A total of 663 pupils at four schools were studied serologically and clinically during a period of large sequential and/or mixed epidemics of infection with two subtypes of influenza A virus, H3N2 and H1N1. Of 91 middle-school pupils infected with H3N2 virus shortly before and 82 pupils not previously infected with this subtype, 59% and 91% became infected with H1N1 virus, respectively; this difference was significant. Similar results were obtained at the two primary schools studied. At a high school where epidemics due to the H3N2 and H1N1 subtypes occurred concurrently, the rate of infection of individual pupils with both viruses (2%) was significantly lower than those at the other three schools (21%, 23%, and 31%, respectively), where an epidemic caused by the H3N2 subtype appeared first and was then partially overlapped and succeeded by an epidemic caused by the H1N1 subtype. These findings suggest the existence of cross-subtype protection in humans during sequential and/or concurrent epidemics caused by two viral subtypes.

We have studied influenza in Tailand since 1979'-'8). This report describes the isolation of influenza viruses in Bangkok and Chiang Mai in July andAugust, 1989.Five strains of influenza viruses were isolated from throat garglings of suspected influenza sufferers in Bangkok by using MDCK cell cultures16). Throat swab specimens were collected from 132 out-patients suspected of bearig influenza in the Department of Pediatrics, Chiang Mai University Hospital from 23rd to 29th August, and they were inoculated into MDCK cell cultures16). Sixty-seven (50.8%) cultures showed positive haemagglutination with chicken red cells after one to two passages of the cultures.The five isolates from Bangkok and the 67 from Chiang Mai were subjected to haemaglutination-inhibition (HI) tests with reference mouse antisera raised to A/Yamagata/120/86 (H1N1), A/Cichuan/2/87 (H3N2) and B/Nagasaki/1/87 of influenza viruses, produced at the Chiba Prefectural Serum Institute. Tables 1 and 2 show the results. Five isolates from Bangkok consisted of three (H3N2 subtype) type A and two type B. Sixty-seven isolates from Chiang Mai consisted of 36 (53.7%) H1N1, 21 (31.3%) H3N2 and 10 (14.9%) type B.During the period from October, 1988 through August, 1989, 1570 strains of influenza viruses were isolated in Japan, among which 1490 (94.9%) were H1N1, 47 (3.0%) H3N2 and 33 (2.1%) type B. Furthermore only one isolate of influenza virus was reported in 1989 and none in 1988. During the period from September, 1989 through April, 1990, 2393 strains of influenza viruses were isolated in Japan, among which none were H1N1, 1547 (64.6%) were H3N2 and 846 (35.4%) type B. These figures in Japan coincide with the pattern of influenza epidemics in many parts of the world in the 1988/89 and 1989/90 seasons19).