Modified Vaccinia Ankara: Potential as an Alternative Smallpox Vaccine
- Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland
- Dr. Barney S. Graham, Vaccine Research Center/NIAID/NIH, 40 Convent Dr., MSC 3017, Bldg. 40, Rm. 2502, Bethesda, MD 20892-3017 (bgraham{at}nih.gov).
Abstract
Despite the declaration of smallpox eradication in 1980, the existence of variola stockpiles and the threat of bioterrorism demand that immunity to smallpox through vaccination be maintained. Although the currently available vaccine was used for the most successful medical intervention ever accomplished, it also is associated with side effects that are difficult to accept in a vaccine for a disease that has not been present for >25 years. Herein, we review alternative approaches to maintaining immunity to smallpox through vaccination with attenuated poxviruses, and we suggest modified vaccinia Ankara (MVA) as a leading candidate for an alternative smallpox vaccine.
Recent events in the world have elicited concern over the introduction of poxviruses into the human population. In response to the need for enhanced immunity in our communities, vaccination programs and new strategies to protect against smallpox and other orthopoxviruses have emerged. The search for a safer, equally efficacious vaccine has led to the investigation of an attenuated vaccinia as a potential second-generation smallpox vaccine.
After the events of 11 September 2001, concern over the use of bioweapons as agents of terrorism increased. This trepidation became reality only months later with the diagnosis of 22 cases of anthrax in 7 states, including 5 fatalities [1]. Although multiple infectious organisms have been discussed as potential agents of bioterrorism, the possibility of a release of variola virus, the agent of smallpox, has elicited the most fear among scientists and political advisors. Although the World Health Organization declared successful global eradication of variola in 1980, there is information that suggests that several countries possess stockpiles of variola [2]. The reintroduction of variola virus could lead to substantial mortality: historical data suggest that death occurs in up to 30% of exposed nonimmune persons [3]. With the discontinuation of routine smallpox vaccination in the United States in 1972, nearly one-half of the estimated 293 million Americans (those aged <31 years) are likely to be vaccinia naive and vulnerable to a smallpox outbreak (US Census [http://www.census.org]). In addition, waning immunity in previously vaccinated individuals may leave the overall community even more susceptible in the event of a terrorist attack.
Many discussions have been held regarding the best approach to protect vulnerable citizens from a potential smallpox outbreak. With the successful eradication of naturally occurring smallpox, an outbreak of smallpox would imply that an intentional release occurred in the setting of either warfare or bioterrorism. On 13 December 2002, US President George W. Bush outlined a national plan that involves the initial vaccination of military personnel and health care first-responders followed by voluntary vaccination of the US population in 2004. Many concerns have been raised about the safety of the widespread use of the US Food and Drug Administration (FDA)–licensed vaccine Dryvax (Wyeth) [4]. Although it is proven to protect against smallpox, studies from the 1960s indicate a rate of serious complications after Dryvax vaccination of 1 in 13,000 primary vaccinees to 1 in 800 primary vaccinees, including death in one per million vaccinees [5, 6]. Some medical providers are fearful of an even higher rate of complications if mass vaccination were instituted today because of the increased number of persons for whom Dryvax is contraindicated, including persons with eczematous skin disease and immunocompromised persons who have HIV infection or cancer or have received organ transplants [7].
The considerations for protecting the civilian population from acts of bioterrorism are distinct from those for protecting soldiers from biowarfare [8]. The general public lives in diverse geographic regions with diverse population densities and has different educational backgrounds and different degrees of access to health care services. Our citizenry is also diverse in age and underlying disease conditions. Therefore, we need a greater number of options for protecting the public from smallpox than we need for protecting the military and first-responder emergency personnel. But even among the healthiest individuals there are idiosyncratic reactions, such as encephalitis and myocarditis, that have limited enthusiasm for Dryvax. These concerns have prompted renewed interest in the development of a less virulent vaccine that can provide protection for the general population, including persons who are at risk for complications with the present day replication-competent vaccines.
Interest in the development of safer approaches to protect against smallpox is not a novel concept (figure 1). This is, in fact, what Edward Jenner did in 1796 by instituting the practice of vaccination, as opposed to the previously used method of variolation [9]. In the 1930s, Rivers [10] reported his investigation of an attenuated strain of vaccinia. Kempe et al. [11] successfully used this strain of vaccinia in young children with eczema during the 1960s. The vaccine was well tolerated and immunogenic in this at-risk population. Scientists have investigated other approaches to minimize the complications of routine vaccination, including alterations in the method of vaccine administration [12, 13], the use of less virulent replication-competent vaccinia strains, such as CVI-78 and LC16m8 [11–14], and the development of attenuated replication-defective virus strains, including NYVAC and modified vaccinia Ankara (MVA) [15, 16, 18]. In the 1970s, German scientists performed a series of studies in animals and humans using the attenuated vaccinia strain MVA [16–20]. Their experience with >120,000 humans suggests that vaccination with MVA is well tolerated, provides protection against a subsequent intradermal replication–competent vaccinia challenge, and, therefore, may provide a potential alternative to Dryvax as a smallpox vaccine [16]. However, as work on MVA progressed, the threat of smallpox waned. As a result, evaluation of MVA as an alternative smallpox vaccine was never accomplished in the setting of a smallpox epidemic. Now, with the risk of smallpox and concern for the safety of Dryvax use in the general population growing, the National Institutes of Health have developed a plan to further investigate MVA in phase I trials as a potential second generation smallpox vaccine.
Figure 1
Genealogy of vaccinia virus. Both vaccinia and variola are members of the orthopoxvirus genus in the family poxviridae (A). The significant cross-reactive immunity between members of the orthopoxviruses is the basis by which vaccinia virus immunization protects against smallpox. The commonly published strains of vaccinia are listed in panel B. The New York City Board of Health (NYCBH) strain was obtained from England shortly after Jenner's work on vaccinia in 1796; the Rivers and Wyeth strains were derived from the NYCBH strain. CVI-78 is a derivative of the Rivers strain. The WR strain was derived from the NYCBH strain after multiple passages in mouse brain. NYVAC was derived from Copenhagen strain. The LC16m8 strain was derived from the Lister strain (also called “Elstree strain”).
MVA was derived from CVA dermovaccinia that was originally passaged in Ankara, Turkey, alternating between donkeys and calves [17]. In 1953, German scientists obtained the CVA strain, and 5 years later, they began serial passages in chicken embryo fibroblasts. After >300 passages, changes in the plaque morphology of the attenuated strain were observed, and the virus was noted to be less virulent in mice than its parent strain CVA [17]. After the 516th passage, the virus was renamed “MVA” and has subsequently been passaged >570 times. During the process of attenuation, MVA lost ∼15% of its parent genome, including specific genes that regulate viral host range and other genes responsible for evasion of the host immune response [21–23]. As a result, MVA replication is extremely limited in mammalian cells [21, 24]. In vitro MVA infection demonstrates replication of both early and late genes and synthesis of viral proteins; however, in nonpermissive human cell lines, only immature virus particles are formed, and dissemination within the host does not occur [25, 26]. This characteristic of MVA offers a distinct safety advantage over replication-competent vaccinia strains that are able to disseminate within a host and also have the potential to be transmitted between hosts.
Much work has been performed that demonstrates the improved safety of MVA in vivo. A hallmark of vaccinia virulence is its neurovirulence, with death occurring after intracerebral challenge of mice. Multiple studies have demonstrated that intracerebral inoculation of MVA results in no obvious disease and that prior MVA immunization can, in fact, prevent the development of neurovirulence after replication-competent vaccinia challenge [18, 27]. Furthermore, the safety of MVA has been demonstrated in immunosuppressed animal models. Several studies have involved administration of high doses of MVA to irradiated mice and rabbits without the development of subsequent illness or detection of viral replication [27, 28]. Irradiated mice challenged with intravenous MVA show a lack of illness, yet they develop a delayed vaccinia-specific antibody response and are protected from challenge with the Elstree strain of vaccinia [27]. Safety of a high-dose MVA challenge has been demonstrated in immunocompromised rhesus macaques after total body irradiation, CD4 cell depletion, and measles infection. After exposure to MVA by various routes, there were no ill effects noted in the macaques, and autopsy evaluation demonstrated no replication of the virus [29]. These data suggest that the attenuation of MVA and subsequent loss of host range genes significantly reduce its virulence and pathogenesis in animals, and these data support the future evaluation of MVA in both healthy and immunocompromised human hosts.
Although an improved safety profile is paramount, the development of a new generation vaccine against smallpox will require demonstration of immunogenicity and clinical efficacy. The genome of poxviruses encodes soluble inhibitors of cytokines and chemokines, including IFN-γ, IFN-α/β, and TNF-α, which prevent appropriate receptor interactions and thereby allows for evasion of the host immune response [30–32]. Compared with other poxviruses, studies of MVA have demonstrated a loss in several genes that encode these inhibitors [22, 23]; however, most structural genes that encode for epitopes known to elicit neutralizing antibodies are maintained, including B5R, H3L, A17L, A27L, and L1R [33–35]. In addition, 3 human CD8+ cytotoxic T lymphocyte epitopes restricted to HLA-A*0201 that have recently been identified in the Copenhagen strain of vaccinia and variola are also found in MVA [36, 37]. Therefore, it raises the question of whether MVA may in fact be more immunogenic than Dryvax and supports the concept that MVA could be developed as an effective vaccine for the general population.
Prior studies of MVA in both small animal and primate models demonstrate the development of immunity and clinical protection against heterologous orthopox virus challenges, including variola challenges [16, 18–20]. Animal studies demonstrate a dose response following MVA immunization and suggest that, unlike replication-competent vaccinia, immunization by routes other than intradermal are effective [19–20, 38]. Studies involving rabbits demonstrate that a multidose vaccination regimen can enhance both immunogenicity and protection from illness [19]. Similarly, macaques immunized with a single intramuscular dose of MVA develop an attenuated illness on intravenous variola challenge. However, administration of 2 injections of MVA produces a greater humoral immune response and is more effective in preventing disease [20]. The optimal dosing regimen for MVA remains an unanswered question and one to be addressed by future studies.
Although most studies have assessed protection from systemic poxvirus infection through the evaluation of skin lesions and illness, another concern is the potential risk of an aerosolized smallpox attack by terrorists [39]. Predictions by mathematical models suggest widespread infection and morbidity would occur after successful aerosolization of the variola virus [40]. A recent study has compared the protection elicited by MVA and Wyeth vaccinia in a pulmonary model of vaccinia infection in mice [37]. With weight loss used as a marker of illness, MVA-immunized mice demonstrated a graded dose response following receipt of a single immunization of MVA and showed protection from an intranasal vaccinia challenge equivalent to that in mice immunized with Dryvax. In addition, models to evaluate poxvirus infection by aerosolization have been developed using macaques [41], and a recent study has importantly shown that MVA immunization offers significant protection against a lethal monkeypox challenge in macaques [42].
The human experience with MVA is drawn from German studies conducted at a time when smallpox had been eradicated from Europe [16, 18]. As a result, MVA was never field-tested in the face of a natural smallpox epidemic. Although the efficacy of MVA in the face of natural disease remains in question, the existing data suggest that MVA vaccination is immunogenic and is able to protect against a subsequent orthopoxvirus challenge. There are reports of >120,000 persons, including children, elderly individuals, and those with skin conditions, who safely received injections of MVA [18]. In a population that included persons at increased risk for complications, no serious adverse events were noted [16]. Compared with replication-competent vaccinia, there was minimal local reaction and significantly reduced fever and systemic symptoms. MVA was administered by various routes (intracutaneously, intradermally, and intramuscularly) and in combination with replication-competent vaccinia as part of a 2-step vaccination regimen [16, 18]. Vaccinees developed evidence of humoral immunity measured by neutralizing antibody that was not restricted by route of administration [18]. Perhaps most suggestive of successful immunity was the mitigation of take responses in MVA recipients administered Elstree vaccinia weeks to months later. Vaccinia-immune persons have an attenuated response following revaccination as a result of prior immunity [4, 43]. In the German studies, nearly 75% of MVA recipients had an attenuated response, demonstrated by either a more rapid evolution of the typical vaccinia pustule or the development of a papule alone, after receiving the Elstree strain of vaccinia [16]. Although protection from an intradermal vaccinia challenge is different from a smallpox exposure, the ability of MVA to elicit an immune response that can inhibit the replication of a poxvirus in humans is encouraging.
Focus on the development of a safer vaccinia vaccine has been heightened with the emerging threat of bioterrorism. Evaluation of MVA as an effective poxvirus vaccine, however, has greater implications than just developing a safer smallpox vaccine, because there are other scenarios in which MVA could be used. Laboratory and health care personnel working with vaccinia are presently vaccinated with Dryvax. However, these persons are often young and vaccinia naive and, therefore, at increased risk for the severe complications associated with the present vaccine. A second scenario relates to the need for protection from zoonotic infections caused by other potentially pathogenic orthopoxviruses. Recently, the first outbreak of monkeypox infection, which was related to transmission of the orthopoxvirus from exotic pets, was reported in the United States [44]. In addition, there have been reports from the Democratic Republic of the Congo describing outbreaks of monkeypox infection among humans that may relate to the overall waning immunity against poxviruses within the world community. However, there are serious concerns about the reintroduction of replication-competent vaccinia into African populations where the prevalence of HIV infection/AIDS is high [45]. As personal travel and exchange of goods throughout the world community continues to increase, the potential for the introduction and transmission of other pathogens is increased and thus provides further impetus to identify alternative options for active immunization against poxviruses.
The attenuation of MVA has resulted in a profile that appears optimal for vaccine development—diminished virulence and enhanced immunogenicity. Clinical trials are now underway to evaluate the safety and immunogenicity of multiple doses of MVA, compared with the traditional Dryvax scarification, and future trials will be performed to determine the optimal dose, duration of immunity, and safety among subpopulations at risk for Dryvax immunization. Licensure of MVA as an alternative to Dryvax will be challenging because of the inability to define efficacy in the absence of natural disease. It will require extensive evidence in animal models, following the guidance of the FDA's “2 animal rule,” demonstrating efficacy against multiple heterologous orthopox virus challenges [46, 47] in parallel with safety and comparative immunogenicity studies in humans. Although MVA is a promising vaccine candidate for safely inducing and maintaining immunity against orthopoxviruses, our hope remains that protection against a smallpox outbreak will never be necessary.
Acknowledgments
We would like to thank John R. Mascola and Gary J. Nabel for their critical review of the manuscript.
- Received October 16, 2003.
- Accepted January 7, 2004.
- © 2004 by the Infectious Diseases Society of America
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