Concerns have arisen recently about the possible use of
smallpox for a bioterrorism attack. Routine
smallpox vaccination was discontinued in Japan in 1976; however, it is uncertain exactly how long vaccination-induced immunity lasts. We sought to evaluate the seroprevalence and intensity of anti-
smallpox immunity among representatives of the present Japanese population. The subjects included 876 individuals who were born between 1937 and 1982. Vaccinia virus-specific
immunoglobulin G (
IgG) levels were measured by
enzyme-linked
immunosorbent assay (ELISA), and 152 of 876 samples were also tested for the presence of
neutralizing antibodies. Of the subjects who were born before 1962, between 1962 and 1968, and between 1969 and 1975, 98.6, 98.6, and 66.0%, respectively, still retained the vaccinia virus-specific
IgG with ELISA values for optical density at 405 nm (OD(405)) of > or = 0.10. The corresponding figures for retained IgGs with OD405 values of > or = 0.30 were 91.0, 90.3, and 58.2%, respectively.
Neutralizing antibodies were also maintained. The sera with OD(405) values of > or = 0.30 showed 89% sensitivity and a 93% positive predictive value for detection of
neutralizing antibodies (> or = 4). Thus, approximately 80% of persons born before 1969 and 50% of those born between 1969 and 1975 were also found to have maintained
neutralizing antibodies against
smallpox. A considerable proportion of the previous vaccinated individuals still retain significant levels of
antiviral immunity. This long-lasting immunity may provide some protective benefits in the case of reemergence of
smallpox, and the disease may not spread as widely and fatally as generally expected.