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Anticipating smallpox and monkeypox outbreaks: complications of the smallpox vaccine.

BACKGROUND: The recent outbreak in the Midwest of monkeypox, as well as the continued fears of a terrorist-induced epidemic of smallpox, prompted the authors' review of the literature regarding past and current experiences with smallpox vaccination. The smallpox vaccine, which is highly effective in preventing the spread of both these orthopoxvirus infectious illnesses, might be administered to numerous health care workers and, in the event of a smallpox attack, millions of other citizens. However, vaccinees would be at risk for several vaccine-related neurologic complications. REVIEW SUMMARY: According to prior reports, neurologic complications have occurred in 2.5 per million US individuals, with the most common being postvaccinal encephalomyelitis (PVEM). In older children and adults, PVEM causes stupor and coma, seizures, paraparesis, and other neurologic and mental abnormalities, and, in 16% of cases, permanent neurologic sequelae. The overall mortality rate of neurologic complications is approximately 1.5 per million vaccinees. Risk factors for PVEM were age younger than 1 year and no previous smallpox vaccination, but not a prior episode of PVEM or other preexisting neurologic illnesses. Neither the current smallpox vaccination campaigns in Israel nor the one in the United States has had comparable complications, but the US campaign has been associated with myocarditis and myopericarditis. CONCLUSION: Although the potential neurologic complications of the smallpox vaccine must be weighed against the threat of monkeypox and smallpox, current experience with vaccination suggests it carries a very low risk of neurologic complications and does not lead to exacerbations of chronic neurologic illnesses.

Animals↗

A comprehensive overview of monkeypox virus disease.

BACKGROUND: Monkeypox (mpox), caused by monkeypox virus (MPXV), re-emerged as a major global public health concern in 2022, resulting in widespread transmission beyond traditionally endemic regions. As of March 2026, 181,164 confirmed cases and 492 deaths had been reported across 144 countries globally. The unprecedented geographic spread of the outbreak highlighted important knowledge gaps in disease surveillance, prevention, and control. Given the ongoing global circulation of MPXV and the risk of future outbreaks, this review provides a comprehensive synthesis of current evidence on MPXV and mpox. METHODS: The literature, surveillance data, and public health reports available up to March 2026 were systematically reviewed and synthesized. The review comprehensively assesses viral biology, genetic diversity, epidemiology, transmission dynamics, clinical manifestations, pathogenesis, laboratory diagnosis, infection during pregnancy, host immune responses, immune evasion mechanisms, therapeutic interventions, and prevention strategies. FINDINGS AND CONCLUSIONS: Globally, the decline in public immunity following the cessation of routine smallpox vaccination, together with ongoing viral evolution, may have contributed to the resurgence of mpox. Advances in genomic surveillance, diagnostics, and public health preparedness have strengthened outbreak response; however, important gaps remain in understanding long-term immunity and optimal treatment strategies. This review summarizes current evidence on MPXV and mpox and highlights priorities for future research and public health interventions.

Antiviral therapy↗

Outbreak of human monkeypox, Democratic Republic of Congo, 1996 to 1997.

Human monkeypox is a zoonotic smallpox-like disease caused by an orthopoxvirus of interhuman transmissibility too low to sustain spread in susceptible populations. In February 1997, 88 cases of febrile pustular rash were identified for the previous 12 months in 12 villages of the Katako-Kombe Health Zone, Democratic Republic of Congo (attack rate = 22 per 1,000; case-fatality rate = 3.7%). Seven were active cases confirmed by virus isolation. Orthopoxvirus- neutralizing antibodies were detected in 54% of 72 patients who provided serum and 25% of 59 wild-caught animals, mainly squirrels. Hemagglutination-inhibition assays and Western blotting detected antibodies in 68% and 73% of patients, respectively. Vaccinia vaccination, which protects against monkeypox, ceased by 1983 after global smallpox eradication, leading to an increase in the proportion of susceptible people.

Adolescent↗

Update: multistate outbreak of monkeypox--Illinois, Indiana, Kansas, Missouri, Ohio, and Wisconsin, 2003.

CDC and state and local health departments continue to investigate cases of monkeypox among persons who had contact with wild or exotic mammalian pets or persons with monkeypox. This report updates epidemiologic, laboratory, and smallpox vaccine use data for U.S. cases, and summarizes the laboratory-based evidence implicating imported African rodents as the probable source of the outbreak.

Adolescent↗

Update: multistate outbreak of monkeypox--Illinois, Indiana, Kansas, Missouri, Ohio, and Wisconsin, 2003.

CDC and state and local health departments continue to investigate cases of monkeypox among persons in the United States who had contact with wild or exotic mammalian pets or with persons with monkeypox. This report updates results of the epidemiologic investigation, provides information on the use of smallpox vaccine during the outbreak, and summarizes the animal tracing activities to identify the origin and subsequent distribution of infected animals.

Adolescent↗

Human infection with monkeypox virus: laboratory investigation of six cases in West Africa.

Between September 1970 and May 1971 six cases of human infection with monkeypox virus were identified in three West African countries-Liberia, Sierra Leone, and Nigeria. Four of the cases were confirmed by viral isolation, and two were diagnosed on epidemiological and serological evidence. Poxvirus strains isolated from the four cases were indistinguishable from reference monkeypox strains (Copenhagen and Utrecht), and all were easily differentiated from variola and vaccinia viruses. The isolated strains produced small necrotic haemorrhagic pocks on CAM, grew well at 39.0 degrees C, formed large plaques in Vero cell cultures, showed markedly more virulence for chick embryos and mice than do variola strains, and produced large necrotic haemorrhagic local lesions with generalized illness and florid secondary exanthem when inoculated into rabbit skin.The finding of smallpox-like illness in humans resulting from infection with a poxvirus of lower animal origin serves to emphasize the importance of thorough epidemiological and laboratory evaluation of all suspect smallpox cases occurring in areas where smallpox has been or is about to be eradicated.

Africa, Western↗

Insights into the Life Cycle and Therapeutic Agents for Monkeypox Virus Infection.

Since the first confirmed case in 1970, the monkeypox virus (MPXV) has emerged as a significant threat to global public health. The World Health Organization (WHO) has declared it a Public Health Emergency of International Concern (PHEIC) on two occasions. Despite decades of research, only tecovirimat has been approved by the European Medicines Agency (EMA) for the treatment of MPXV infection. The genome and structure are similar between MPXV and other orthopoxviruses (OPXVs), suggesting that the strategies used for other OPXVs may be applicable to MPXV. This review systematically summarizes the genome, structure, and critical stages in the life cycle of OPXVs, especially MPXV. A variety of antiviral agents against MPXV and other OPXVs are discussed according to their distinct mechanisms of action: 1) blocking viral entry and fusion, 2) inhibiting DNA replication and processing, 3) disrupting transcription and mRNA processing, 4) preventing virion assembly, maturation and release, 5) modulating immune responses, and 6) mechanism unknown. Overall, this article provides a systematic review of current research progress on potential therapeutic targets and agent for MPXV, aiming to offer innovative insights and strategies for the development of effective therapeutic agents against mpox.

Animals↗

Immunogenicity of a highly attenuated MVA smallpox vaccine and protection against monkeypox.

The potential use of smallpox as a biological weapon has led to the production and stockpiling of smallpox vaccine and the immunization of some healthcare workers. Another public health goal is the licensing of a safer vaccine that could benefit the millions of people advised not to take the current one because they or their contacts have increased susceptibility to severe vaccine side effects. As vaccines can no longer be tested for their ability to prevent smallpox, licensing will necessarily include comparative immunogenicity and protection studies in non-human primates. Here we compare the highly attenuated modified vaccinia virus Ankara (MVA) with the licensed Dryvax vaccine in a monkey model. After two doses of MVA or one dose of MVA followed by Dryvax, antibody binding and neutralizing titres and T-cell responses were equivalent or higher than those induced by Dryvax alone. After challenge with monkeypox virus, unimmunized animals developed more than 500 pustular skin lesions and became gravely ill or died, whereas vaccinated animals were healthy and asymptomatic, except for a small number of transient skin lesions in animals immunized only with MVA.

Animals↗

Smallpox vaccine does not protect macaques with AIDS from a lethal monkeypox virus challenge.

It is unknown whether smallpox vaccination would protect human immunodeficiency virus type 1 (HIV-1)-infected individuals, because helper CD4(+) cells, the targets of HIV-1 infection, are necessary for the induction of both adaptive CD8(+) cell and B cell responses. We have addressed this question in macaques and have demonstrated that, although smallpox vaccination is safe in immunodeficient macaques when it is preceded by immunization with highly attenuated vaccinia strains, the macaques were not protected against lethal monkeypox virus challenge if their CD4(+) cell count was <300 cells/mm(3). The lack of protection appeared to be associated with a defect in vaccinia-specific immunoglobulin (Ig) switching from IgM to IgG. Thus, vaccination strategies that bypass CD4(+) cell help are needed to elicit IgG antibodies with high affinity and adequate tissue distribution and to restore protection against smallpox in severely immunocompromised individuals.

Animals↗

Experimental infection of ground squirrels (Spermophilus tridecemlineatus) with monkeypox virus.

A proposed new small-animal (rodent) model for studying the pathogenesis and treatment of severe orthopoxvirus infections is described. Thirteen-lined ground squirrels (Spermophilus tridecemlineatus) were infected intraperitoneally and intranasally with monkeypox virus (MPXV). A fulminant illness developed in all animals, and they died 6-9 days after infection. Virus was cultured from the blood and oropharynx several days before death; at necropsy, all of the organs tested contained relatively high titers of MPXV. The major pathologic findings were in the liver, which showed centrilobular necrosis, steatosis, and basophilic inclusion bodies in hepatocytes. Splenic necrosis was also observed, as well as interstitial inflammation in the lungs. The pathologic features of MPXV in ground squirrels are similar to that described with MPXV in macaques and severe variola (smallpox) virus infection in humans.

Animals↗

Serological relatedness of monkeypox, variola, and vaccinia viruses.

Closely related human and monkey orthopoxviruses were differentiated by serologic techniques. Antiviral sera were tested by immunodiffusion for reactivity against six different viral antigens prepared from either infected cell cultures or infected chorioallantoic membranes (CAMs) of embryonated eggs. Portions of each antiserum were separately absorbed with heterologous antigens from infected CAMs to remove common reactivity. The absorbed sera formed immunodiffusion precipitates with both types of antigen preparation and revealed specific-character differences that made it possible to classify the viruses as variola, vaccinia, or monkeypox. Cross-complement fixation tests were also used to examine the immunologic reactivities of antisera to detergent-treated, purified preparations of three orthopoxviruses. Only common reactivities were detected by this method, however, and differentiating reactivities were not observed.

Complement Fixation Tests↗

Analysis of the monkeypox virus genome.

Monkeypox virus (MPV) belongs to the orthopoxvirus genus of the family Poxviridae, is endemic in parts of Africa, and causes a human disease that resembles smallpox. The 196,858-bp MPV genome was analyzed with regard to structural features and open reading frames. Each end of the genome contains an identical but oppositely oriented 6379-bp terminal inverted repetition, which similar to that of other orthopoxviruses, includes a putative telomere resolution sequence and short tandem repeats. Computer-assisted analysis was used to identify 190 open reading frames containing >/=60 amino acid residues. Of these, four were present within the inverted terminal repetition. MPV contained the known essential orthopoxvirus genes but only a subset of the putative immunomodulatory and host range genes. Sequence comparisons confirmed the assignment of MPV as a distinct species of orthopoxvirus that is not a direct ancestor or a direct descendent of variola virus, the causative agent of smallpox.

Animals↗

Properties of the recombinant TNF-binding proteins from variola, monkeypox, and cowpox viruses are different.

Tumor necrosis factor (TNF), a potent proinflammatory and antiviral cytokine, is a critical extracellular immune regulator targeted by poxviruses through the activity of virus-encoded family of TNF-binding proteins (CrmB, CrmC, CrmD, and CrmE). The only TNF-binding protein from variola virus (VARV), the causative agent of smallpox, infecting exclusively humans, is CrmB. Here we have aligned the amino acid sequences of CrmB proteins from 10 VARV, 14 cowpox virus (CPXV), and 22 monkeypox virus (MPXV) strains. Sequence analyses demonstrated a high homology of these proteins. The regions homologous to cd00185 domain of the TNF receptor family, determining the specificity of ligand-receptor binding, were found in the sequences of CrmB proteins. In addition, a comparative analysis of the C-terminal SECRET domain sequences of CrmB proteins was performed. The differences in the amino acid sequences of these domains characteristic of each particular orthopoxvirus species were detected. It was assumed that the species-specific distinctions between the CrmB proteins might underlie the differences in these physicochemical and biological properties. The individual recombinant proteins VARV-CrmB, MPXV-CrmB, and CPXV-CrmB were synthesized in a baculovirus expression system in insect cells and isolated. Purified VARV-CrmB was detectable as a dimer with a molecular weight of 90 kDa, while MPXV- and CPXV-CrmBs, as monomers when fractioned by non-reducing SDS-PAGE. The CrmB proteins of VARV, MPXV, and CPXV differed in the efficiencies of inhibition of the cytotoxic effects of human, mouse, or rabbit TNFs in L929 mouse fibroblast cell line. Testing of CrmBs in the experimental model of LPS-induced shock using SPF BALB/c mice detected a pronounced protective effect of VARV-CrmB. Thus, our data demonstrated the difference in anti-TNF activities of VARV-, MPXV-, and CPXV-CrmBs and efficiency of VARV-CrmB rather than CPXV- or MPXV-CrmBs against LPS-induced mortality in mice.

Amino Acid Sequence↗

Emerging infections and pregnancy: West Nile virus, monkeypox, severe acute respiratory syndrome, and bioterrorism.

As new infectious diseases, such as West Nile virus, monkeypox, and severe acute respiratory syndrome (SARS) are recognized in the United States, there are critical questions about how these infectious diseases will affect pregnant women and their infants. In addition, the implications of bioterrorist attacks for exposed pregnant women need to be considered. In this article, the authors address the following questions for a number of infectious disease threats: (1) does pregnancy affect the clinical course of these novel infectious diseases?, (2) what are the implications for prophylaxis and treatment of exposed or infected pregnant women?, and (3) are these novel infectious diseases transmitted during pregnancy, labor and delivery, or breastfeeding?

Bioterrorism↗

Specific detection of monkeypox virus by polymerase chain reaction.

The open reading frame coding for the A-type inclusion body protein (ATI) of monkeypox virus (MPV) was identified and sequenced for two strains. Nucleotide sequence comparison revealed 72-95.3% homology with the reported open reading frame sequences of the ATIs of other orthopoxvirus species, such as variola, vaccinia, cowpox, ectromelia, and camelpox viruses. Each MPV strain contained an 8-bp deletion, which caused a frameshift that introduced a premature stop in the open reading frame at base 2091 relative to the ATI open reading frame of cowpox virus strain Brighton. The sequences enabled a primer pair to be designed that flanked the deletion and specifically amplified a 601-bp fragment that identified and differentiated 19 MPV strains examined from five other Old World orthopoxvirus species examined. The specificity was confirmed by cleavage of the 19 MPV strain amplicons with BglII, which produced three subfragments of expected sized, based on the determined MPV sequences.

Animals↗