The 1965 A2 influenza epidemic in the USSR and the natural history of Asian influenza.

An epidemic of Asian (A2) influenza that occurred in the USSR early in 1965 was the fourth wave of Asian influenza there since 1957, when the disease became pandemic, spreading through all countries and continents. The 1965 epidemic was severe, in several cities even more so than the 1957 pandemic. Study of the antigenic structure of the viruses isolated during the latest epidemic indicated that the viruses were similar to those isolated in western Europe in 1964 (A2/England 12/64) but that their structure showed a further shift away from that of the prototype virus (A2/Singapore 1/57). The 1957 and 1959 pandemics followed similar courses, starting in China. The third wave affected Europe. North Africa and Asia in winter 1960-61, but did not reach the USSR until the following winter. The fourth wave was more complex, spreading in Asia in 1962-63 and in Europe and America in 1963-64; it reached the USSR at the beginning of 1965. The way in which the structure of the A2 virus has changed since 1957 is discussed.