Persistence of anti‐HBs and protective efficacy after administration of hepatitis B vaccine

To evaluate the effect of hepatitis B vaccine on the persistence of anti-HBs and its efficacy in preventing hepatitis type B, anti-HBs and anti-HBc levels were studied over a period of 5.5 years. Plasma-derived hepatitis B vaccine, containing 20 μg of HBsAg protein, was injected subcutaneously in 122 healthy medical staff members, followed by two identical injections 1 and 6 months later. Anti-HBs and anti-HBc levels were then measured by radio-immunoassay. The anti-HBs titres were expressed as the sample/negative (S/N) ratios, and an S/N ratio of more than 2.1 was considered positive. The mean (and s.d.) anti-HBs titre peaked 7 months after the first vaccination with an S/N ratio of 153.6 ±149.8, after which it decreased with time. The mean anti-HBs titre dropped to an S/N ratio of 8.0 ±5.1 5.5 years after the first vaccination. The percentage of vaccinees who were anti-HBs positive also gradually decreased with time after a peak of 84.2% at 7 months following the first vaccination. The percentage of vaccinees who were anti-HBs positive was 38.9% 5.5 years after the first vaccination. The anti-HBc level was not positive in all subjects during the observation period. Five vaccinated volunteers who had developed anti-HBs after the basic vaccination, but whose acquired antibody level became negative within 4.5 years following the first vaccination, were administered a booster dose of 20 μg of HBsAg 4.5 years after the first vaccination. Only one of these subjects did not respond to the booster vaccination. The peak anti-HBs titre of this subject was very low following the initial vaccination. From these observations, it was concluded that, although the acquired antibody titres and percentage of vaccinees who were anti-HBs positive decreased with time, the incidence of hepatitis B infection would be lower in vaccinees than that in other non-vaccinated general medical staff members in Japan. This was probably due to the persistence of immunological memory in vaccinees who had responded well after basic vaccination.