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Influenza outbreak slow in arriving due to warm weather

An injection is given at a hospital in Fukuoka. Medical officials are calling for people to get vaccinated early. (Mainichi)

A warm winter is delaying the arrival of the seasonal influenza epidemic in Japan, forcing the government not to announce a flu outbreak even after the onset of the new year for the first time in nine years. Normally, a seasonal flu outbreak is announced in December.

    But the latest number of flu patients released by the Health, Labor and Welfare Ministry on Jan. 7 does not indicate signs of an outbreak. Delayed epidemics led to a surge in the number of patients in the past, and medical experts are recommending people to get flu shots at an early date.

    ''Usually, there are many influenza patients during the year-end and New Year periods, but the clinic is almost empty this year,'' said Chiharu Hosobe, director of Hosobe Pediatrics Clinic in Tokyo's Bunkyo Ward, on Jan. 7. Hosobe says the number of flu patients is extremely low, although the number of infectious gastroenteritis patients is about the same as in the past.

    The health ministry put the number of flu patients at 3,734 in the Dec. 21-27 period last year, based on data from about 5,000 medical institutions. That means 0.76 patients per institution, up from 0.46 from the preceding week but lower than the yardstick figure of one patient per institution to determine the arrival of the flu epidemic. In the past five years, the number of flu patients topped 10,000, except for 2013 when the figure was over 9,000. The figure in 2014 totaled more than 130,000 due to a pandemic. During that year, the flu outbreak began as early as November while the other years experienced the onset of such epidemics in December.

    Normally, an influenza epidemic occurs between December and March when it is an extremely cold and dry winter. Tomimasa Sunagawa of the Infectious Disease Surveillance Center at the National Institute of Infectious Diseases (NIID) in Tokyo says a flu outbreak is likely to happen when humidity is low and dry conditions continue, lowering the immune functions for the throat and bronchi.

    Yukitaka Ohashi, associate professor at Okayama University of Science who is studying correlations between influenza viruses and weather conditions, attributes the delay of the flu outbreak this winter to extended periods of high temperatures and heavy downpours across Japan from November to December. He says rainfall in December was much heavier than previous years and daytime temperatures were high, boosting vapor amounts. Such high humid conditions have curbed the spread of the flu virus.

    According to the NIID, flu epidemics took place after the start of the new year in 2004-2005 and 2006-2007. The number of flu patients soared after late January in 2005 and a flu epidemic in 2007 continued well into spring. Sunagawa says it takes time to develop an antibody against influenza and urges people to receive shots as early as possible.

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