Mpox in depth.

Overview

Mpox (monkeypox) is a viral zoonotic disease causing a distinctive pustular rash. Caused by Monkeypox virus (Poxviridae, Clade I or IIb), it spreads through close physical contact with infectious rash, bodily fluids, or respiratory secretions. Key symptoms: fever, swollen lymph nodes, muscle aches, and a rash that progresses from macules to pustules. Case fatality rates range from below 0.1% (Clade IIb in high-income settings) to up to 10% (Clade I in Central Africa).

Mpox was first identified in humans in 1970 in a 9-month-old boy in the DRC. For decades it was considered a rare zoonosis of Central and West Africa. This changed dramatically in May 2022, when mpox began spreading rapidly across Europe, the Americas, and other regions — leading the WHO to declare it a PHEIC on 23 July 2022 (lifted February 2023). Over 95,000 cases were confirmed in 110+ countries.

In August 2024, the WHO declared a second mpox PHEIC, driven by Clade Ib — a new Clade I subvariant in the DRC showing enhanced human-to-human transmission, spreading to Uganda, Rwanda, Burundi, and Kenya. Clade Ib has a significantly higher case fatality rate than Clade IIb and disproportionately affects children and immunocompromised individuals.

History & Origin

Monkeypox was first identified in monkeys kept for research in 1958 — though rodents are the primary natural reservoir. The first human case was identified in 1970 in a child in DRC. The withdrawal of smallpox vaccination following eradication in 1980 (which provided ~85% cross-protection against mpox) has increased population susceptibility. In 2022, WHO renamed the disease "mpox" to replace stigmatizing language.

The 2003 US outbreak (70+ cases from prairie dogs imported from Africa) was the first outside Africa. The 2022 global outbreak was driven by a distinct Clade IIb lineage acquiring mutations enhancing human-to-human spread, initially amplified in international sexual networks at gatherings in Europe.

Transmission

• Animal-to-human: Direct contact with blood, body fluids, or skin lesions of infected animals (rodents, primates). Handling or consuming insufficiently cooked bushmeat in endemic regions.
• Human-to-human (close contact): Direct contact with skin lesions, body fluids, respiratory secretions, or contaminated materials (bedding, clothing). Requires close physical contact.
• Sexual transmission: The 2022 outbreak demonstrated efficient sexual transmission through direct contact with genital lesions. Mpox lesions anywhere on the body are infectious throughout the entire lesion lifecycle (until all scabs fall off).
• Mother to child: Vertical transmission (congenital mpox) documented — high risk of severe disease in newborns.
• Fomites: The poxvirus can survive on surfaces for weeks; contaminated objects can transmit virus.

Symptom Timeline

Incubation period: 5–21 days (most commonly 6–13 days). A person is infectious from onset of symptoms until ALL scabs fall off.

Diagnosis

• PCR (mpox-specific): Gold standard. Swabs from lesion fluid or crusts. Clade identification requires sequencing.
• Clinical diagnosis: Characteristic rash + lymphadenopathy + epidemiological link is strongly suggestive.
• Blood tests: Leukocytosis, elevated CRP common in systemic disease.

Treatment

• Tecovirimat (TPOXX): FDA-approved antiviral active against orthopoxviruses including MPXV. Used under EUA/compassionate use for severe mpox. 14-day oral course. Benefit most clear in immunocompromised patients.
• Brincidofovir: Alternative antiviral for severe disease; IV formulation.
• Supportive care: Pain management (lesions are very painful — consider prescription analgesics); wound care; antibiotics for secondary bacterial infection; trifluridine eye drops for ocular mpox.
• Isolation: Full lesion isolation (home or hospital) until all scabs fall off — typically 2–4 weeks.

Prevention & Vaccines

• JYNNEOS (Imvamune/Imvanex): Two-dose live attenuated Modified Vaccinia Ankara vaccine (~85% efficacy). FDA-approved for smallpox and mpox. Recommended for high-risk individuals. Post-exposure vaccination within 4 days may prevent disease; within 14 days may reduce severity.
• ACAM2000: Older smallpox vaccine with ~85% cross-protection; contraindicated in immunocompromised, pregnant women, eczema patients.
• Infection control: Avoid close contact with anyone with unexplained new rash; use condoms; reduce number of sexual partners during outbreaks; PPE for healthcare workers.

Global Impact

The 2022 mpox global outbreak confirmed mpox as a sexually transmissible disease capable of spreading in global sexual networks. Over 95,000 cases in 110+ countries, with low mortality (~0.1% CFR) in immunocompetent individuals in high-income countries. Rapid JYNNEOS deployment, behaviour change, and community-led responses controlled the outbreak in high-income countries.

The 2024 Clade I emergency in Central Africa is far more severe. Clade Ib MPXV has a CFR of 3–10% and disproportionately affects children under 5. Vaccine supplies are severely inadequate relative to need in DRC and neighbouring countries. The spread of Clade Ib to East Africa represents a significant escalation of the global mpox situation.

History: From Obscure Zoonosis to Global Outbreak

Mpox was first identified in 1958 in laboratory monkeys in Denmark — giving it the historical (and now changed) name "monkeypox" — though rodents are the primary reservoir, not monkeys. The first human case was identified in 1970 in a 9-year-old boy in the DRC (then Zaire). For decades, mpox remained a rare zoonotic disease confined to Central and West Africa, with periodic outbreaks in DRC, Republic of Congo, and Nigeria.

The disease received limited international attention until 2003, when a US outbreak was traced to imported Gambian giant pouched rats infecting prairie dogs sold as pets — 47 confirmed US cases, no deaths. The 2017 Nigeria outbreak (200+ cases) was the largest in decades and introduced a now-dominant clade IIb lineage. In May 2022, mpox suddenly appeared in multiple non-endemic countries simultaneously — with over 100,000 cases globally in 2022 from a predominantly sexually transmitted outbreak in networks of men who have sex with men (MSM), triggering WHO's PHEIC declaration in July 2022.

Clade I vs Clade II: Critical Differences

Virology & Pathophysiology

Monkeypox virus (MPXV) is a double-stranded DNA virus of the genus Orthopoxvirus (family Poxviridae). It is closely related to variola virus (smallpox), vaccinia (used in smallpox vaccine), and cowpox. The MPXV genome (~197 kb) encodes ~190 proteins, including immune evasion factors that help the virus evade innate and adaptive immune responses. Two main MPXV clades exist: Clade I (formerly Congo Basin clade) and Clade II (formerly West Africa clade), further subdivided into IIa and IIb.

Pathogenesis: Entry is through skin abrasions, mucosal surfaces, or respiratory tract. After local replication and lymph node spread, primary viremia seeds liver and spleen. Secondary viremia spreads virus to skin, causing the characteristic centrifugal rash progression (from face to trunk to extremities in classic presentation). The rash progresses through stages: macule → papule → vesicle → pustule → crust. Lesions are typically deeper and more uniform at the same stage than varicella (chickenpox). Pro-inflammatory cytokines drive fever and systemic symptoms. Secondary bacterial infection of skin lesions is a common complication.

Treatments & Post-Exposure Prophylaxis

Antivirals

• Tecovirimat (TPOXX, ST-246): FDA-approved (2018) for smallpox; used under IND/EAP for mpox. VP37 inhibitor blocking viral egress. Most widely used antiviral in 2022 outbreak. Efficacy data from RCTs still emerging — PALM-007 trial (DRC Clade I) did not show mortality benefit; community-based studies suggest symptom duration reduction in mild Clade IIb.
• Brincidofovir: Prodrug of cidofovir; FDA-approved for smallpox. Available for severe mpox; renal toxicity limits use.
• Cidofovir: Broad-spectrum antiviral; limited to severe or immunocompromised cases due to nephrotoxicity.
• Vaccinia immunoglobulin (VIG): Used for severe cases, immunocompromised patients, or vaccine complications.

Vaccines

• Jynneos (Imvamune/Imvanex, Bavarian Nordic): Attenuated Modified Vaccinia Ankara (MVA) — non-replicating. Two-dose regimen (28 days apart). FDA-approved for mpox and smallpox in adults. Preferred due to safety profile (no replication; safe in immunocompromised). 66–89% effectiveness against mpox in 2022 outbreak studies.
• ACAM2000: Replicating vaccinia virus; FDA-approved for smallpox; single-dose; provides rapid protection but has more adverse effects; used in some settings.
• Post-exposure prophylaxis (PEP): Jynneos within 4 days of exposure can prevent disease; within 14 days can attenuate severity. Pre-exposure prophylaxis (PrEP) recommended for high-risk groups in endemic regions.

Sexual Transmission & 2022 Outbreak Dynamics

The 2022 global mpox outbreak represented a major shift in transmission dynamics. Prior outbreaks were primarily zoonotic (animal-to-human) or household contact (skin lesion, respiratory secretions, fomites). In 2022, the virus spread efficiently through sexual networks — primarily in MSM communities globally — with genital, anal, and oral lesions as the primary presentation, consistent with sexual transmission. The 2022 outbreak demonstrated MPXV can sustain prolonged human-to-human chains of sexual transmission.

Molecular analysis showed the 2022 Clade IIb strains had accumulated an unusually high number of APOBEC3 (host immune enzyme) signature mutations — suggesting the virus had been circulating in humans for some time before detection. This was a fundamentally different epidemiology from classic African mpox outbreaks.

Country-Specific Information

Frequently Asked Questions

Sources & Citations

Mpox Diagnosis

• PCR (gold standard): Swab of skin lesion exudate or crust — most sensitive and specific. Blood PCR less sensitive. WHO-referenced orthopoxvirus generic PCR followed by MPXV-specific PCR. Determines clade (Clade I vs II) for epidemiological response.
• Electron microscopy: Can visualize poxvirus particles (brick-shaped, ~200–300 nm); does not differentiate MPXV from other poxviruses.
• IgM/IgG serology: Cross-reactive with other orthopoxviruses (vaccinia, cowpox); not useful in vaccinated individuals; used for seroprevalence studies.
• Clinical diagnosis: In outbreak context, classic rash presentation (single-stage pustular lesions, palms and soles) is highly specific; rash on genitals/perianal in 2022 outbreak also characteristic.
• Differential diagnosis: Chickenpox (varicella) — lesions in multiple stages simultaneously; genital herpes — fewer lesions, cluster on genitals only; hand-foot-mouth disease — vesicular, different age group; syphilis — variable rash, serology diagnostic.

Mpox in Children & Vulnerable Populations

Children (Clade I DRC Context)

In the DRC, children under 15 account for the majority of Clade I mpox cases and deaths. In endemic areas, children are infected through zoonotic contact (hunting, handling forest animals) and household transmission. Clade I causes more severe disease in children than Clade IIb — severe rash, ocular involvement, and opportunistic secondary infections are more common in pediatric cases with malnutrition or HIV. Zabdeno+Mvabea (the two-dose vaccine regimen) is approved for children ≥1 year and is being deployed in DRC.

Immunocompromised Individuals

People with HIV — particularly those with CD4 counts <200 — experience more severe mpox, disseminated rash, atypical presentations (necrotic lesions, encephalitis), and higher mortality. In the 2022 outbreak, nearly 50% of hospitalized mpox patients in some settings had HIV. Antiretroviral therapy (ART) reduces mpox severity in HIV+ individuals. Tecovirimat may be prioritized for immunocompromised patients with severe mpox.

Pregnancy

Mpox in pregnancy is associated with adverse outcomes including preterm delivery, stillbirth, and congenital mpox. WHO recommends pregnant women avoid close contact with confirmed mpox cases. Jynneos vaccine safety data in pregnancy are limited; vaccination during pregnancy may be considered on a case-by-case basis in high-risk scenarios.

Global Mpox Response: 2022 Outbreak Lessons

The 2022 global mpox outbreak provided important lessons for epidemic response:

• Community trust: Partnerships with LGBTQ+ community organizations and health advocates were critical for efficient vaccine deployment and reducing stigma; parallel to HIV epidemic response lessons
• Nomenclature: WHO renamed "monkeypox" to "mpox" in November 2022 to address stigmatizing connotations and avoid associations with African countries or monkeys — the animal is not the primary reservoir
• Vaccine equity: The 2022 outbreak exposed deep inequities in vaccine access — high-income countries rapidly secured Jynneos supplies while DRC (endemic for Clade I) had limited access. Gavi vaccine access programs for Clade I outbreak now established.
• Sexual health integration: Mpox vaccination integrated into sexual health clinic services; PrEP (HIV pre-exposure prophylaxis) clinics used as outreach points for mpox vaccination in MSM communities
• Genomic surveillance: Rapid sharing of MPXV sequences on GISAID enabled regularly updated tracking of outbreak lineages and detection of novel Clade Ib emergence in 2024

Related Diseases

Key Terms: Mpox

• MPXV: Monkeypox virus — the causative orthopoxvirus of mpox disease
• Orthopoxvirus: A genus of poxviruses including variola (smallpox), vaccinia, cowpox, and monkeypox viruses; double-stranded DNA viruses with characteristic "brick-shaped" appearance
• Clade I: Central African (Congo Basin) mpox clade; higher CFR (1-10%); primarily zoonotic and household transmission; subclade Ib emerging as sexually transmitted 2024
• Clade IIb: The lineage responsible for the 2022 global outbreak; predominantly sexually transmitted in MSM networks; lower CFR (<0.1%)
• Jynneos (Imvamune): Modified Vaccinia Ankara (MVA)-based mpox and smallpox vaccine; non-replicating; two doses; preferred for immunocompromised and those at risk; FDA-approved for adults
• Tecovirimat (TPOXX): An antiviral drug that inhibits the VP37 protein needed for viral egress; FDA-approved for smallpox; used under compassionate use / EUA for mpox
• APOBEC3: A host cell DNA-editing enzyme that introduces C-to-T mutations in viral DNA as part of innate immunity; the 2022 Clade IIb strain showed accelerated APOBEC3 signature mutations suggesting prolonged human adaptation
• Smallpox immunity: Prior smallpox vaccination (before 1980 eradication) provides significant cross-protection against mpox due to orthopoxvirus antigen similarity; people vaccinated against smallpox before 1980 have partial mpox immunity
• Post-exposure prophylaxis (PEP): Jynneos given within 4 days of a known mpox exposure to prevent disease; within 14 days to attenuate severity
• MSM: Men who have sex with men — the population disproportionately affected by the 2022 global Clade IIb mpox outbreak, driven by sexual network transmission dynamics

More Mpox Questions

Epidemiology at a Glance: Mpox

Mpox Prevention Checklist

• Know your risk: Current high-risk groups include men who have sex with men (MSM) with multiple partners, and people in active outbreak areas with known case exposure. Risk is low for general public in non-endemic countries.
• Vaccination (Jynneos): Two-dose series (28 days apart) for high-risk individuals. Contact your local health department or sexual health clinic. Pre-exposure vaccination recommended for MSM with multiple partners in areas with ongoing Clade IIb transmission.
• Reduce skin-to-skin contact with unknown individuals — especially if they have unexplained rash, lesions, or have been diagnosed with mpox. Mpox spreads by direct contact with lesions, secretions, and respiratory droplets at close range.
• Isolate if you develop symptoms: Fever + rash (anywhere on body, but especially genitals/perianal) in someone with recent exposure — isolate at home, contact your healthcare provider, and avoid close physical contact with others.
• Bedding/clothing hygiene: Wash bedding, towels, and clothing used by an infected person separately at high temperature. Disinfect surfaces with household disinfectant.
• In endemic regions (DRC and neighbors): Avoid contact with wild animals (rodents, primates); avoid bushmeat; cook all meat thoroughly; avoid contact with sick or dead animals.

Mpox: WHO Guidance Highlights