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Serologic responses and patterns of antibody persistence in children given HPV-77 DE-5 or RA27/3 vaccine were compared with those in children who had experienced natural infection. The results indicated that both vaccines induce long-lasting immunity in most individuals. RA27/3 vaccine proved more immunogenic and resulted in higher antibody titers; after 11 years 95% of those who seroconverted who were tested had hemagglutination-inhibiting (HAI) antibodies and 100% had neutralizing (NT) antibodies. Titers in HPV-77 DE-5 vaccinees were lower, and in 16% of those whose immune responses were feeble, HAI titers decreased to less than 1:8 after nine to 12 years; however, when their sera were examined by the more sensitive latex-agglutination test, all but two were found to have specific rubella antibody at levels of 1:1 to 1:16. Natural infection with wild virus was shown to induce more vigorous immune responses than did either vaccine: antibody titers were higher, decreases were smaller, and none fell to less than 1:8 (HAI) or less than 1:4 (NT).

In general, hemagglutination-inhibiting (HAI) and neutralizing (NT) antibodies to rubella virus tend to parallel one another, and the presence of either antibody has been interpreted as reflecting protective immunity. It has recently been shown that persons who have HAI antibody but lack NT antibody may be subject to reinfection. In the present study these two antibodies were compared with regard to their appearance and persistence in vaccines and in those who have experienced natural infection. NT antibody appeared more slowly following immunization with HPV77DE5 or Cendehill vaccines than after natural infection or immunization with RA 27/3 vaccine. As with natural infection, initial NT antibody responses to RA 27/3 vaccine were of higher titer and persisted at higher levels for three years than was the case with either of the other vaccines. NT testing procedures were found to differ in sensitivity, depending on several factors including the cells in which the virus was grown.