The Infectious Disease Surveillance Center issued on Wednesday this winter's first flu warning to residents of Yamagata Prefecture and cautioned residents of Hokkaido, indicating the nationwide flu season has begun.
The affiliate of the National Institute of Infectious Diseases upgraded a caution issued earlier for Yamagata.
Type-A and type-B Hong Kong flu may prevail in Japan and the same types of vaccines as last season have been used to limit the spread so far, according to the IDSC.