Influenza subunit vaccine: antibody responses to one and two doses of vaccine and length of response, with particular reference to the elderly.

Antibody responses to subunit influenza vaccine prepared against A2/England/42/72 (h3n2) were studied in 69 volunteers aged 60 and over and 231 aged 59 and below over 12 months in 1973 and 1974. After two doses of vaccine seroconversion frequencies and geometric mean haemagglutination-inhibition (HI) titres were higher in the elderly, but no differences were observed between the two groups in the length of their responses. Sixteen (23%) of the elderly volunteers seroconverted only after receiving a second dose of vaccine or seroconverted twice after receiving both doses of vaccine. It was considered justifiable, therefore, to recommend the continuation of a two-dose schedule for patients in a high-risk category. Within 30 weeks of vaccination 87 (29%) volunteers had considerably reduced HI titres (less than 48), which might indicate potential susceptibility to influenza during an epidemic, and the number had risen to 132 (44%) by 50 weeks. It was suggested that high-risk patients should receive annnual vaccination two to four months before the possible epidemic period.