[Influenza surveillance].

From 1982 to June 1990, seven influenza A (H3N2) epidemics and four influenza A (H1N1) epidemics occurred in Shanghai, and several new variants of influenza virus were isolated. During that period, influenza A (H3N2) and influenza A (H1N1) appeared alternatively while each lasted for one to two years. The epidemic peak of influenza usually was seen from July to August and late winter to early spring. The subtype which appears in July and August usually starting in late winter and early spring in Shanghai causes epidemics in the northern regions of China and some other countries in the northern hemisphere. If minor epidemics occur only in July and August, further antigenic changes may cause moderate epidemics in late winter and next early spring. Information supplied for prediction of influenza epidemics and timely preparation of new vaccines may lead to better control of influenza.