Measles virus

Measles is a highly contagious viral exanthem of both historical and contemporary importance, a disease that has shaped epidemics, eradication strategy, and vaccinology since its formal description as distinct from smallpox in the ninth century. Despite a safe and effective live attenuated vaccine in use since the 1960s, measles remains a leading cause of vaccine-preventable childhood mortality. The 2025 New England Journal of Medicine review by Do and colleagues recorded over 395,000 laboratory-confirmed cases in 2024 alone, a resurgence reflecting falling vaccine coverage, COVID-era programmatic disruption, and growing vaccine hesitancy.

Discovery and historical significance

The Persian physician Rhazes first distinguished measles from smallpox in a ninth-century treatise. In 1846 the Danish physician Peter Panum documented the disease’s basic epidemiology (the 14-day incubation period, the lifelong immunity, the explosive contagion) during an outbreak in the Faroe Islands that had been measles-free for sixty-five years. Measles virus itself was isolated and propagated in cell culture in 1954 by John Enders and Thomas Peebles, work that laid the foundation for the first attenuated vaccines licensed in the 1960s. The taxonomic name has changed several times as ICTV nomenclature has evolved: from Measles virus through Measles morbillivirus (2020) and most recently to Morbillivirus hominis in the 2024 Master Species List (MSL40), though the common name “measles virus” (MeV) remains the working term in clinical and laboratory practice.

Classification, structure, and genome

Classification

Measles virus belongs to the genus Morbillivirus, family Paramyxoviridae, order Mononegavirales. Its closest relatives include rinderpest virus (officially eradicated in 2011) and canine distemper virus, reflecting a shared origin in ungulate hosts that crossed to humans some five to ten thousand years ago, when Middle Eastern populations grew large enough to sustain continuous chains of transmission.

The WHO recognises eight clades (A to H) and twenty-four genotypes, distinguished by sequencing the C-terminal region of the N gene. These genotypes are not antigenic variants: measles has remained monotypic despite centuries of immune pressure. They are invaluable for molecular outbreak surveillance and source attribution, supporting the WHO Measles Nucleotide Surveillance (MeaNS) database that underpins elimination verification.

Virion structure

The virion is enveloped, spherical to pleomorphic, and 100 to 300 nm in diameter. A helical nucleocapsid coils within a host-derived lipid envelope studded with two transmembrane glycoproteins, haemagglutinin (H) and fusion (F), and lined internally by the matrix protein (M).

H is the receptor-binding glycoprotein and the dominant target of neutralising antibodies. It engages three known cellular receptors: CD150 (SLAM) on activated lymphocytes, macrophages and dendritic cells (the primary wild-type receptor); Nectin-4 on epithelial cells (important for respiratory shedding and the late phase of dissemination); and CD46 (preferentially used by attenuated vaccine strains, but poorly by wild-type virus). F mediates the actual fusion event: it is synthesised as inactive F₀ and cleaved by host furin into the active disulfide-linked F₁/F₂ form before reaching the cell surface; fusion also requires coexpression of H, whose receptor binding triggers the conformational change that exposes the F₁ fusion peptide.

Genome organisation

The genome is a single, linear, negative-sense single-stranded RNA of approximately 15,900 nucleotides, encoding six structural proteins (N, P, M, F, H and L) and two non-structural proteins (C and V, alternatively translated from the P gene) that antagonise host interferon responses. The gene order runs 3’-N-P-M-F-H-L-5’. A single 3’ promoter directs monocistronic transcription of each gene, with periodic polymerase release at gene junctions producing the characteristic descending mRNA abundance gradient (N most abundant, L least), a hallmark of the Mononegavirales.

Replication cycle

Replication is entirely cytoplasmic; there is no nuclear phase and no DNA intermediate. After H binds its receptor on a susceptible cell, F mediates pH-independent fusion of the viral envelope with the plasma membrane, releasing the nucleocapsid into the cytoplasm. The viral RNA-dependent RNA polymerase (L, with P as cofactor) transcribes capped, polyadenylated monocistronic mRNAs from the negative-sense genome, working sequentially from N at the 3’ end to L at the 5’ end. Periodic polymerase detachment between genes produces the characteristic 3’ to 5’ mRNA abundance gradient (N most abundant, L least).

As cytoplasmic N protein accumulates and reaches a threshold, the polymerase switches from transcription to genome replication: nascent RNA is encapsidated by N as it is synthesised, suppressing transcription termination signals, and the polymerase reads through gene boundaries to produce full-length positive-sense antigenome and, in turn, negative-sense progeny genomes. H and F glycoproteins are translated and glycosylated in the endoplasmic reticulum and trafficked through the Golgi to the plasma membrane, where M binds progeny nucleocapsids and links them to the cytoplasmic tails of H and F, excluding host membrane proteins from patches of bilayer destined for budding. Progeny virions bud from the plasma membrane and are immediately infectious: furin cleavage of F takes place before assembly, so no post-budding maturation step is required.

Two practical consequences of this cycle deserve emphasis. Surface expression of H and F drives cell-to-cell fusion with neighbouring cells, generating the multinucleated giant cells (Warthin-Finkeldey cells) that are characteristic on histology. And in subacute sclerosing panencephalitis, mutations in the M, H and F genes abolish virion assembly while still permitting cell-to-cell viral spread within the CNS, the molecular basis for the persistent CNS infection that defines that disease.

Pathogenesis

Measles is acquired by inhalation of respiratory droplets or by direct contact with respiratory secretions. The earliest cellular targets are alveolar macrophages and dendritic cells expressing SLAM, which carry virus from the airway mucosa to draining lymph nodes within two to four days. A primary viraemia carried by infected monocytes then disseminates virus through the reticuloendothelial system: tonsils, spleen, thymus, lymph nodes and Peyer’s patches all become major sites of replication, with the characteristic Warthin-Finkeldey giant cells and eosinophilic Cowdry type A intranuclear inclusions appearing throughout these tissues. A secondary viraemia, beginning five to seven days after infection, spreads measles to epithelial surfaces via Nectin-4 (skin, conjunctivae, oropharynx, respiratory tract, gastrointestinal tract, kidneys and CNS), producing the prodrome and exanthem that bring the patient to medical attention.

The dominant immunological signature is a profound but transient immunosuppression. Infection of lymphocytes causes lymphopenia and the selective depletion of pre-existing memory T and B cells, a phenomenon now widely called immune amnesia, leaving the patient susceptible to unrelated infections for weeks to months. This explains why most measles deaths are caused not by the virus itself but by bacterial pneumonia, gastroenteritis, and other secondary infections. Paradoxically, a concurrent robust MeV-specific antibody and cellular response is mounted and confers lifelong immunity. In the severely immunocompromised (advanced HIV, primary immunodeficiency, post-anti-CD20 therapy), the rash may be absent, viral shedding is prolonged, and the virus can persist in the CNS to cause measles inclusion-body encephalitis months later, or subacute sclerosing panencephalitis years later.

Epidemiology

Measles is among the most contagious infectious diseases known to medicine, with a basic reproduction number (R₀) of 12 to 18, exceeding even smallpox and rubella. There is no animal reservoir and no chronic carrier state; the virus is sustained solely by an unbroken chain of acute infections in human populations, requiring a critical community size of roughly 300,000 to 500,000 to maintain endemic transmission. In smaller populations, episodic outbreaks depend on importation.

Globally, measles morbidity declined substantially after the introduction of mass measles vaccination in the 1960s: from an estimated 130 million cases and 7 to 8 million deaths annually in the pre-vaccine era, to fewer than 115,000 deaths in 2014. That trajectory has reversed in the past half-decade. The Do et al. NEJM 2025 review records that by 2024 all WHO regions reported rising case numbers, with over 395,000 laboratory-confirmed cases globally and a further 16,000 in the first two months of 2025; hospitalisation was required in more than half the reported cases, so the true burden is likely substantially higher.

Genotype B3 dominates current African circulation; genotype D8 has driven recent European and North American outbreaks; genotypes imported into elimination-status countries are catalogued in the WHO MeaNS database and used for source attribution of every sporadic case.

Natural history

The natural history of an uncomplicated case follows a predictable sequence. After exposure, a 10 to 14-day silent incubation gives way to the 2 to 4-day prodrome with the three C’s; Koplik’s spots appear 1 to 2 days before the rash. The exanthem then evolves cephalo-caudally over three to four days, fades in the same direction over a similar period, and the patient becomes clinically well during the subsequent week, with cough sometimes persisting longer than other features.

Most well-nourished immunocompetent patients recover fully. However, the heightened susceptibility to secondary infection due to immune amnesia persists for weeks to months, accounting for most measles mortality. In a small subset, CNS complications (ADEM, MIBE, SSPE) develop on timelines ranging from days to years after acute infection. There is no chronic measles infection of the immunocompetent host: either the virus is cleared (with lifelong immunity), or it kills, or it persists quietly in the CNS to cause delayed neurological disease.

Clinical presentations and complications

Measles produces three clinically recognised syndromes that differ by host immune status and prior vaccine type.

Classical (typical) measles

The classical syndrome begins after the 10 to 14-day incubation with a 2 to 4-day prodrome of rising fever and the three “C’s” (cough, coryza and conjunctivitis). The pathognomonic Koplik’s spots appear on the buccal mucosa opposite the lower molars 1 to 2 days before the exanthem and last 1 to 2 days after rash onset: small bluish-white papules on an erythematous base, easily missed unless actively looked for.

The exanthem is an erythematous maculopapular rash that begins behind the ears and on the hairline, spreads cephalo-caudally over three to four days to involve the trunk, limbs, palms and soles, and fades in the same direction, often with fine desquamation. Patients are infectious from approximately four days before to four days after rash onset, a window that substantially predates clinical recognition and is the principal reason quarantine alone is ineffective.

Atypical measles syndrome

Atypical measles syndrome is now rare but historically and diagnostically important. It was first described in the 1960s in children who had received the killed (inactivated) measles vaccine in use between 1963 and 1967; on subsequent exposure to wild-type measles, these children developed a hypersensitivity-style illness with high fever, severe headache, abdominal pain, interstitial pneumonia with pleural effusion, and a rash that began on the extremities and spread centripetally (the opposite of the classic pattern), sometimes vesicular, petechial, or purpuric.

Modified measles

Modified measles is the attenuated form seen in persons with partial immunity: infants aged 6 to 9 months with waning maternal antibody, recipients of recent post-exposure normal human immunoglobulin, and vaccinated individuals with breakthrough infection. The clinical course is milder (shorter prodrome, less pronounced and faster-fading rash, Koplik’s spots often absent) and serology can be ambiguous, with a lower IgM:IgG ratio characteristic of a secondary immune response. Modified cases remain infectious and contribute to under-recognised onward transmission in vaccinated populations.

Acute complications

Acute complications are driven by direct epithelial injury and by the prolonged immunosuppression that follows acute infection. The commonest acute complication is otitis media, which can lead to sensorineural hearing loss. Pneumonia (either the classic giant-cell pneumonia of immunocompetent children or secondary bacterial pneumonia) is the leading cause of measles death worldwide. Other acute complications include laryngotracheobronchitis (croup), diarrhoea with dehydration (particularly in malnourished children), and keratoconjunctivitis with the attendant risk of corneal scarring and blindness in vitamin-A-deficient settings. Myocarditis occurs but is rare.

Neurological complications

The neurological complications form a striking spectrum, defined by the timing after infection, the host immune status, and the presence and form of virus in the CNS.

Acute postinfectious encephalomyelitis (ADEM) is the commonest, occurring in approximately one in a thousand cases: a post-infectious autoimmune demyelinating process arising days to weeks after rash, with no detectable virus in the brain and a mortality of 10 to 20 per cent.

Measles inclusion-body encephalitis (MIBE) strikes the immunocompromised, develops over weeks to months, demonstrates active CNS virus replication, and is almost universally fatal.

Subacute sclerosing panencephalitis (SSPE) is the late, devastating complication of measles acquired in early childhood (particularly under two years of age), appearing six to ten years after the primary infection. The recent NEJM review reports an incidence of 7 to 11 per 100,000 cases, possibly higher per recent surveillance. The diagnostic triad is gradual neurological decline with myoclonus, characteristic periodic high-amplitude slow-wave EEG complexes, and intrathecal measles antibody with oligoclonal IgG bands.

Beyond these acute events, the immune amnesia of measles continues to impair host defence against unrelated pathogens for months to years, a non-rash mortality burden that vaccination largely abrogates, and one of the strongest population-level arguments for high vaccination coverage.

Diagnosis

The diagnostic approach is matched to the clinical phase of illness, the patient’s immune status, and the public-health question being asked. Measles-specific IgM by enzyme immunoassay on serum is the routine surveillance test; it becomes detectable approximately four to five days after rash onset, peaks at one to three weeks, and falls to undetectable within four to eight weeks. A positive IgM in a clinically compatible case is sufficient to confirm acute infection. A four-fold rise in IgG between paired sera collected two to four weeks apart is the alternative serological confirmation, and low-avidity IgG indicates a recent primary response.

Several caveats matter: recent measles-containing vaccine (within approximately six weeks) can produce a false-positive IgM; in a secondary immune response in a vaccinated person, IgM may be positive at a lower IgM:IgG ratio than in primary infection, requiring avidity testing for resolution; and in immunocompromised patients IgM may be negative altogether.

Reverse-transcription PCR is therefore essential, particularly in early disease, in the immunocompromised, in suspected CNS disease, and in any outbreak investigation. Appropriate specimens are a throat or nasopharyngeal swab (or oral fluid) and / or a urine sample, both in the first four days of rash for maximum yield (useful up to seven days), and cerebrospinal fluid where central nervous system disease is suspected. Sensitivity is highest when multiple specimen types are submitted early.

In any confirmed case, genotyping at the NICD reference laboratory (sequencing the C-terminal 450 nucleotides of the N gene, sometimes with the H gene) assigns the isolate to one of the WHO-recognised genotypes and is submitted to the global WHO Measles Nucleotide Surveillance (MeaNS) database. Genotyping enables source attribution (imported versus endemic), transmission-chain mapping, distinction of wild-type from vaccine-strain infection in a vaccinee with an IgM-positive post-vaccination rash, and the documentation of elimination (continuous interruption of endemic genotype circulation for at least twelve months is a WHO elimination criterion).

Management

There is no approved antiviral medication for measles. Management therefore rests on supportive care, vitamin A, the early treatment of complications, and isolation. Supportive care is straightforward but critical: oral or intravenous rehydration; paracetamol for fever (aspirin is contraindicated in children because of Reye’s syndrome risk); nutritional support (measles is profoundly catabolic and malnutrition worsens prognosis); oxygen and ventilation as required for pneumonia; and gentle eye care.

Vitamin A is the single intervention with strong randomised-trial support. WHO has recommended it for all children with acute measles since 1993, regardless of suspected deficiency, after randomised trials in the 1980s and 1990s in low- and middle-income countries demonstrated a 34 to 50 per cent reduction in measles mortality in children aged one to five years. The dose is administered orally on two consecutive days (50,000 IU for infants under six months, 100,000 IU for infants 6 to 11 months, and 200,000 IU for children of twelve months and over), with a third dose two to four weeks later in the presence of clinical eye signs of vitamin A deficiency or severe malnutrition. Strict adherence to WHO-recommended doses matters: high-dose unsupervised vitamin A is toxic, and the NEJM 2025 review documents recent toxicity cases linked to public misinformation during US outbreaks.

Ribavirin is not approved and not routinely recommended. Anecdotal use exists in severe disease in the immunocompromised, in MIBE, and in some severe pneumonias, but no randomised trial supports it. Complications-directed treatment includes empirical antibiotics for secondary bacterial pneumonia (not prophylactically to all measles cases); amoxicillin first-line for otitis media; supportive care with nebulised adrenaline and a corticosteroid for severe croup; oral rehydration and zinc for diarrhoea per WHO paediatric protocols; supportive ICU care for acute encephalitis or ADEM; and palliative care for the inevitably fatal MIBE and SSPE.

Hospitalised patients should be managed under airborne precautions, ideally a negative-pressure room with N95 or FFP2 respirators for staff; the NICD’s pre-hospital guidance specifies standard, contact and droplet precautions. The patient is isolated and excluded from school, work, healthcare facilities and public gatherings until four days after rash onset.

Prevention and public health

Vaccination

Two doses of a measles-containing vaccine are the cornerstone of prevention. MMR combines live-attenuated measles, mumps and rubella vaccines; MMRV adds varicella; MR (measles-rubella only) is the WHO-preferred low-cost product for high-burden settings. All are live attenuated and pharmacologically similar; MMRV is contraindicated in HIV-infected persons regardless of CD4 status (only MMR may be used).

The clinically important contraindications to a measles-containing vaccine are pregnancy (including planning pregnancy within three months, with pregnancy to be avoided for one month after vaccination), severe immunosuppression, and a severe allergic reaction to a previous dose or to vaccine components, notably neomycin or gelatin. Egg allergy is no longer a contraindication: residual ovalbumin from the chick embryo fibroblast substrate is minimal. HIV-infected children with adequate CD4 thresholds (at least 15 per cent in those aged five years and younger, and at least 200 cells/µL in those over five) should be vaccinated.

Two-dose schedules and herd-immunity threshold of 95 per cent make universal childhood immunisation the only viable elimination strategy. Most national programmes give the first dose at 12 to 15 months and the second at 4 to 6 years; high-transmission settings give an earlier first dose (6 to 9 months) plus the two routine doses.

Post-exposure prophylaxis

Post-exposure prophylaxis follows the NICD framework. For an immunocompetent contact, measles-containing vaccine within 72 hours of exposure is most effective, though it remains worth giving thereafter (it is safe in someone already incubating and will boost immunity in the already-immune).

For a vaccine-contraindicated contact (severe immunosuppression, infants under six months whose mothers are non-immune, pregnant women without documented immunity, or HIV-infected contacts with CD4 below the thresholds above), normal human immunoglobulin 0.5 mL/kg intramuscularly (maximum 15 mL) within six days of exposure is the alternative. Both the NICD FAQ and the NEJM 2025 review caution that the measles-neutralising antibody content of contemporary NHIG preparations has declined as donor populations have shifted toward vaccine-acquired rather than naturally-acquired immunity; efficacy is correspondingly uncertain and the recommended dose is under periodic review.

Surveillance and notification

Measles is a globally notifiable condition under the WHO elimination framework. National surveillance systems collect clinical, laboratory and genotype data on every suspected case and feed them into regional and global databases. WHO MeaNS catalogues genotyped isolates worldwide, supporting source attribution and elimination verification. The measles and rubella laboratory network coordinates standardised testing methodology, proficiency testing, and reference confirmation. In elimination-status countries, a single confirmed case triggers a mandatory public health response.

Outbreak response

Outbreak response combines rapid case investigation, identification of all susceptible contacts (including healthcare and household contacts), prompt PEP within the 72-hour or 6-day windows, catch-up and supplementary immunisation activities in under-vaccinated communities, exclusion of cases until four days after rash, and intensified surveillance for secondary cases. Healthcare-associated outbreaks are particularly important because they expose immunosuppressed patients to the highest-risk inoculum.

Elimination and eradication

Measles meets every one of the Dahlem Conference (1997) preconditions for the eradication of an infectious disease: there is no animal reservoir; the virus is antigenically monotypic; natural infection and complete vaccination both confer lifelong immunity; a highly immunogenic vaccine exists; the disease is clinically recognisable; and sensitive laboratory tools support confirmation and source attribution. Eradication has nevertheless eluded decades of effort because of operational rather than biological obstacles: a herd-immunity threshold of at least 95 per cent two-dose coverage that few countries achieve and sustain, growing vaccine hesitancy (much of it still attributable to the fraudulent and long-retracted 1998 Wakefield paper linking MMR to autism, despite that link being comprehensively disproven by large cohort studies in multiple countries), COVID-era disruption of routine immunisation services, recent declines in WHO and Gavi funding, and the young-infant immunity gap that current vaccines cannot close.

Five of the six WHO regions have set measles elimination targets, but only the Americas have approached interruption of endemic transmission, and even that achievement has been challenged by recent importations and outbreaks. Global elimination is a multi-decade endeavour requiring sustained political, financial, and operational investment.

South African context

Measles is endemic in South Africa with intermittent outbreaks between calmer interepidemic years. Most cases occur in children under five years of age, and the majority of these in children under one year, the rationale for the early-onset first dose of the South African EPI.

Vaccination in the SA EPI. Since 2023 to 2024 the public-sector Expanded Programme on Immunisation has used the Measles-Rubella (MR) vaccine manufactured by Cipla, given at 6 and 12 months of age. The 6-month dose is administered under NDoH policy in alignment with WHO guidance for high-transmission settings, despite the product’s Patient Information Leaflet stating a minimum age of 9 months. The private sector uses MMR or, less commonly, MMRV (with added varicella), typically at 12 months and again at 4 to 6 years.

Outbreak history. South Africa experienced a major outbreak between October 2009 and August 2010 with approximately 18,000 laboratory-confirmed cases, dominated by genotype B3 and concentrated in under-vaccinated communities in Gauteng, KwaZulu-Natal, and the North West. A follow-up outbreak in 2022 to 2023 produced over 1,000 laboratory-confirmed cases, with clusters in Limpopo, Mpumalanga, and the North West, attributed to COVID-era programmatic disruption and pre-existing under-vaccinated cohorts. The Western Cape recorded a localised outbreak in 2024 to 2025 in the Cape Metro and West Coast districts, requiring intensified vaccination response.

Notification. Measles is a Category 1 Notifiable Medical Condition in South Africa: suspected cases must be notified to the NICD within 24 hours, and a single laboratory-confirmed case in a previously elimination-status area triggers an outbreak response. The NICD Centre for Vaccines and Immunology is the national reference laboratory, performing serology, RT-PCR, and genotyping (including submission to MeaNS).

HIV co-infection. The high HIV prevalence in South African children shapes the local vaccinology and the outbreak response. HIV-infected children remain at high risk of severe and persistent measles infection; PEP and supplementary immunisation activities prioritise this group. Adequate CD4 thresholds permit vaccination; MMRV (with varicella) is contraindicated in any HIV-infected individual regardless of CD4. NHIG is preferred for PEP in the severely immunocompromised contact.

Service-delivery considerations. Routine first-dose MR coverage has historically tracked at 80 to 90 per cent, with substantial provincial variation; second-dose coverage is typically 5 to 10 percentage points lower. The 6-month dose counts toward routine surveillance but not as a WHO-counted “first routine dose”; the WHO target of 95 per cent two-dose coverage refers to MCV1 (at 9 to 12 months) and MCV2 (at 4 to 6 years).

References and recommended reading

• Moss WJ, Griffin DE. Measles. In: Richman DD, Whitley RJ, Hayden FG, editors. Clinical Virology, 4th ed. ASM Press; 2016. Chapter 38.
• Do LAH, et al. Measles. N Engl J Med 2025;393:2447 to 2458. DOI: 10.1056/NEJMra2504516.
• National Institute for Communicable Diseases (Centre for Vaccines and Immunology). Measles: Prevention of secondary cases (FAQ document FAQ_Measles_20170111). Compiled January 2017, reviewed June 2022.
• Cipla. Measles Rubella Cipla: Patient Information Leaflet, Version 2, May 2023.
• DeStefano F, Shimabukuro TT. The MMR vaccine and autism. Annu Rev Virol 2019;6:585 to 600.
• Korsman SNJ, van Zyl GU, Nutt L, Andersson MI, Preiser W. Virology: An Illustrated Colour Text (Churchill Livingstone / Elsevier, 2012).