The results of the current study suggest that IMVAMUNE®
is safe, well-tolerated and immunogenic at doses of 2 × 107
to 1 × 108
and attenuates the cutaneous Dryvax®
reaction when given by either the SC or IM routes. In general, there were no significant differences between 1 × 108
per ml IMVAMUNE®
when given by the IM or SC routes except for a significant difference in the number with mild induration in SC group. Slightly fewer cases of erythema and induration were observed after the first or second IMVAMUNE®
injections given IM compared to SC. In a recent trial [9
] to evaluate two doses of IMVAMUNE®
given subcutaneously or intramuscularly on Days 0 and 28, IMVAMUNE®
was found to be generally well-tolerated albeit, a higher incidence of systemic symptoms was reported in the IM versus the SC group.
given prior to Dryvax®
reduces virus replication at the site of vaccination, local reactogenicity and the time to healing. Although MVA may reduce the size of a primary cutaneous lesion after vaccination with Dryvax®
, it does not completely ameliorate the response. Partial attenuation of primary takes in some vaccinia experienced individuals receiving MVA [2
] or replicating virus [11
] vaccination has been documented in previous studies and may depend on the level of pre-existing immunity. While heterologous vaccination protects animals from a variety of generalized orthopoxvirus infections, Dryvax®
does not completely protect against the formation of skin lesions after a second vaccination [15
] nor did it protect against monkeypox during the Wisconsin outbreak [16
]. Lack of complete protection against skin lesions after vaccination may be dependent on the high dose of inoculum in the skin (e.g. vaccinia vaccination) versus a small dose via the respiratory tract (e.g. variola infection) [15
]. Nonetheless the partial amelioration of the Dryvax®
lesions provides limited evidence that two doses of MVA may be protective against vaccinia [17
]. Further studies to determine the efficacy of MVA against other orthopox viruses such as variola and monkeypox are warranted.
In this study a dose response effect was observed for both PRNT and ELISA titers after each vaccination. For the IFN-γ producing ELISPOT assay, a dose response was apparent for the first and second vaccination but not the third vaccination. The CMI response after two doses of IMVAMUNE® compared favorably with one dose of Dryvax® alone, indicating that IMVAMUNE® inoculation did not attenuate T-cell IFN-γ responses to Dryvax®, and that diminished skin response to challenge with Dryvax® suggests that the T-cell responses generated during priming were protective in nature. In addition, following two priming doses of IMVAMUNE®, Dryvax® did not boost T-cell responses after 60 days between vaccinations, however, earlier intervals when a boost in T-cell responses may have been detectable were not evaluated.
When comparing assays, the antigen chosen to perform the antibody assay, i.e. MVA or Dryvax®, affects the magnitude of the GMTs after IMVAMUNE® or Dryvax® vaccination. When MVA is used as the assay antigen, significantly higher GMTs occur post first and second dose of IMVAMUNE® compared to responses when Dryvax® is used as the assay antigen, and a diminished response is observed in subjects receiving Dryvax® alone. When Dryvax® is used as the assay antigen, there is a significantly higher antibody response in naive individuals receiving a single dose of Dryvax® compared to individuals receiving an IMVAMUNE® prime.
One subject in the placebo/placebo/Dryvax® group appeared to be antibody positive after the second dose of placebo, but this phenomenon only occurred when MVA was used as the neutralizing antigen. Although the antibody titer to MVA was low (49) a PRNT titer of ≥20 was considered to be seropositive, regardless of the antigen used for the PRNT assay. Previously, the appropriate threshold to use to consider a serum sample positive for vaccinia antibody was evaluated. It was found that the sensitivity and specificity were very similar regardless of whether <20 or < 50 was used as the cut-off value to be considered seronegative. The more conservative value of 20 is used as the threshold to determine serostatus, although occasionally a higher value is observed in individuals with no known exposure or vaccination with vaccinia.
These data may suggest that immunologic determinants for humoral immunity may be different for MVA versus replication efficient vaccinia such as Dryvax®
. Recent studies suggest that MVA does not produce all the major vaccinia proteins targeted by VIG [18
]. These investigators determined one protein to be the vaccinia gene fragment corresponding to the cowpox A-type inclusion body. They showed that this cowpox gene is highly conserved across poxvirus strains including variola major virus, India strain (92% homology). These data combined with the recent work on variola survivors [19
] suggest that there are differences in the humoral immune responses in humans following exposure to MVA, Dryvax®
or variola that may have important implications to the degree and duration of protective immunity. Robust cellular responses in the immediate years following vaccination may compensate for diminished humoral response, but the waning of circulating memory T-cells following MVA or other modified vaccinia-based approaches might result in less durable or less prolonged immunity seen historically in vaccinated individuals.
IMVAMUNE® priming acts to limit local reactogenicity to Dryvax® challenge. The power of the study was too small to evaluate whether IMVAMUNE® could decrease both systemic and local reactogenicity as well as serious adverse events. Larger studies would be needed to determine the timing of vaccination and the number of doses of IMVAMUNE® that are sufficient to protect against significant adverse events due to vaccination with replicating virus. However, studies to evaluate whether priming with MVA reduces the rate of uncommon or life threatening adverse effects of Dryvax® are not currently feasible. Clinical trials using MVA conducted during the 1970’s in Germany reported no severe adverse events in children and did not reveal an increased risk for immunocompromised persons. Further trials are under development to evaluate the optimal route, dose and schedule for IMVAMUNE® priming followed by replicating vaccinia boost and to evaluate the protective immunity of IMVAMUNE® as a stand-alone smallpox vaccine without replicating vaccinia boost.