[An analysis of the 2002-2003 influenza epidemic focusing on patients infected with types A and B in the same season sequentially].

A total of 2,320 cases of influenza A (1,517 cases) and B (803 cases) in the 2002-2003 influenza season were analyzed. Influenza infection was confirmed by a rapid diagnosis kit, based on the immunochromatography method, at 24 clinics in 18 of the 47 prefectures of Japan. Influenza A/H3N2 was reported between November 22 and April 12 (the median at January 21), and influenza B was reported between December 24 and April 20 (the median at February 16). The mean age of type B patients (16.7 years old) was significantly younger than that of type A patients (26.7 years old) (p < 0.001). Pneumonia was more frequently a complication of type A (0.63%) than of type B (0%, p < 0.05). Although 5 type A patients needed hospitalization, neither hospitalization nor death was associated with type B. Of the 2,293 cases, 27 (1.2%) were infected with both influenza of A and B. The age of these 27 patients ranged from 2 to 51 years (mean of 11.2 years), but 20 of the 27 patients were 9 years of age or under. Type B followed type A in 25 patients. The median date of these 25 type A patients was January 22, similar to that of all cases of type A, January 21. However, the median date of type B infection was March 3, 15 days later than that of all patients with type B, February 16. The mean age of the 27 cases was significantly younger than that of all cases of type A (p < 0.001) and type B (p < 0.05). In epidemics consisting of multiple types of influenza viruses, patients may be infected by more than one virus. This is especially true for children.