The influenza virus is spreading rapidly across Japan, with health care institutions reporting an average of 12.09 flu patients in the week ending Jan. 16, more than doubling the 5.06 seen the previous week, the Infectious Disease Surveillance Center said.
The center is warning that the rise in flu patients may continue, and it advised early vaccination especially among pregnant women and people with chronic diseases, who are at higher risk of complications.
The figures were based on data provided by 5,000 designated health care institutions across the nation.
The center estimates 780,000 flu patients visited doctors in the latest reporting week, with people aged 20 or older comprising 57.7 percent.
Yoshinori Yasui, a researcher at the center, said many of this season's flu patients are adults who have not developed strong immunity, as most of them escaped infection during the last flu season.
In the last season, the H1N1 influenza epidemic swept the country from August 2009 until around last March. It mainly affected infants and children up to age 19, while those in the 20 to 59 age category comprised less than 30 percent of the total.
"It is a matter of concern that the pace of the spread of the epidemic is faster than the previous season," Yasui said.
Past data show that there are cases in which the number of deaths is higher in the second year of a new flu epidemic.