Epidemiological research on the phenomenon of the influenza herald wave.

Among the many kinds of respiratory infectious diseases caused by viruses, influenza is thought to be one of the most important factors affecting human social life. Since 1980, we have continued surveillance on influenza epidemiology among children in part of a thickly inhabited area of Tokyo Metropolis, around Kamata in Ota Ward. Influenza epidemics occurred every winter in this surveillance period all over Japan including around Kamata. In those epidemics, type A (H1N1), A (H3N2) and B influenza viruses were involved either as a single causative agent, or two or three of them cocirculated in a single epidemic. During the past eight years, we observed the so-called influenza herald waves several times. These were detected as small epidemics or sporadic infections after the main epidemics around Kamata. The influenza herald wave is considered to be an indicator of what the prevailing influenza virus in the next epidemic season will be. The materials examined in this study were collected from out-patients who visited the pediatric clinic of the Social Insurance Associations Kamata General Hospital. Virus isolation and identification were carried out by the methods described previously. The sole exception was that the nasopharyngeal swabs were inoculated into monolayer cultures of primary cynomolgus monkey kidney cells and/or Madin-Darby Canine Kidney (MDCK) cells. Patients' acute and convalescent sera were examined for the HI antibody titers with the micromethod after RDEtreatment. As antigens for HI test, both prototype . strains of currently circulated viruses, inside Japan or outside when an epidemic of Kamata occurred, and one of the isolated strains in the epidemic were used. Case numbers of influenza detected in this surveillance are shown in Fig. 1. In the winter of 1979/80, an influenza epidemic caused by three types of A (H1N1), A (H3N2) and B virus broke out at the end of February, 1980 and continued until April of the same year, and each type of virus disappeared from the surveillance population almost simultaneously. In the winter of 1980/81, an influenza outbreak occurred at the beginning of February, 1981, when both A type viruses were isolated but B type virus infections were identified only by a serological test. Epidemics caused by both A type viruses almost ceased in mid-March. However, type B virus was isolated first from a patient in March, and the virus prevailed and then