Immunofluorescence test for persistent poxvirus antibodies

Summary

A collection of 408 human sera and 165 monkey sera were examined for vaccinia and monkeypox antibodies respectively by immunofluorescence (IF). Neutralization (N) was performed for comparison in 266 of the human and all monkey sera.

Sera of persons examined within 10 years after vaccination showed similar results with IF and N. Only one of 93 sera tested in this period showed discordant results, being IF−N+; 91 sera were IF+N+ and one IF−N−.

Out of 73 samples, which were taken more than 10 years after vaccination, 74 per cent were IF+ and 77 per cent N+. Discordant results were obtained almost exclusively in these late samples, 8 of which were IF−N+ and 7 IF+ N−. The median titre of IF decreased from 512 in the most recently vaccinated group to 32 in the group which was vaccinated ≥51 years previously. The median titre of N decreased from 361 to 28 in the same period.

No positive reactions were found in 138 human pre-immunization sera by IF, neither in 100 of these pre-sera when tested by neutralization. Sera of 154 wild-caught monkeys were negative with one exception being IF+ N−in dilutions up to 8.

Immunofluorescence appeared a specific and sensitive test for persistent vaccinia antibodies.