passengers and crew members on board, made headlines after an outbreak of COVID-19 in January and February. The coronavirus infected more than 700 passengers and crew members.

The patients in the study were transported to the medical center where they were put under continued observation in February. The results of the study showed that the time it took to recover from the infection increased with age. The average age of the patients was 59.5 years.

The researchers explained that the 11 patients who did develop symptoms later had been pre-symptomatic, rather than asymptomatic for COVID-19.

The findings add to the ongoing debate on the role of asymptomatic patients in transmission of the coronavirus.

“We still don’t know enough on the link between the proportion of asymptomatic carriers and their capability of transmitting the coronavirus in comparison to people with (COVID-19) symptoms, as their role remains under debate,” Doi said.

The New England Journal of Medicine previously called transmission by seemingly healthy people “the Achilles’ heel of COVID-19 pandemic control.”

The World Health Organization earlier in June sparked controversy over coronavirus transmission routes when a top official said that such transmission is “very rare.”

Maria Van Kerkhove, head of WHO’s emerging diseases and zoonosis unit, later backtracked from her statement, noting that there is not enough data to conclude how infectious asymptomatic carriers are.

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