The prevalence of antibodies against A/New Jersey / 76, A / Victoria / 3 / 75 and B / Hong Kong/5/72 influenza viruses in the population of Milan examined late in the spring of 1976 and evaluation of the past experience of this population of the probable agent of the 1918-20 pandemic.

: Hemagglutination inhibiting antibodies (h.i.a.) to influenza viruses A/New Jersey/76, A/Victoria/3/75 and B/Hong Kong/5/72 were titrated in 227 serum samples collected in Milan in the late spring 1976 from healthy subjects of 0 to 90 years of age. In 185 of these neuraminidase-inhibiting antibodies (n.i.a.) for the type A strains were also titrated. Subjects up to 40 years of age lack h.i.a. to A/New Jersey/76 strain; antibody levels equal to or greater than 1:40 are occasionally present in the 41-50 year group and the incidence rises to 75% and more after 60 years. N.i.a. are absent in the sera from persons of 0-30 years and titers equal to or greater than 1:100 were found only in the age group over 40. Of the sera examined 44% is lacking in h.i.a. to A/Victoria/3/75 strain and antibodies at protective levels were found in 14% of the subjects, usually among the youngest (0-15 years) and the elderly (over 60 years). N.i.a. for this strain are present in 40% of the samples and 10% have titers equal to or greater than 1.100. Half of the sera under test has h.i.a. to B/Hong Kong/5/72 strain, but only 19% with titers equal to or greater than 1:40. The age distribution of h.i. and n.i. antibodies to A/New Jersey/76 virus suggests that strains antigenically related to the swine virus were widespread in our population until about 1929. The presence of n.i.a., but not of h.i.a. in subjects born between 1935 and 1946 supports the hypothesis of cross-reaction between the neuraminidases of the swine and of the A-O strains.