Effect of low level immunity on response to live rubella virus vaccine

Serum specimens of 1075 young adults representing nursing and medical staff were collected for determination of rubella immunity using the radial haemolysis (RH) technique, also know as the haemolysis in gel (HIG) test. Of the sera 84 (7.8%) were negative (RH titre less than 4 mm) and an additional 93 (8.7%) gave an RH titre of 4-5 mm, which was regarded as the limit of immunity. Initially, 64 persons of these 177 were vaccinated with the RA27/3 strain of live attenuated rubella virus. Their serum samples were collected at the time of vaccination and at three weeks and three months after vaccination. Altogether 54 vaccinees could be followed for their immune response throughout the study. It became obvious that vaccination generally caused seroconversion only after three months rather than within three weeks. Only one person remained seronegative - even after a booster dose of vaccine. The mean final antibody titres in individuals with pre-existing low level immunity was 6 mm whereas in initially seronegative persons the antibody titre after vaccination was 8.5 mm on average. Thus a pre-existing low level immunity will effectively block the immune response to live rubella virus vaccine and this phenomenon may explain some apparent vaccination failures.