The old is stronger than the young: Age-specific incidence/fatality of 2009 novel influenza epidemic in Japan

[Purpose] To analyze the age difference of incidence and fatality of novel (swine) influenza. [Data and Methods] The surveillance data of novel (swine) influenza during the 2009 epidemic in Japan were analyzed. As soon as WHO raised the level of alert to phase 4 on 28th April, Japan required all doctors to report confirmed cases. The first case was confirmed on 16th May followed by a large epidemic, then the mandatory reporting was abandoned in the middle July and only hospitalized and fatal cases were reported afterward. A total of 4986 confirmed cases were reported by 24th July and a total of 17640 hospitalized cases were reported between 16 July and 23rd March 2010 with 198 fatalities (27 of them were outpatient). [Results] Age-specific incidence was highest in high teenagers (15-19 years old) :195/100000 population and lowest among the elderly: 8.2 for 50-59 yo and 1.3 for >=60yo. Admission rate was highest among 5-9 yo :1239.8/100000 population followed by low teenagers: 427.8. Remarkably, the admission rate of the elderly was lower than high teenagers (49.4 for >=80yo vs. 91.3 for 15-19 yo). As for fatality of admitted patients, after excluding those with underlying diseases, the middle age (40-49 yo) showed the highest case-fatality (8 deaths/408 admissions or 2%) followed by 30-39 and 50-59 yo groups (1.2%). However no fatalities were observed with 60-69 and >=80 yo groups and only 0.4% for 70-79 yo group (2 deaths/505 admissions) [Conclusion] The elderly showed stronger resistance against novel influenza than young population.