Rapid protection in a monkeypox model by a single injection of a replication-deficient vaccinia virus

Abstract

The success of the World Health Organization smallpox eradication program three decades ago resulted in termination of routine vaccination and consequent decline in population immunity. Despite concerns regarding the reintroduction of smallpox, there is little enthusiasm for large-scale redeployment of licensed live vaccinia virus vaccines because of medical contraindications and anticipated serious side effects. Therefore, highly attenuated strains such as modified vaccinia virus Ankara (MVA) are under evaluation in humans and animal models. Previous studies showed that priming and boosting with MVA provided protection for >2 years in a monkeypox virus challenge model. If variola virus were used as a biological weapon, however, the ability of a vaccine to quickly induce immunity would be essential. Here, we demonstrate more rapid immune responses after a single vaccination with MVA compared to the licensed Dryvax vaccine. To determine the kinetics of protection of the two vaccines, macaques were challenged intravenously with monkeypox virus at 4, 6, 10, and 30 days after immunization. At 6 or more days after vaccination with MVA or Dryvax, the monkeys were clinically protected (except for 1 of 16 animals vaccinated with MVA), although viral loads and number of skin lesions were generally higher in the MVA vaccinated group. With only 4 days between immunization and intravenous challenge, however, MVA still protected whereas Dryvax failed. Protection correlated with the more rapid immune response to MVA compared to Dryvax, which may be related to the higher dose of MVA that can be tolerated safely.

Fig. 1.

Vaccine-induced immune responses. Monkeys received MVA (1 × 10⁸ pfu) intramuscularly or Dryvax (2.5 × 10⁵) by skin scratch. The animals were bled on the indicated days and serum and mononuclear cells were obtained for antibody and T cell analysis, respectively. (A) Serum ELISA titers were determined by using 96-well plates coated with purified VACV. The numbers of animals at each time point were 4 (M and D) and 2 (N). Groups determined by vaccination status: M, MVA; D, Dryvax; N, naïve unvaccinated. Averages with standard error are plotted. (B) Neutralizing antibodies were determined by using a flow cytometer to measure reduction in infectivity of VACV-expressing green fluorescent protein. (C) Fresh peripheral blood mononuclear cells were infected with VACV in the presence of brefeldin A overnight and then stained with antibodies to CD3, CD8, and IFN-γ conjugated to phycoerythrin, fluorescein isothyocyanate, and allophycocyanin, respectively, for analysis on a cytometer.

Fig. 2.

Viral load and survival after MPXV challenge. (A–E) The number of MPXV genomes per ml of blood was determined by quantitative TaqMan 3′-minor groove binder PCR. The lower limit of detection was 5000 genomes/ml. Average values with standard error are shown. Survival curves for each group are shown in the Insets. Groups determined by vaccination status: M, MVA; D, Dryvax; N, naïve unvaccinated. Challenge with 5 × 10⁷ pfu of MPXV at 30 (A) and 10 (B) days after vaccination. Four naïve unvaccinated animals were challenged with this dose of MPXV and are included in the above panels. Challenge with 5 × 10⁶ pfu of MPXV at 10 (C), 6 (D), and 4 (E) days after vaccination. Six naïve unvaccinated animals were challenged with this dose of MPXV and are included in panels C–E. The immune responses in Fig. 1 were determined from the same animals used in panel A of this figure.

Fig. 3.

Antibody responses after MPXV challenge. ELISA titers were performed as in Fig. 1 with samples from monkeys challenged 4 days after immunization. Groups determined by vaccination status: M, MVA; D, Dryvax; N, naïve unvaccinated.