Comparison of the Effectiveness of Transepidemal and Intradermal Immunization of Mice with the Vacinia Virus.

The spread of the monkeypox virus infection among humans in many countries outside of Africa, which started in 2022, is now drawing the attention of the medical and scientific communities to the fact that immunization against this infection is sorely needed. According to current guidelines, immunization of people with the first-generation smallpox vaccine based on the vaccinia virus (VACV) LIVP strain, which is licensed in Russia, should be performed via transepidermal inoculation (skin scarification, s.s.). However, the long past experience of using this vaccination technique suggests that it does not ensure virus inoculation into patients' skin with enough reliability. The procedure of intradermal (i.d.) injection of a vaccine can be an alternative to s.s. inoculation. The effectiveness of i.d. vaccination can depend on the virus injection site on the body. Therefore, the aim of this study was to compare the development of the humoral and cellular immune responses in BALB/c mice immunized with the LIVP VACV strain, which was administered either by s.s. inoculation or i.d. injection into the same tail region of the animal. A virus dose of 105 pfu was used in both cases. ELISA of serum samples revealed no significant difference in the dynamics and level of production of VACV-specific IgM and IgG after i.d. or s.s. vaccination. A ELISpot analysis of splenocytes from the vaccinated mice showed that i.d. administration of VACV LIVP to mice induces a significantly greater T-cell immune response compared to s.s. inoculation. In order to assess the protective potency, on day 45 post immunization, mice were intranasally infected with lethal doses of either the cowpox virus (CPXV) or the ectromelia virus (ECTV), which is evolutionarily distant from the VACV and CPXV. Both vaccination techniques ensured complete protection of mice against infection with the CPXV. However, when mice were infected with a highly virulent strain of ECTV, 50% survived in the i.d. immunized group, whereas only 17% survived in the s.s. immunized group. It appears, therefore, that i.d. injection of the VACV can elicit a more potent protective immunity against orthopoxviruses compared to the conventional s.s. technique.