Acta path. microbiol. scand. Sect. B, 84: 373-378, 1976

ANTIBODIES TO INFLUENZA A/SWINE-LIKE VIRUSES (HswlNl) IN HUMAN SERA: ANTIGENIC STIMULATION AND CHANGES IN ANTIBODY STATUS REIJOPYHALA Central Public Health Laboratory, Department of Virology, 00280 Helsinki 28, Finland

Pyhala, R. Antibodies to influenza uswine-like viruses (HswlNl) in human sera: antigenic stimulation and changes in antibody status. Acta path. microbiol. scand. Sect. B, 84: 373-378, 1976. Human sera collected at the end of 1975 were studied for H I antibodies to influenza u M a y o Clinic/103/74 ( H s w l N l ) . The frequent Occurrence of these antibodies in subjects born before 1930 suggested that they are present in about 25 per cent of the Finnish population. During the H3N2 epidemic in winter 1975-76 a low response of antibodies to uswine-like viruses was recorded in about half of the influenza patients with a pre-existing titre, but not in any of the patients without pre-existing antibodies. A comparison of samples collected in 1969 and 1975 suggested that at the community level the antibodies to uswine-like viruses have not decreased during the era of the H3N2 subtype. Key words: Influenza A (HswlNl) ; antibody status; antigenic stimulation.

R. Pyhala, Central Public Health Laboratory, Department of Virology, 00280 Helsinki 28, Finland.

Received 22.vi.76

Accepted 6.vii.76

Haemagglutination-inhibiting (HI) antibodies to influenza A /swine-like viruses (HswlN1 subtype) are frequently found in sera of humans born before 1930 (5, 8, 9, 14, 18). There is a general agreement that these antibodies originally resulted from infections with the A /swine-like viruses, which are believed to have caused the 1918 pandemic of influenza. According to the principle of the “original antigenic sin” (5, 7, 8), subsequent infections with viruses of HONl and H l N l subtypes maintained the level of anti-A/ swine-like antibodies in the subpopulation of subjects born before 1930. I t was reported, however, that exposure to viruses of H2N2

and H3N2 subtypes, responsible for influenza A epidemics since 1957, did not evoke an antibody response to earlier subtypes (13, 14, 15, 21, 22). Somewhat contrary to these negative findings, a weak increase of anti-A/ swine-like antibodies was depicted as a response to H2N2 influenza vaccine in about half of 47 vaccinated volunteers (10). Due to the lack of a proper antigenic stimulus one would expect to find a decrease in the level of antibodies to HswlNl viruses in the abovementioned subpopulation. Indeed, this was demonstrated by comparing samples of sera collected in 1958 and 1967 (14). Because of the outbreak of an HswlNl infection in a military camp in New Jersey in

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Fig. 1 . H I antibodies to influenza A/Mayo Clinic/103/74 ( H s w l N l ) and A/Port Chalmers/l/73 H3N2) in sera collected in autumn 1975. Geometric mean titres ( G M T ) connected with lines (individual titres of