Influenza A (H3N2) in Finland: Changes of Antibody Status during 1968-72

Abstract

Haemagglutination-inhibiting (HI) antibodies to influenza A (H3N2) were determined in sera from adults and children, collected in 4 successive years before the epidemic season. In addition, influenza A antibody levels were determined by HI and complement fixation (CF) techniques in paired sera of pregnant Rh-negative women before and at the end of the 1971–72 epidemic. The percentage of seronegative (HI<12) adults fell from 92% in 1968 to 24% in 1970. The children exhibited a similar pattern. In autumn 1971, before the third Hong Kong epidemic, the antibody status resembled the situation before the second epidemic in 1969. Seroconversion or 4-fold titre rise was seen in 28.5% of the serum pairs collected before and after the epidemic of 1971–72. All significant antibody responses were seen in persons with a pre-epidemic HI titre of ≤ 24 or CF titre of ≤ 8.