Alternative immunological markers to document successful multiple smallpox revaccinations

Successful smallpox vaccination is traditionally defined by clinical response ("take"). Nevertheless, only 60% of subjects in the 2002 Israeli smallpox revaccination campaign developed clinical take. More sensitive immunological markers are needed to document successful revaccination. We compared the level of vaccinia-specific immune markers among subjects who did or did not develop clinical take following revaccination.

Forty subjects who participated in 2002 smallpox revaccination campaign and developed clinical take were individually matched for age, sex, and smallpox vaccinations with subjects who did not develop clinical take ("no-take"). Vaccinia immunity markers were examined prior to and 14 days and 2 years after revaccination.

Higher levels of total immunoglobulin (Ig) G, IgG1, and neutralizing antibodies (highest dilution of serum that inhibited the cytopathic effect by at least 50% [PRNT50]) were observed in the no-take group before vaccination (166 vs 94.3 ELISAU/mL [P<.05], 53.2 vs 34.5 ELISAU/mL [ P<.05], and 30% vs 19.7% [P<.05], respectively). The mean time since last smallpox vaccination was longer in the take group than in the no-take group. Total IgG, IgG subclasses, avidity index, and PRNT50 levels were higher among "take" than "no-take" volunteers 14 days after revaccination. The no-take group was not inferior to the take group in all vaccinia immune marker levels measured 24 months after vaccination. Moreover, mean interferon-γ secretion and the percentage of serum samples with PRNT50 levels ≥1:32 were significantly higher in the No-take group than in the take group (677.5 vs 282.7 pg/mL [P < .05] and 95% vs 80% [P<.05], respectively).

The overwhelming majority of subjects in the no-take group were successfully revaccinated against smallpox. Under circumstances of emergent smallpox mass immunization, there is no need for revaccination success assessment among individuals who have received multiple previous smallpox vaccinations.