Influenza A2 and B in the USSR in 1966-67, and some aspects of global epidemiology of influenza

An epidemic wave of influenza caused by the A2 virus was recorded in the USSR in the winter of 1966-67; this was the fifth wave since this pandemic strain first appeared. In contrast to the last wave of 1965, it developed simultaneously with, or followed, an epidemic of influenza B. The epidemic spread of influenza was observed for a period of 2 years in the USSR; influenza B prevailed in 1966, influenza A2 in 1967, and the two viruses spread at different speeds. A slow spread and a gradual increase in intensity during most of the year was characteristic of influenza B while influenza A2 spread rapidly and involved a considerable part of the country within 3 months. Although the influenza B started to spread almost 9 months before influenza A2 became active, the periods of maximum intensity of both outbreaks coincided in most parts of the USSR in the winter of 1966-67. The approach of summer temporarily stopped the spread of influenza B but did not bring the outbreak to an end nor lessen the intensity of the next wave in the autumn. Among the haemagglutinating agents isolated in the USSR in 1966-67 were 121 strains of virus B and 132 strains of virus A2. A study of the antigenic structure of the B strains showed that they were not all similar; some of them were closely related to B/Johannesburg/33/58, a greater number were more closely related to B/Singapore/3/64 and a number of strains differed from both varieties. Among the A2 viruses isolated and investigated were strains similar to those of pandemic prototype A2/Singapore/1/57, strains close to A2/England/12/64 and A2/USSR, Gor/62/65, strains antigenically different from all those strains but still related to the A2 subtype and strains that had S-antigen of A-type but which were not inhibited by antisera to the prototypes A2/57, A2/64 or A2/67. Outbreaks of influenza A2 and B recorded in the USSR are considered to be parts of the global influenza epidemics.