Abstract
In the early stage of the Asian influenza pandemic of 1957, several virus strains were isolated from patients among the inhabitants in Singapore and among the American marines of the Far East bases. Hemagglutination-inhibition (HI) test for influenza virus showed that these strains did not share a common antigen with any other type A or B influenza virus which had been isolated before 1957. However, in sera of the patients significant antibody rises in complement-fixation (CF) test were observed when type A soluble (S) antigen was used (Meyer et al., 1957) . Afterwards, laboratory tests for diagnosis of Asian influenza were carried out as a rule throughout the world using Asian strain as antigen for HI test or S antigen of type A for CF test.
In 1958, Asian influenza virus was designated A2 sub-group of influenza type A virus by the Expert Committee on Respiratory Virus Diseases (Expert Committee on Respiratory Virus Diseases, 1959) .
Later, Choppin et al. (1958) reported the existence of a minor common antigen between A2 virus and other type A viruses, which was discovered by HI test. They also observed that a few patients infected with A2 influenza were found to have a positive antibody response in HI test not only to A2 but also to Swine influenza or FM1 strain. Jensen et al. (1958) also reported that in a few patients infected with A2 influenza rises of antibody against A1 or A were proved in HI test or in CF test by virus (V) antigen. At the same time they reported that some patients did not show any antibody response to A2 but, in a certain grade, positive response to some other type A virus. Boger et al. (1957) observed that a few individuals among a group of people who received a polyvalent influenza vaccine which did not contain A2 strain showed antibody rises even to A2. On the contrary, Hilleman et al. (1958) reported a few positive cases even to A1 or Swine virus among a group of people who received an A2 monovalent vaccine.
At the outbreaks of influenza epidemic of 1960 in Japan, the author obtained similar results as mentioned above that, in testing some paired sera collected from patients, rises of antibody were proved against both A1 and A2 by HI test. However, all viruses isolated from these patients were found to belong to A2, and no A1 virus was isolated at all. From the foregoing, it seemed to be one of the most important problems to clarify whether or not a common antigen really exists between A1 and A2 viruses, although sera of patients indicated antibody rises to both antigens as mentioned above.
The present paper describes the results of an antigenic analysis of A1 and A2 influenza viruses carried out in our laboratory using HI test and neutralization test in eggs. From these results it can safely be said that these viruses of two sub-groups may possess a minor common antigen as expected. In addition to the results, the cases which were positive to A 1 and A 2 in HI test during the epidemic of influenza from 1957 to 1962 in Japan are presented and discussed.
