Authorities are warning of an outbreak of H1N1 swine flu, with the number of patients in the country surging since the onset of the new year.
The National Institute of Infectious Diseases suggested that those in their 20s, 30s and 40s as well as people under age 5 get vaccinated early, as the number of new flu patients in these age groups was relatively small last season.
"The new flu is beginning to spread on a full-fledged scale," said Yoshinori Yasui, a senior researcher at the institute's Infection Disease Surveillance Center. Locally, the relatively new strain is known as "shingata infuruenza" (new-type influenza).
"The new influenza can cause even adults to develop pneumonia that can become serious rapidly, so we want people to go to see doctors without delay if they develop symptoms of the new flu," Yasui said.
Cases of the type-A Hong Kong influenza were initially the mainstream shortly after the influenza season came into full force late last year.
But the speed at which H1N1 flu cases has increased has since become conspicuous in urban areas in Tokyo, Osaka, Kanagawa, Aichi, Fukuoka and Miyagi prefectures, the center said.
The number of reported flu cases increased conspicuously Tuesday, with a survey indicating most of the cases are the new flu, it said.
Although more than 20 million people contracted the new flu last winter in Japan, the virus proved to be of low virulence.
But analysis of past data on new flu strain outbreaks has shown that more people could die in a second bout of the disease spreading, the institute cautioned.