MVA was well tolerated among the 99 subjects who received a total of 179 injections of 106
pfu and no serious adverse events were attributed to MVA. Importantly, prior vaccination with MVA decreased the reactogenicity and improved the immunogenicity of Dryvax®
. This is consistent with the limited ability of MVA to replicate in mammalian cells [16
] and with the extensive evaluation of MVA in humans in the mid 20th
century that reportedly demonstrated MVA to have an excellent safety profile[5
The local and systemic reactogenicity of MVA was minimal. None of the rare idiosyncratic serious events associated with Dryvax®
inoculation were observed in these studies after either MVA or Dryvax®
. In particular, there was no evidence of cardiac complications that have been raised as a theoretical concern because of the myopericarditis occasionally seen after Dryvax®
]. The safety record of MVA should allow immunization of expanded populations of subjects including groups excluded from Dryvax®
inoculation. Larger trials should be performed including subjects at the extremes of age and those with eczema, atopy, or immunodeficiency in addition to larger numbers of healthy adults.
The efficacy of MVA was tested in this study by Dryvax®
challenge at 3 months after the vaccine regimen. Vaccinia-naïve volunteers who received Dryvax®
alone had responses typical of non-immune persons with scab formation by the end of the second week and scab separation by the end of the third week according to CDC guidelines [18
]. In a prior study, scab separation occurred closer to the end of the 4th
]. The difference in duration of lesion healing may have been affected by the dressing technique. In our subjects a sterile transparent dressing was initially applied and replaced by gauze pad under an occlusive dressing when the lesion became purulent. In the Lancet paper a simple non-occlusive dressing was applied.
Prior MVA immunization significantly reduced lesion severity and systemic reactogenicity induced by the Dryvax® challenge in vaccinia-naïve subjects, and two or three doses of MVA reduced the duration of vaccinia replication after Dryvax® challenge. These effects were less dramatic in vaccinia-immune subjects because of the significant preexisting residual immunity from prior Dryvax® vaccination. However, two doses of MVA, even in vaccinia-immune subjects, resulted in diminished vaccinia replication following Dryvax® challenge. Therefore, a 2-dose regimen is recommended for future studies that evaluate duration of protective immunity.
It was shown in reports from the 1970s that prior MVA immunization could reduce the reactogenicity of vaccinia inoculation. Those studies typically combined MVA with live vaccinia virus administered 1–2 weeks later as a porposed vaccination schedule. A theoretical concern of combining MVA with Dryvax is that diminished vaccinia-induced inflammation may reduce the immunity afforded by vaccinia vaccination. In the present studies, subjects were inoculated with vaccinia 3 months after completing the MVA vaccine regimen with the intent of challenging with live vaccinia virus in the memory phase of the initial MVA-induced immune response. We found in both vaccinia-naïve and vaccinia-immune subjects in these studies, two MVA doses of 106
pfus (adjusted dose) induced detectable T cell responses in the large majority of individuals, and post Dryvax®
inoculation the magnitude of vaccinia-specific CD8+ T response was significantly increased above subjects who received Dryvax®
alone. While the neutralizing antibody titer was not different between groups, there was an increase in the response to EEV proteins in the vaccinia-naïve individuals who received MVA. The EEV virion form is believed to be a determinant of long-range spread of the virus within the host[21
] and may be an important target for vaccine-induced immunity. The increased vaccinia-specific CD8+ T cell response and EEV antibody response indicates that 2 injections with this dose of MVA not only attenuates the clinical reaction to Dryvax®
inoculation but enhances Dryvax®
immunogenicity. A detailed characterization of the phenotype and functionality of the T cell response will be described elsewhere (Precopio et al., in submitted).
The dose of MVA may affect the antibody response to MVA and consequently the level of protection from the immediate Dryvax® challenge. The 106 adjusted dose did not elicit an appreciable antibody response prior to challenge which allowed limited vaccinia replication to occur which may account for the higher CD8+ T cell responses post-challenge in MVA recipients. Complete protection against vaccinia would be desirable for laboratory workers using recombinant viruses, and therefore higher doses of MVA should particularly be considered for occupational indications. However, partial immunity as noted in this study may have advantages if Dryvax® inoculations are needed to control a future smallpox outbreak. Since this was a schedule-finding study evaluating the number of MVA injections, future studies should include formal dose escalation using 2 MVA injections. These studies will be necessary to determine whether a higher dose of MVA will induce better antibody responses, to assess the level of protection against Dryvax®, and to define the duration of immunity.
We were not able to precisely define an immune correlate of protection, but induction of CD8+ T cell responses post MVA immunization and the magnitude of the CD8+ T cell response post Dryvax® challenge were associated with reduced severity of lesion formation following Dryvax® inoculation. Vaccine-induced immune protection from orthopoxviruses in mice involves both antibody and CD8+ T cell responses, which are dependent on or improved by CD4+ T cell responses. Immunity is lost in murine models only when CD4+ T cells, CD8+ T cells, and antibody are all defective or depleted [8
]. Antibodies have been shown to be an important factor in controlling monkeypox infection after intravenous challenge in macaques, but the relative importance of antibody versus CD8+ T cell for protection from a local inoculation was not evaluated [23
]. Additional studies will be needed to establish the respective roles of CD8+ T cells and antibody in the speed of resolution of skin lesions caused by cutaneous inoculation or protection from a mucosal or aerosol challenge with orthopoxviruses.
While the efficacy of MVA in protecting against a challenge with a live virus vaccinia vaccination was tested in these trials, the efficacy of MVA against smallpox cannot be evaluated in humans. Therefore the FDA has adopted the “Animal Rule” which allows for potential licensure of products when efficacy testing is not possible in humans. This process involves extensive efficacy testing in well characterized animal models and defining correlates of protection that can be used as endpoints for evaluating the product in humans [24
]. As an adjunct to the “Animal Rule” approach, consideration should be given for testing MVA as a vaccine against monkeypox which is still endemic in parts of equatorial Africa. During the 2003 outbreak of human monkeypox in the Republic of Congo, 11 confirmed and probable monkeypox cases were observed along with human-to-human transmission of disease. Although no healthcare workers were infected during this outbreak, there was hospital-associated transmission of disease, indicative of the possibility of larger hospital-associated monkeypox outbreaks in the future which would leave healthcare workers and their families particularly vulnerable [25
]. Similar examples of extended serial human-to-human transmission have been demonstrated in Democratic Republic of Congo (personal communication, J.J. Muyembe.) MVA has an excellent safety profile and is replication deficient; it would be expected to be safe in populations where there is a high prevalence of HIV or other conditions that would require pre-screening for Dryvax®
inoculation. Evaluating MVA immunization in a setting with a high incidence of monkeypox would provide additional guidance for how to use MVA either alone or in combination with Dryvax®
for protection against orthopoxviruses.
Our data support further evaluation of immunizing the general population with 2 doses of MVA as a safe way to provide a platform of orthopoxvirus immunity. MVA should also be considered as a method of protecting the growing number of vaccinia-naïve laboratory workers from recombinant orthopoxviruses in biomedical research. Whether MVA immunization alone would be sufficient to protect against variola as it does against monkeypox or variola in macaques would not be known until faced with a crisis. Therefore, emergency plans should also be established for Dryvax® immunization of those persons without medical contraindications in the event of a smallpox outbreak. Our data suggest that prior MVA immunization will make vaccination with Dryvax® safer and more immunogenic.