The humoral and cellular immune response to the administration of OrthopoxVac vaccine to volunteers

OrthopoxVac, a fourth-generation smallpox vaccine, was the first of its kind registered worldwide in 2022, and it has been shown to be both safe and to induce only a mild reaction. A six-month clinical study confirmed its immunogenicity as compared to the first-generation live smallpox vaccine. Our study aimed to determine the levels of specific humoral and T-cell immune responses in volunteers following intradermal OrthopoxVac vaccine administration either in a single dose of 107 PoFU or in two doses of 106 PoFU, at 1.5, 3, and 5 years after initial vaccination. Following the immunization of volunteers with the OrthopoxVac vaccine at a dosage of 107 PoFU, the T-helper response remained at a relatively high level for three years, before it significantly dropped. Administration of the same vaccine twice at a dose of 106 PoFU resulted in a considerable decrease in the level of T-helpers, after 1.5 years. Additionally, some patients exhibited a reduction in viral neutralizing antibody (VNA) titers after 1.5 years of OrthopoxVac vaccine administration. When OrthopoxVac was administered at a dosage of 107 PoFU, no substantial differences were noted between groups at the 1.5-, 3-, and 5-year marks. In contrast, in the groups receiving two doses of 106 PoFU, VNA titers showed a significant reduction after 1.5 years. These findings indicate that a single intradermal dose of 107 PoFU of the OrthopoxVac vaccine elicits a significant and lasting immune response involving both antibodies and T-cells for a minimum of three years.