Viruela en Santiago, Concepción y Santafé: comparación de las estrategias higienistas ilustradas (1782-1807) / Smallpox in Santiago, Concepcion and Santafe: A Comparisson of the Enlightened Sanitary Strategies (1782-1807) / Varíola em Santiago, Concepcion e Santafe: Um comparisson das estratégias sanitárias esclarecidas (1782-1807)

Introducción: este artículo pretende estudiar las medidas tomadas en las epidemias de viruela de Santiago (1787), Concepción (1789) y Santafé (1782/1802), para compararlas y entender cómo el estudio de estas epidemias nos puede dar luces para el abordaje del reto de salud pública actual: la pandemia de covid-19. Desarrollo: el artículo está dividido en tres partes: en la primera se exponen las medidas de higiene que se tomaron para subsanar y prevenir estas epidemias, previas a la llegada de la vacunación, comparando el rol desempeñado por los actores locales en cada región; en la segunda se relata y se compara el proceso de llegada y búsqueda de la vacuna contra la viruela en cada territorio, y en la tercera se reflexiona brevemente sobre la pandemia actual. Conclusiones: en el análisis comparativo, se destaca la manera en la que la agenda transcolonial incluye una serie de similitudes para su aplicación en cada territorio, pero también las diferencias que los procesos locales y transcoloniales plantean para su domesticación en cada uno de ellos. Además, se resaltan las particularidades que ha tenido la pandemia de covid-19 y las lecciones que este estudio de caso deja para pensar en la necesidad de enfrentarla desde una perspectiva global.

Introduction: This study aimed to investigate and compare measures implemented during the smallpox epidemics in Santiago (1787), Concepción (1789), and Santafé (1782/1802). In addition, we also tried to understand how the study of these epidemics could help identify an approach for managing the current public health challenge, i.e., the covid-19 pandemic. Development: The article is divided into three parts: the first part studies the hygienic measures that were taken to face and prevent the epidemics as well as compares the role played by local actors in each region; the second part studies the processes of searching and acquiring smallpox vaccine in Santafé and Santiago; and the third part reflects on the current pandemic scenario. Conclusions: Through comparative analysis, we evaluated the similarities in the application of transcolonial agenda in each territory and the differences brought about by local and transcolonial processes implemented for its domestication. Furthermore, we highlighted particular processes conducted for managing and treating covid-19 as well as lessons learnt from this case study about the need of dealing with covid-19 from a global perspective.

Introdução: este artigo tem como objetivo estudar as medidas tomadas nas epidemias de varíola de Santiago (1787), Concepción (1789) e Santafé (1782/1802), compará-las e compreender como o estudo des-sas epidemias pode lançar luz sobre a abordagem do desafio atual da saúde pública: a pandemia covid-19. Desenvolvimento: o artigo está dividido em três partes: na primeira, são expostas as medidas de higiene que foram tomadas para corrigir e prevenir estas epidemias, antes da chegada da vacinação, comparando o papel desempenhado pelos atores locais em cada região; na segunda, relaciona-se e compara-se o processo de chegada e busca da vacina contra a varíola em cada território; e, na terceira, faz uma breve refle-xão sobre a atual pandemia. Conclusões: na análise comparativa, destacamos a forma como a agenda transcolonial suscita uma série de semelhanças para a sua aplicação em cada território, mas também as diferenças que os processos locais e transcoloniais colocam para a sua domesticação em cada um deles. Além disso, destacamos as particularidades que a pandemia covid-19 teve e as lições que este estudo de caso deixa para pensar a necessidade de enfrentá-la a partir de uma perspectiva global.

CRISPR/Cas9 as an antiviral against Orthopoxviruses using an AAV vector.

A vaccine for smallpox is no longer administered to the general public, and there is no proven, safe treatment specific to poxvirus infections, leaving people susceptible to infections by smallpox and other zoonotic Orthopoxviruses such as monkeypox. Using vaccinia virus (VACV) as a model organism for other Orthopoxviruses, CRISPR-Cas9 technology was used to target three essential genes that are conserved across the genus, including A17L, E3L, and I2L. Three individual single guide RNAs (sgRNAs) were designed per gene to facilitate redundancy in rendering the genes inactive, thereby reducing the reproduction of the virus. The efficacy of the CRISPR targets was tested by transfecting human embryonic kidney (HEK293) cells with plasmids encoding both SaCas9 and an individual sgRNA. This resulted in a reduction of VACV titer by up to 93.19% per target. Following the verification of CRISPR targets, safe and targeted delivery of the VACV CRISPR antivirals was tested using adeno-associated virus (AAV) as a packaging vector for both SaCas9 and sgRNA. Similarly, AAV delivery of the CRISPR antivirals resulted in a reduction of viral titer by up to 92.97% for an individual target. Overall, we have identified highly specific CRISPR targets that significantly reduce VACV titer as well as an appropriate vector for delivering these CRISPR antiviral components to host cells in vitro.

Phage display antibodies against ectromelia virus that neutralize variola virus: Selection and implementation for p35 neutralizing epitope mapping.

In this study, five phage display antibodies (pdAbs) against ectromelia virus (ECTV) were selected from vaccinia virus (VACV)-immune phage-display library of human single chain variable fragments (scFv). ELISA demonstrated that selected pdAbs could recognize ECTV, VACV, and cowpox virus (CPXV). Atomic force microscopy visualized binding of the pdAbs to VACV. Three of the selected pdAbs neutralized variola virus (VARV) in the plaque reduction neutralization test. Western blot analysis of ECTV, VARV, VACV, and CPXV proteins indicated that neutralizing pdAbs bound orthopoxvirus 35 kDa proteins, which are encoded by the open reading frames orthologous to the ORF H3L in VACV. The fully human antibody fh1A was constructed on the base of the VH and VL domains of pdAb, which demonstrated a dose-dependent inhibition of plaque formation after infection with VARV, VACV, and CPXV. To determine the p35 region responsible for binding to neutralizing pdAbs, a panel of truncated p35 proteins was designed and expressed in Escherichia coli cells, and a minimal p35 fragment recognized by selected neutralizing pdAbs was identified. In addition, peptide phage-display combinatorial libraries were applied to localize the epitope. The obtained data indicated that the epitope responsible for recognition by the neutralizing pdAbs is discontinuous and amino acid residues located within two p35 regions, 15-19 aa and 232-237 aa, are involved in binding with neutralizing anti-p35 antibodies.

Buccal viral DNA as a trigger for brincidofovir therapy in the mousepox model of smallpox.

Orthopoxviruses continue to pose a significant threat to the population as potential agents of bioterrorism. An intentional release of natural or engineered variola virus (VARV) or monkeypox viruses would cause mortality and morbidity in the target population. To address this, antivirals have been developed and evaluated in animal models of smallpox and monkeypox. One such antiviral, brincidofovir (BCV, previously CMX001), has demonstrated high levels of efficacy against orthopoxviruses in animal models and is currently under clinical evaluation for prevention and treatment of diseases caused by cytomegaloviruses and adenoviruses. In this study we use the mousepox model of smallpox to evaluate the relationship between the magnitude of the infectious virus dose and an efficacious BCV therapy outcome when treatment is initiated concomitant with detection of ectromelia virus viral DNA (vDNA) in mouse buccal swabs. We found that vDNA could be detected in buccal swabs of some, but not all infected mice over a range of challenge doses by day 3 or 4 postexposure, when initiation of BCV treatment was efficacious, suggesting that detection of vDNA in buccal swabs could be used as a trigger to initiate BCV treatment of an entire potentially exposed population. However, buccal swabs of some mice did not become positive until 5 days postexposure, when initiation of BCV therapy failed to protect mice that received high doses of virus. And finally, the data suggest that the therapeutic window for efficacious BCV treatment decreases as the virus infectious dose increases. Extrapolating these findings to VARV, the data suggest that treatment should be initiated as soon as possible after exposure and not rely on a diagnostic tool such as the measurement of vDNA in buccal cavity swabs; however, consideration should be given to the fact that the behavior/disease-course of VARV in humans is different from that of ectromelia virus in the mouse.

[EVALUATION OF THE HUMAN SENSITIVITY TO SMALLPOX VIRUS BY THE PRIMARY CULTURES OF THE MONOCYTE-MACROPHAGES].

Studies of the primary cultures of granulocytes, mononuclear, and monocyte-macrophage cells derived from human blood were performed using variola virus (VARV) in the doses of 0.001-0.021 PFU/cell (plaques-forming units per cell). Positive dynamics of the virus accumulation was observed only in the monocyte-macrophages with maximum values of virus concentration (5.0-5.5 Ig PFU/ml) mainly within six days after the infection. The fact of VARV replication in the monocyte-macrophages was confirmed by the data of electron microscopy. At the same time, virus vaccines when tested in doses 3.3 and 4.2 Ig PFU/ml did not show the ability to reproduce in these human cells. The people sensitivity to VARV as assessed from the data obtained on human monocyte-macrophages corresponded to -1 PFU (taking into account the smooth interaction of the virus in the body to the cells of this type), which is consistent to previously found theoretical data on the virus sensitivity. The human susceptibility to VARV assessed experimentally can be used to predict the adequacy of developed smallpox models (in vivo) based on susceptible animals. This is necessary for reliable assessment of the efficiency of development of drugs for treatment and prophylaxis of the smallpox.

[THE USE OF THE MODEL MOUSE ICR--VARIOLA VIRUS FOR EVALUATION OF ANTIVIRAL DRUG EFFICACY].

Mice of the ICR outbred population were infected intranasally (i/n) with the variola virus (VARV, strain Ind-3a). Clinical signs of the disease did not appear even at the maximum possible dose of the virus 5.2 lg PFU/head (plaque-forming units per head). In this case, 50% infective dose (ID50) of VARV estimated by the presence or absence of the virus in the lungs three days after infection (p.i.) was equal to 2.7 ± 0.4 lg PFU/head. Taking into account the 10% application of the virus in the lungs during the intranasal infection of the mice, it was adequate to 1.7 lg PFU/lungs. This indicates a high infectivity of the VARV for mice comparable to its infectivity for humans. After the i/n infection of mice with the VARV at a dose 30 ID50/ head the highest concentration of the virus detected in the lungs (4.9 ± 0.0 lg PFU/ml of homogenate) and in nasal cavity tissues (4.8 ± 0.0 lg PFU/ml) were observed. The pathomorphological changes in the respiratory organs of the mice infected with the VARV appeared at 3-5 days p.i., and the VARV reproduction noted in the epithelial cells and macrophages were noticed. When the preparations ST-246 and NIOCH-14 were administered orally at a dose of 60 µg/g of mouse weight up to one day before infection, after 2 hours, 1 and 2 days p.i., the VARV reproduction in the lungs after 3 days p.i. decreased by an order of magnitude. Thus, outbred ICR mice infected with the VARV can be used as a laboratory model of the smallpox when evaluating the therapeutic and prophylactic efficacy of the antismallpox drugs.

[THERMAL STABILITY AS A PROGNOSTIC INDICATOR OF CONSERVATION OF LIVE EMBRYONIC SMALLPOX VACCINE (TEOVAC) DURING STORAGE].

AIM: Determination of values of coefficients of thermal stability of TEOVac for prognosis of conservation of the vaccine (specific biological activity) during the process of warranty period storage. MATERIALS AND METHOD: TEOVac (masticatory tablets) in primary packaging was kept at increased temperature (accelerated and stress-tests) and at the conditions established by PAP for the preparation (long-term tests). Biological activity of the vaccine was determined by titration on 12-day chicken embryos. RESULTS: A correlation between the value of coefficients of thermal stability and conservation of the prepared series of the condition preparation at the final date of storage was experimentally established. CONCLUSION: Coefficients of thermal stability could be used as a prognostic indicator of quality of the produced pelleted formulation of the preparation for evaluation of conservation of the vaccine during warranty period storage.

Vaccinia virus strain LC16m8 defective in the B5R gene keeps strong protection comparable to its parental strain Lister in immunodeficient mice.

BACKGROUND: Attenuated vaccinia virus strain, LC16m8, defective in the B5R envelope protein gene, is used as a stockpile smallpox vaccine strain in Japan against bioterrorism: the defect in the B5R gene mainly contributes to its highly attenuated properties. METHODS: The protective activity of LC16m8 vaccine against challenge with a lethal dose of vaccinia Western Reserve strain was assessed in wild-type and immunodeficient mice lacking CD4, MHC class I, MHC class II or MHC class I and II antigens. RESULTS: The immunization with LC16m8 induced strong protective activity comparable to that of its parent strain, Lister (Elstree) strain, in wild-type mice from 2 days to 1 year after vaccination, as well as in immunodeficient mice at 2 or 3 weeks after vaccination. These results implicated that the defect in the B5R gene hardly affected the potential activity of LC16m8 to induce innate, cell-mediated and humoral immunity, and that LC16m8 could be effective in immunodeficient patients. CONCLUSION: LC16m8 with truncated B5 protein has an activity to induce immunity, such as innate immunity and subsequent cell-mediated and humoral immunity almost completely comparable to the activity of its parental strain Lister.

Rapid spreading and immune evasion by vaccinia virus.

Vaccinia virus (VACV) is the live vaccine that was used to eradicate smallpox, a feat achieved in 1977 and certified by the World Health Organization in 1980. Since 1980, research with VACV has continued in part because of the development of techniques to genetically manipulate VACV and create live VACV strains expressing foreign genes. These recombinant VACVs can be used as live vaccines against other infectious diseases and cancers, and as a powerful tool to study virus pathogenesis, immunology, cell biology, and virus-host interactions. This short article describes two examples of how enduring interest in VACV has revealed new features of VACV biology and the immune system.

Evaluation of a non-viral vaccine in smallpox-vaccinated individuals and immunized HLA-transgenic mice.

The current poxvirus vaccine is associated with rare, but serious adverse events. Therefore, we investigated a non-replicating approach to vaccine design. Peptides encoding potential HLA-binding motifs were derived from the orthopoxvirus genes, D8L, A27L, and C12L (the IL-18-binding protein [vIL18BP105]), all of which are preserved among poxviruses that infect humans, and which may be a target of host immunity. The peptides were tested with poxvirus-vaccinated human PBMC and serum for eliciting memory responses, as well as with splenocytes and serum from peptide-immunized, human HLA-DR04 transgenic (HLA tg) mice. vIL18BP105 induced 5-fold proliferation of vaccinated-donor PBMC over non-vaccinated (P<0.001), including IL-2-producing CD8+ cells. Serum IgG recognizing vIL18BP105 was detected (P<0.002 vs non-vaccinated) by ELISA. Viral peptides were conjugated to the HLA-targeting mAb, L243, for immunization of HLA tg mice. Splenocytes from vIL18BP105-L243-immunized mice proliferated upon exposure to vIL18BP105 (P<0.001). Proliferating splenocytes were interferon-γ-producing CD4(+)CD45RA(neg). vIL18BP105-L243-immunized mice generated IgG more rapidly than free-peptide-immunized mice. Peptide-specific antibody was also detected when different L243-peptide conjugates were combined. vIL18BP, by eliciting human memory responses, is a viable antigen for inclusion in a virus-free vaccine. The immunogenicity of peptides was boosted by conjugation to L243, whether administered alone or combined.

Osteomyelitis variolosa: a case report.

Osteomyelitis variolosa is an infection of bone and joints by smallpox virus variola major, most commonly in the elbows, wrists, ankles, hands, and feet. We report one such case in a 70-year-old woman who presented with deformities of the right knee, both elbows and ankles, and the left hand, and a history of childhood fever with rashes. Her lateral femoral condyle of the right knee was hypoplastic with patella baja. Her right elbow was ankylosed and her left elbow was dislocated with multidirectional instability. Her third and fourth metacarpals on the left hand were shortened. Both ankles were stiff with valgus deformity; both taluses were destroyed.

Adverse events following smallpox vaccination with ACAM2000 in a military population.

BACKGROUND: Generalized vaccinia and benign exanthems are 2 adverse events that have been associated with the smallpox vaccination. Accurate incidence and prevalence rates of each are not readily available, but these events are thought to be uncommon. To our knowledge, this is the first case series to provide clinical as well as pathologic descriptions of multiple papulovesicular eruptions occurring after receiving the second-generation smallpox vaccine, ACAM2000 (Acambis, Canton, Massachusetts), among a vaccinia-naïve military population. In addition, we report the first confirmed case, to our knowledge, of generalized vaccinia following administration of the ACAM2000 vaccine. OBSERVATIONS: All patients received primary smallpox immunization as well as 1 to 3 concurrent or near-concurrent (within the preceding 21 days) immunizations for typhoid, anthrax, hepatitis B, and/or seasonal influenza. One patient presented with a flulike prodrome and diffuse vesiclopustules covering the face, neck, chest, back, and upper and lower extremities. Vaccinia polymerase chain reaction confirmed generalized vaccinia. The remaining 7 patients presented with unusual, painful, and pruritic papulovesicular eruptions occurring on the extensor surfaces of their upper and lower extremities without systemic symptoms. Histologic findings revealed 2 general patterns, including a dermal hypersensitivity reaction with lymphocytic vasculitis and a vesicular spongiotic dermatitis with eosinophils. CONCLUSIONS: We present the first confirmed case of generalized vaccinia following immunization with the second-generation smallpox vaccine ACAM2000. In addition, we describe 7 cases of benign, acral, papulovesicular eruptions thought to be associated with ACAM2000 administration. Further research is needed to discern the pathogenesis of these benign eruptions as well as their incidence and prevalence and that of generalized vaccinia with ACAM2000.

Immunosuppressive treatments reduce long-term immunity to smallpox among patients with breast cancer.

BACKGROUND: Mass vaccination is the principal preventive measure against a smallpox outbreak after an act of bioterrorism. Vaccination of subjects who received immunosuppressive therapies is problematic because of smallpox vaccine reactogenicity. Moreover, long-term immunity to vaccinia might be affected. OBJECTIVE: The objective of the study was to examine the effect of cytotoxic chemotherapy on long-term immunity to vaccinia. METHODS: In a case-control study, 67 patients with breast cancer who received cytotoxic chemotherapy and who were disease free for at least 1 year were matched with healthy controls according to age, sex, and the number of smallpox vaccinations received. Markers of immunity to smallpox were examined. Forty-one patients with breast cancer who did not receive chemotherapy were used to assess the affect of cancer and radiotherapy on immunity to smallpox. RESULTS: Patients with breast cancer who received chemotherapy had lower levels of vaccinia total immunoglobulin G and immunoglobulin G1 (expressed as enzyme-linked immunosorbent assay units per milliliter), neutralizing antibodies, vaccinia:memory B cell ratio (expressed as a percentage), and interferon-gamma level (expressed as picograms per milliliter), compared with healthy control individuals. CONCLUSIONS: Immunity to smallpox is reduced after receipt of chemotherapy for breast cancer. This finding should be considered when planning smallpox vaccination campaigns. The effect of immunosuppressive treatments on persistence of immunity should be tested with respect to additional vaccines or natural infections.

Tecovirimat, a p37 envelope protein inhibitor for the treatment of smallpox infection.

Since the eradication of naturally occurring smallpox in 1980, the fear that variola virus could be used as a biological weapon has become real. Over the last 10 years, emergency preparedness programs have been launched to protect populations against a smallpox outbreak or the possible emergence in humans of other orthopoxvirus infections, such as monkeypox. Vaccination against smallpox was responsible for its eradication, but was linked with high rates of adverse events and contraindications. In this context, intensive research in the poxvirus field has led to the development of safer vaccines and to an increase in the number of anti-poxvirus agents in the pipeline. SIGA Technologies Inc, under license from ViroPharma Inc, is developing tecovirimat (ST-246). Tecovirimat is a novel antiviral that inhibits the egress of orthopoxviruses by targeting viral p37 protein orthologs. The development of tecovirimat during the last 5 years for the treatment of smallpox and for its potential use as adjunct to smallpox vaccine is reviewed here.

Smallpox virus plaque phenotypes: genetic, geographical and case fatality relationships.

Smallpox (infection with Orthopoxvirus variola) remains a feared illness more than 25 years after its eradication. Historically, case-fatality rates (CFRs) varied between outbreaks (<1 to approximately 40 %), the reasons for which are incompletely understood. The extracellular enveloped virus (EEV) form of orthopoxvirus progeny is hypothesized to disseminate infection. Investigations with the closely related Orthopoxvirus vaccinia have associated increased comet formation (EEV production) with increased mouse mortality (pathogenicity). Other vaccinia virus genetic manipulations which affect EEV production inconsistently support this association. However, antisera against vaccinia virus envelope protect mice from lethal challenge, further supporting a critical role for EEV in pathogenicity. Here, we show that the increased comet formation phenotypes of a diverse collection of variola viruses associate with strain phylogeny and geographical origin, but not with increased outbreak-related CFRs; within clades, there may be an association of plaque size with CFR. The mechanisms for variola virus pathogenicity probably involves multiple host and pathogen factors.

Real time PCR for the assessment of CD8+ T cellular immune response after prophylactic vaccinia vaccination.

BACKGROUND: The magnitude of specific CD8+ T cell reactivity responsible for vaccine-induced protection against smallpox infection has not yet been fully elucidated. Among other techniques, RT-PCR for the monitoring of cytokine release in effector T cells against tumor and viral antigens has demonstrated a novel promising method. OBJECTIVE: To determine the functional status of antigen specific CD8+ T cells in healthy participants before and 4 weeks after prophylactic vaccination (Lister strain) against smallpox using quantitative real-time PCR (qRT-PCR). STUDY DESIGN: Changes of interferon-gamma (IFNgamma) mRNA expression levels on short term ex vivo peptide antigen stimulation were measured. The corresponding specific CD8+ T cell reactivity was then displayed as CD8-normalized IFN-gamma levels (IFN-gamma/CD8 ratio). RESULTS: We found a 5-9 fold increase of CD8+ T cell reactivity in three out of four vaccinated individuals. The kinetics and strength determined in responders reveal a virus specific T cell effector repertoire pre-vaccination and a corresponding functional state after immunization comparable also to data obtained from tetramer- and ELISPOT analysis. CONCLUSIONS: Apart from protective vaccinia-specific neutralizing antibodies, the presence of antigen-specific CD8+ T-cells has been demonstrated after vaccinia vaccination. In concordance with others, results from this PCR-based study indicate that this smallpox vaccine induces strong vaccinia virus-specific CD8+ and IFN-gamma producing T cell responses.

A chemokine-binding domain in the tumor necrosis factor receptor from variola (smallpox) virus.

Variola virus (VaV) is the causative agent of smallpox, one of the most devastating diseases encountered by man, that was eradicated in 1980. The deliberate release of VaV would have catastrophic consequences on global public health. However, the mechanisms that contribute to smallpox pathogenesis are poorly understood at the molecular level. The ability of viruses to evade the host defense mechanisms is an important determinant of viral pathogenesis. Here we show that the tumor necrosis factor receptor (TNFR) homologue CrmB encoded by VaV functions not only as a soluble decoy TNFR but also as a highly specific binding protein for several chemokines that mediate recruitment of immune cells to mucosal surfaces and the skin, sites of virus entry and viral replication at late stages of smallpox. CrmB binds chemokines through its C-terminal domain, which is unrelated to TNFRs, was named smallpox virus-encoded chemokine receptor (SECRET) domain and uncovers a family of poxvirus chemokine inhibitors. An active SECRET domain was found in another viral TNFR (CrmD) and three secreted proteins encoded by orthopoxviruses. These findings identify a previously undescribed chemokine-binding and inhibitory domain unrelated to host chemokine receptors and a mechanism of immune modulation in VaV that may influence smallpox pathogenesis.

Stealth nucleosides: mode of action and potential use in the treatment of viral diseases.

Riboviruses and retroviruses have been shown to spontaneously mutate at an extraordinarily high rate. While this genetic diversity allows viral subpopulations to escape conventional antivirals, it also has a cost. Indeed, this high mutation rate results in the synthesis of many defective virions. Stealth nucleosides are nucleoside analogues that are designed to increase the already high spontaneous mutation rate of viruses to the point where the virus cannot further replicate, a process known as "lethal mutagenesis". Rather than causing chain termination and attempting to immediately halt viral replication, as with conventional nucleoside reverse transcriptase inhibitors (NRTI), stealth nucleosides are incorporated into the viral genome during replication and, by mispairing, cause mutations to the viral genome. These mutations affect all viral proteins and cumulatively, over a number of replication cycles, are lethal to the virus. There are two distinct stealth nucleoside platforms: DNA stealth nucleosides and RNA stealth nucleosides. DNA stealth nucleosides are currently being screened for activity against HIV and may have activity against hepatitis B virus and smallpox virus, with the clinical lead DNA stealth nucleoside demonstrating activity in the low nanomolar range. In addition, DNA stealth nucleosides have been shown to be able to effectively treat NRTI-resistant HIV strains in vitro, which is not surprising given that the two principal modes of resistance (low affinity of reverse transcriptase for a modified sugar or pyrophosphorolysis) should not be applicable to DNA stealth nucleosides. RNA stealth nucleosides are being developed for the treatment of ribovirus infections, and particularly hepatitis C virus infection. RNA stealth nucleosides are selected for their broad spectrum of antiviral activity, and current lead RNA stealth nucleosides have potency in the same range as ribavirin.