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Coronavirus symptoms taking longer to appear in patients in Nagoya since August

A file photo of the Nagoya Municipal Government building is shown here. (Mainichi/Hiroki Sameshima)

NAGOYA -- The Nagoya Municipal Government announced on Oct. 19 that there has been a growing number of cases since August where residents don't show signs of COVID-19 symptoms until at least 10 days after coming into contact with the source of coronavirus infection.

    The coronavirus is said to have an incubation period of five or six days on average. However, a total of 19 people were reported to have shown symptoms after an interval of 10 days or more during August and September. It's possible that such findings will prompt officials to review the current self-isolation period of 14 days. The Nagoya government plans to move forward with analysis and research together with the National Institute of Infectious Diseases and other bodies.

    Although the number of infected patients in Nagoya whose incubation period was 10 days or more had not been recorded at all until July, the tally rose to eight in August, and 11 in September. Such individuals accounted for 0.6% of all 1,435 people that tested positive for the coronavirus in Nagoya in August, and 2.5% of all 439 patients in September. During the two months, cases where symptoms appeared over 14 days later were seen in a total of five individuals.

    As for whether there are common factors of age or region among patients with long incubation periods, a city official commented that they "have not yet been able to analyze the matter thoroughly." Meanwhile, the municipal government said that they intend to keep the self-isolation period as it is now for the time being, as incubation periods greater than 10 days are apparently "rare cases."

    Kazunori Oishi, head of the Toyama Institute of Health and an expert on infectious diseases, commented, "An increase in the number of tests, which has enabled us to grasp larger numbers of infected individuals with mild symptoms, may also be an influencing factor. However, a rise in the number of individuals with mild symptoms has been seen since around June, so another reason appears to be behind these figures if cases with an incubation period of 10 days or more emerged in August."

    The coronavirus is said to have an incubation period of between one and 14 days, and under 5% of patients reportedly show symptoms after five to seven days or later from infection. "It is also possible that there have been changes in virulence caused by mutations of the virus," said Oishi.

    (Japanese original by Masakatsu Oka, Nagoya News Center and Yuki Ogawa, Lifestyle and Medical News Department)

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