Duration of neutralizing antibody persisting in Thai individuals after childhood vaccination against smallpox.

BACKGROUND: Although smallpox was completely eliminated by 1980, it remains possible that variola virus could be intentionally released in an act of bioterrorism. Thus, several studies have been performed to detect antibody levels after smallpox vaccination of the current population in various countries to indicate the duration of maintenance of immunological memory. Our study endeavored to investigate the level of neutralizing (Nt) antibody responses of Thai individuals who had been immunized with smallpox vaccine during childhood.

METHODS: The plaque reduction neutralization test (PRNT) was used to study vaccinia Nt antibody responses in sera of individuals ranging in age from 35-4, 45-54, 55-64, 65-74, 75-84 and > 84 years old, referred to as groups 1-6, respectively. Each group included 200 sera: 100 male sera and 100 female sera.

RESULTS: An incubation time of 15 hours for sera and vaccinia virus was confirmed to be the optimal incubation period for PRNT. Positive Nt antibody titers (≥32) were detected in 135 (11.25%) of 1,200 sera: 81 (6.75%) male sera and 54 (4.5%) female sera. There were 4 (2%), 11 (5.5%), 19 (9.5%), 16 (8%), 33 (16.5%), and 52 (26%) positive sera in groups 1-6, respectively. Interestingly, the oldest individual with positive Nt antibody was a 98-year-old female. Two males aged 96 and 91 years old had the highest Nt antibody titers.

CONCLUSIONS: Our data suggests that the vaccinia-specific Nt antibody response in the current Thai population could be maintained for more than 90 years after vaccination. However, the majority of the Thai population aged ≥35-74 years old is still highly susceptible to infection.