Sero-epidemiological survey on human and swine influenza A. Situation at the onset of summer 1976.

A serological survey was performed to detect the presence of HI antibodies against the swine-like A/New Jersey/76 (Hsw1 N1) and the human A/Victoria 3/75 (H3N2) virus strains in the sera of 700 subjects of different age, and of 244 swine. HI antibodies against A/New Jersey/76 strain were not detected in 308 subjects younger than 31 years. They turned out to be present in 8% of the subjects from 31 to 43 years, always at low titre, and at a different titre, in 50% of the subjects from 44 to 58 years and in 90% of the subjects older than 58 years. None of the 244 swine tested from breeding farms of Liguria, Piedmont and Lombardy turned out to be endowed with antibodies against A/New Jersey/76 virus. This shows that the swine-like virus, which caused infections in the human population up to about 1930, did not circulate any more subsequently and that the swine also are now free from this infection. As far as the A/Victoria 3/75 strain is concerned, HI antibodies were detected in the different age groups with a frequency ranging between 41 and 69%. The fact that in many subjects antibodies were present at low titre suggests that their presence should be only partially brought back to infections caused by A/Victoria 3/75 but that in many cases it is the expression of a heterologous response to previous infections caused by other A H3N2 strains. HI antibodies against A/Victoria 3/75 were found also in 13% of the swine examined. A group of older people, who had been regularly immunized for years with the inactivated (A H3N2-B vaccine) showed an HI antibody titre significantly higher in comparison with subjects of equivalent age not vaccinated both against A/Victoria 3/75 and A/New Jersey/76.