Orthopoxviruses are a group of large, double-stranded DNA viruses responsible for some of humanity’s most consequential infections, including smallpox, mpox (formerly monkeypox), cowpox, and several lesser-known animal diseases. Though smallpox was eradicated in 1980, the genus has stayed stubbornly relevant: mpox triggered a global outbreak in 2022, new species have been discovered as recently as 2015, and the waning of smallpox-era immunity has left most of the world’s population with little natural defense against any of these viruses. Understanding what orthopoxviruses are, how they move between animals and people, and what medical tools exist to fight them matters more now than at any point since smallpox disappeared.
The Orthopoxvirus Family and the Diseases It Causes
The genus Orthopoxvirus sits within the broader Poxviridae family. Its members share a distinctive brick-shaped virion, a large genome (roughly 170,000 to 230,000 base pairs depending on species), and the unusual trait of replicating entirely in the host cell’s cytoplasm rather than the nucleus. Several species infect humans, each with a different profile of severity and geographic range.
Variola virus, the agent of smallpox, was the deadliest member. The more virulent form, variola major, killed roughly 30 percent of unvaccinated people who contracted it, while variola minor had a fatality rate closer to one percent. Its eradication, certified by the World Health Organization in 1980, remains the single greatest achievement of global public health. Today variola virus exists only in two highly secured laboratories, one in the United States and one in Russia.
Mpox virus, the genus’s most active human pathogen at present, circulates in Central and West Africa and spilled into a worldwide outbreak beginning in 2022. There are two broad genetic groups. Clade I, found mainly in Central Africa, is more virulent, with a fatality rate in the range of five to ten percent. Clade II, which includes the subclades behind recent global outbreaks, is less deadly but spreads more easily beyond endemic regions.1PubMed Central. Comparative analysis of Mpox clades: epidemiology, transmission dynamics, and detection strategies Animal model work supports this hierarchy, showing clade I as the most virulent, followed by clade IIa, then clade IIb, and suggesting that the globally spreading clade IIb lineage may be evolving toward lower virulence.2PubMed Central. Virulence differences of mpox (monkeypox) virus clades I, IIa, and IIb.1 in a small animal model
Cowpox virus, the infection that inspired Edward Jenner’s original vaccine, still circulates in Europe. A typical human case produces a single painful ulcerated blister, usually on the hand, accompanied by fever and swollen lymph nodes, with an incubation period of about seven to twelve days. In people with weakened immune systems or widespread eczema, however, the virus can cause a generalized, potentially fatal rash resembling smallpox.3Emerging Infectious Diseases. Generalized Cowpox Virus Infection in a Patient with HIV, Germany, 2012 Other species in the genus include camelpox, taterapox (a virus of African rodents), and vaccinia virus, the workhorse of smallpox vaccines that has no clearly identified natural reservoir.
Newly Discovered Members
The genus is not a closed set. In 2015, a novel orthopoxvirus was isolated from a resident of Fairbanks, Alaska. Named Alaskapox virus, it turned out to be highly diverged from all previously known species.4PubMed Central. Genome of Alaskapox Virus, A Novel Orthopoxvirus Isolated from Alaska A similar discovery in the Republic of Georgia produced Akhmeta virus, another deeply branching lineage. Phylogenetic estimates suggest these outlier species split from the rest of the orthopoxvirus tree roughly 10,000 to 20,000 years ago, while most of the familiar species diverged from one another within the last several thousand years.5PubMed Central. An Update of Orthopoxvirus Molecular Evolution The existence of these hidden lineages is a reminder that surveillance gaps remain: how many orthopoxviruses circulate in wild rodent populations and occasionally spill into people is genuinely unknown.
How Orthopoxviruses Spread
Every orthopoxvirus that infects humans is either currently zoonotic or was at some point in its history. The animal reservoir for most species is rodents. Surveys in southern Germany found high rates of orthopoxvirus antibodies in bank voles and yellow-necked mice, pointing to these species as the likely natural hosts for cowpox virus in Europe.6PubMed. Patterns of orthopox virus wild rodent hosts in South Germany In central and west Africa, mpox virus maintains itself in a range of small mammals, though the full list of reservoir hosts remains unclear. Cats that hunt infected rodents can carry cowpox to their owners, which is the most common route of human cowpox infection in Europe.
Transmission from animals to humans typically happens through direct contact with an infected animal’s skin lesions, blood, or bodily fluids. Once a person is infected, human-to-human spread can follow, particularly for variola and mpox viruses. Smallpox spread primarily through respiratory droplets during face-to-face contact. The 2022 mpox outbreak revealed that intimate skin-to-skin contact is a key human-to-human route for clade IIb, a pattern not emphasized in earlier descriptions of the disease.1PubMed Central. Comparative analysis of Mpox clades: epidemiology, transmission dynamics, and detection strategies Fomites, meaning contaminated objects like bedding or clothing, are another documented pathway for several orthopoxviruses, including camelpox, cowpox, and possibly Alaskapox.7PubMed Central. The Disease Ecology, Epidemiology, Clinical Manifestations, Management, Prevention, and Control of Increasing Human Infections with Animal Orthopoxviruses
A thread running through all of this is the decline of smallpox-era immunity. Because routine smallpox vaccination ended decades ago, the majority of the global population now has no pre-existing immune defense against any orthopoxvirus. That immunological gap raises the risk that animal-to-human spillovers will gain a foothold in human populations more easily than they once did.7PubMed Central. The Disease Ecology, Epidemiology, Clinical Manifestations, Management, Prevention, and Control of Increasing Human Infections with Animal Orthopoxviruses
How These Viruses Dodge the Immune System
Orthopoxviruses are not passive targets for the immune system. They carry a substantial arsenal of genes devoted to disabling or sidestepping host defenses. Among the most studied is B18R, which encodes a secreted protein that mimics the receptor for type I interferons (the “alarm signals” cells release when they detect a virus). By soaking up those alarm signals before they reach neighboring cells, the virus buys time for replication. Another gene, E3L, encodes a protein that works inside infected cells to block the antiviral machinery that interferons normally activate. Together, these two mechanisms make vaccinia virus highly resistant to the antiviral effects of both type I and type III interferons.8PubMed Central. Inhibition of type III interferon activity by orthopoxvirus immunomodulatory proteins
Interestingly, evolutionary analyses suggest that while some immune evasion genes have been under strong natural selection pressure, the genes most consistently shaped by evolutionary competition are those involved in viral replication and assembly rather than immunomodulation. The mpox virus lineage, though, shows signs of episodic positive selection in a gene encoding a complement activation modulator, hinting at an ongoing arms race with the primate immune system.9Europe PMC. Evolution of the orthopoxvirus core genome
Vaccines Against Orthopoxviruses
The genus’s shared immunology is both its defining advantage and its exploitable weakness. Infection or vaccination with one orthopoxvirus confers meaningful cross-protection against others, a principle Jenner demonstrated in 1796 when cowpox protected against smallpox. Modern vaccines build on that same principle but differ substantially in safety and potency.
Replicating Vaccines
ACAM2000, the current U.S. stockpile vaccine, is a replication-competent vaccinia virus. It is effective, but it comes with real risks. Among roughly 900,000 U.S. military service members who received it, the rate of confirmed heart inflammation (myopericarditis) was about 20 per 100,000 vaccinees, with young males at highest risk.10PubMed. Enhanced safety surveillance study of ACAM2000 smallpox vaccine among US military service members Because the virus in ACAM2000 actively replicates, people with weakened immune systems, eczema, or pregnancy are generally excluded from vaccination. A documented case illustrates the stakes: a young military recruit with undiagnosed HIV developed a disseminated vaccinia infection after smallpox vaccination that, combined with AIDS-related complications, was fatal.3Emerging Infectious Diseases. Generalized Cowpox Virus Infection in a Patient with HIV, Germany, 2012
Non-Replicating Vaccines
JYNNEOS (also marketed as Imvanex or MVA-BN), a modified vaccinia Ankara vaccine, cannot replicate in human cells. That makes it far safer for immunocompromised individuals. A meta-analysis of real-world data found that a single dose of MVA-BN provides about 75 percent efficacy against mpox, rising to roughly 80 percent after two doses.11PubMed. Safety and Efficacy of the Modified Vaccinia Ankara-Bavaria Nordic Vaccine Against Mpox in the Real World: Systematic Review and Meta-Analysis The same analysis found that subcutaneous injection produced fewer side effects than intradermal injection, regardless of whether one or two doses were given. JYNNEOS is currently the primary vaccine used in civilian settings during mpox outbreaks.
How Long Cross-Protection Lasts
One lingering question is how durable the protection is, especially when the vaccine target (vaccinia) is not identical to the threatening pathogen (mpox). A study of adults over 50 in Spain who had been vaccinated against smallpox decades ago found that about 69 percent still had detectable vaccinia virus antibodies.12PubMed Central. Possible Mpox Protection from Smallpox Vaccine-Generated Antibodies among Older Adults However, maintaining detectable antibodies against vaccinia does not guarantee strong cross-neutralization of mpox. Immunological analysis shows that cross-reactive antibody responses weaken as the virus becomes more genetically distant from vaccinia, and JYNNEOS-induced antibody responses wane within roughly a year, while older first-generation vaccine recipients maintained vaccinia-specific neutralization for over 80 years. T-cell cross-reactivity, a separate arm of immunity, proved more durable, detectable up to 70 years after vaccination. Booster doses improved both the strength and breadth of cross-neutralizing responses.13The Lancet Infectious Diseases. Mpox vaccine and infection-driven human immune signatures: an immunological analysis of an observational study The practical takeaway is that old smallpox vaccination likely offers some but not reliable protection against mpox, and newer vaccines may need boosters to maintain coverage.
Antiviral Treatments
Vaccines are the first line of defense, but antivirals matter too, particularly for people who are already infected or cannot be vaccinated. Three drugs dominate the orthopoxvirus antiviral landscape.
Tecovirimat (brand name TPOXX) is the most prominent. It works by targeting a viral protein called F13, which the virus needs to wrap and release new viral particles from infected cells. Structural studies have revealed that tecovirimat acts as a kind of molecular glue, forcing two copies of F13 to lock together in a way that disables the protein’s function.14PubMed Central. Mechanisms of tecovirimat antiviral activity and poxvirus resistance Resistance has been observed in mpox clinical isolates, with escape mutations clustering at the protein interface that tecovirimat is supposed to glue together.
Cidofovir and its oral prodrug brincidofovir attack orthopoxvirus replication at a different point, interfering with the viral DNA polymerase. In animal models, both drugs strongly reduced mpox replication in the respiratory tract when given systemically (cidofovir) or orally (brincidofovir).15PubMed. Treatment efficacy of cidofovir and brincidofovir against clade II Monkeypox virus isolates Brincidofovir is roughly 100-fold more potent than cidofovir against variola virus in cell culture and has the advantage of being taken by mouth without the kidney toxicity that limits cidofovir’s usefulness.16PubMed Central. In vitro efficacy of brincidofovir against variola virus A systematic review concluded that all three drugs are highly effective in laboratory and animal studies, safe in human clinical trials, and that brincidofovir and tecovirimat together produce a strong synergistic effect, which could also reduce the risk of resistance emerging.17Global Biosecurity. Efficacy of three key antiviral drugs used to treat orthopoxvirus infections: a systematic review
There is an important caveat: none of these drugs has been tested in large-scale human clinical trials against active orthopoxvirus disease. Tecovirimat and brincidofovir were approved under the FDA’s Animal Rule, meaning their efficacy was demonstrated in animals and their safety in healthy human volunteers, but controlled trials in actual patients are still lacking. Clinical case reports describe successful use in individual patients, but the evidence base is thinner than many people realize.
Post-Exposure Prevention
If you have been exposed to an orthopoxvirus but are not yet sick, the window for intervention depends on the tool. Vaccination within days of exposure has historically been the mainstay for smallpox contacts. The smallpox eradication campaign ultimately shifted from mass vaccination to a strategy of finding cases, tracing contacts, and vaccinating rings of people around each case, and that approach proved more effective at stamping out outbreaks.18PubMed Central. Mass vaccination and surveillance/containment in the eradication of smallpox Modeling work has confirmed that ring vaccination, when coupled with surveillance, can contain even rapidly spreading smallpox if household contacts are traced aggressively and workplace or social contacts are traced with reasonable thoroughness.19PubMed Central. Logistics of community smallpox control through contact tracing and ring vaccination: a stochastic network model
Vaccinia immune globulin (VIG), a preparation of antibodies harvested from vaccinated donors, offers another post-exposure option. In animal studies, VIG injected within one day of exposure to a lethal orthopoxvirus provided full protection, and even treatment two or three days after infection still saved 60 to 80 percent of animals. VIG can also be given alongside vaccination without significantly interfering with the vaccine’s ability to build a lasting immune response, as long as the antibody dose is not excessive.20PubMed. Effective post-exposure protection against lethal orthopoxviruses infection by vaccinia immune globulin involves induction of adaptive immune response The non-replicating MVA vaccine, by contrast, appears to require administration before or on the day of exposure to be protective; post-exposure vaccination in animal experiments did not prevent disease.21PubMed. Short-term, but not post-exposure, protection against lethal orthopoxvirus challenge after immunization with modified vaccinia virus Ankara
Telling Orthopoxvirus Infections Apart from Look-Alikes
In a clinical setting, orthopoxvirus infections can be surprisingly hard to distinguish from other diseases. The rash of mpox can look strikingly similar to chickenpox (varicella), herpes, syphilis, and various other skin conditions. One useful clinical clue is that mpox lesions tend to all be at the same stage of development at any given time, while chickenpox characteristically shows a mix of macules, vesicles, pustules, and crusts simultaneously.22PubMed. Comparison of human monkeypox, chickenpox and smallpox: a comprehensive review of pathology and dermatological manifestations Mpox patients also tend to have more systemic symptoms, with fever, muscle aches, and especially lymph node swelling appearing far more frequently than in diseases that mimic it visually.23PubMed. How to distinguish mpox from its mimickers: An observational retrospective cohort study
Still, clinical appearance alone is not reliable enough. Laboratory confirmation relies on real-time PCR assays that can detect orthopoxvirus DNA in swab samples from lesions. Some assays target the viral DNA polymerase gene and detect orthopoxviruses broadly (excluding variola), while others target genes specific to a single species like mpox.24PubMed Central. Detection of monkeypox virus with real-time PCR assays During the 2022 outbreak, clinical laboratories worldwide had to rapidly validate and deploy these assays, often adapting previously research-only protocols for diagnostic use.25PubMed Central. Validation and implementation of an orthopoxvirus qualitative real-time PCR for the diagnosis of monkeypox in the clinical laboratory
Biosecurity and Synthetic Biology
A concern that hovers over the entire orthopoxvirus field is the intersection of genomics and biosecurity. In 2017, Canadian researchers announced they had synthesized horsepox virus, a member of the genus thought to be extinct, from commercially available synthetic DNA fragments. The accomplishment was technically impressive and immediately controversial. Some scientists argued it demonstrated that the barrier to recreating variola virus, the deadliest orthopoxvirus, was lower than previously believed.26PubMed. The De Novo Synthesis of Horsepox Virus: Implications for Biosecurity and Recommendations for Preventing the Reemergence of Smallpox
Others pushed back, noting that orthopoxvirus synthesis remains technically challenging and beyond the reach of most actors. A detailed analysis concluded that while the horsepox work was a meaningful step, actual misuse would still require substantial expertise, specialized equipment, and access to DNA synthesis services that screen orders against pathogen sequences.27Health Security. Biosecurity Implications for the Synthesis of Horsepox, an Orthopoxvirus The deeper worry is not one dramatic act of bioterrorism but gradual normalization. If orthopoxvirus synthesis becomes routine for vaccine research and other beneficial applications, the number of laboratories with the knowledge and capability to create infectious orthopoxviruses will grow, and the containment framework that has kept variola virus locked away for 40 years may erode through institutional proliferation rather than through any single breach.
Evolutionary Origins and What They Tell Us
How old is this group of viruses? Estimates vary depending on the method, but molecular clock analyses generally place the divergence of Old World and New World orthopoxviruses somewhere around 40,000 to 50,000 years ago, possibly linked to animal migrations during short warming periods in the last ice age.5PubMed Central. An Update of Orthopoxvirus Molecular Evolution28PubMed. A retrospective study of the orthopoxvirus molecular evolution Variola virus, despite its outsized impact on human history, appears to be a relatively recent species, with one analysis placing its independent evolution at roughly 3,500 years ago and another estimating the most recent common ancestor of modern variola strains around 300 AD.5PubMed Central. An Update of Orthopoxvirus Molecular Evolution The average substitution rate across the genus is roughly six to seven mutations per million nucleotide sites per year, slow by the standards of RNA viruses but fast enough to produce meaningful divergence over thousands of years.29PubMed. Bayesian reconstruction of the evolutionary history and cross-species transition of variola virus and orthopoxviruses
These timelines matter practically because they help predict how well a vaccine based on one species will protect against another. The more evolutionary distance between two orthopoxviruses, the weaker the cross-neutralization of antibodies tends to be. That is exactly what immunological data confirms: cowpox virus, which is more closely related to vaccinia, is neutralized more effectively by vaccinia-induced antibodies than mpox virus is.13The Lancet Infectious Diseases. Mpox vaccine and infection-driven human immune signatures: an immunological analysis of an observational study For public health planners, this means that existing vaccines provide good but imperfect coverage against mpox, and that any future orthopoxvirus to spill out of an animal reservoir will need to be evaluated on its own terms rather than assumed to be covered by the current toolkit.