ABSTRACT
The Hong Kong pandemic was the third of the 20th century. The Spanish influenza in 1918–1919 was a world catastrophe with at least 20 million deaths. In the following years, the virus remained active and caused a number of annual episodes, though it was never as virulent as during the first two waves ending in 1919. Its composition is known today as A(H1N1), but inside this group, many variants appeared, the result of a continuous antigenic drift. For instance a significant seasonal outbreak occurred in 1948 with a variant called A’ (A prime). A few years later, in 1957 a new virus appeared which was completely different from all the viruses [that had] so far been isolated as A or A’. It was understood later that its formula was A(H2N2). This virus caused a sudden and important pandemic with two to three million deaths, as well as a number of more minor episodes during the following years.
