Measles (Rubeola) - Park's Textbook of Preventive and Social Medicine

Definition

Problem Statement

• Measles is endemic virtually worldwide and occurs in epidemics when the proportion of susceptible children reaches ~40%.
• In virgin communities, >90% of individuals will be infected upon introduction.
• In 1980, before widespread vaccination, ~2.6 million measles deaths occurred annually worldwide.
• By 2018, measles-related deaths fell by 73% and cases by 76% compared to 2000, but still accounted for ~9.7 million cases and >140,000 deaths in 2018.
• Measles accounts for ~2% of under-five mortality worldwide.
• In India, it is a significant cause of childhood morbidity, with West Bengal, Assam, J&K, Maharashtra, Delhi, Uttar Pradesh, and Rajasthan being heavily affected states.

Epidemiological Determinants

• In tropical zones: mostly dry season
• In temperate climates: winter disease (crowding indoors)
• India: epidemics peak January to April (winter-early spring)
• Population density and poor socioeconomic conditions lower the average age of attack

Transmission

• Primarily droplet infection and droplet nuclei (person-to-person)
• Portal of entry: respiratory tract (conjunctival route also possible)
• Communicable from 4 days before rash to 4 days after rash onset
• Vaccine recipients are NOT contagious to others

Incubation Period

• 10 days from exposure to onset of fever
• 14 days from exposure to appearance of rash
• When live vaccine is injected (bypassing respiratory tract): shortened to ~7 days

Clinical Features

1. Prodromal Stage (Days 10-14 post-exposure)

• Fever, coryza (sneezing, nasal discharge), cough, red eyes (conjunctivitis), lacrimation, photophobia
• Vomiting or diarrhoea may occur
• Koplik's spots - appear 1-2 days before the rash: small, bluish-white spots on a red base, like "table salt crystals," on buccal mucosa opposite the first and second lower molars - pathognomonic of measles

Koplik's spots on buccal mucosa - Andrews' Diseases of the Skin

2. Eruptive Stage

• Dusky-red macular or maculopapular rash begins behind the ears, spreads over face and neck within hours
• Extends downward over trunk to lower extremities over 2-3 days (cephalocaudal spread)
• Rash may be discrete initially, then becomes confluent and blotchy
• Virus is present in tears, nasal/throat secretions, urine, and blood during the prodrome and first 2-5 days of rash
• As rash appears, circulating antibodies become detectable, viraemia disappears, and fever falls
• Rash results from interaction of immune T-cells with virus-infected cells in small blood vessels; in patients with defective cell-mediated immunity, no rash develops
• Fever and rash clear by day 3-4 of eruptive phase; rash fades leaving brownish discoloration that persists for 2+ months

3. Post-Measles Stage

• Weight loss and weakness persist for days
• Risk of secondary infections: increased susceptibility to bacterial and viral infections
• May develop: growth retardation, diarrhoea, cancrum oris, pyogenic infections, candidiasis, reactivation of pulmonary tuberculosis

Diagnosis

• Clinical: typical rash + Koplik's spots; diagnosis is incorrect in any febrile exanthem without red eyes and cough
• In developed countries (where measles is uncommon):
  • Specific IgM antibodies by ELISA (present at rash onset)
  • RT-PCR (detects measles virus RNA in throat swabs, oral fluid, nasopharyngeal mucus, or urine)
  • RT-PCR can distinguish true measles from IgM elevation due to recent vaccination

Complications

• Developing countries: 3-6%, up to 30% in displaced/isolated populations
• Developed countries: 0.01-0.1%
• Highest mortality: children <1 year and adults >30 years

Treatment

• Relieve fever, cough, nasal congestion, conjunctivitis
• Nutritional support (prevent malnutrition)
• Encourage breast-feeding where appropriate
• Oral rehydration salts to prevent dehydration
• Patient isolation (important to prevent spread)

• All severe cases and all cases in areas with high case-fatality rates
• Dose schedule:
  • <6 months: 50,000 IU/day for 2 days
  • 6-11 months: 100,000 IU/day for 2 days
  • ≥12 months: 200,000 IU/day for 2 days
  • If clinical signs of vitamin A deficiency (e.g., Bitot's spots): give a third dose 4-6 weeks later
• Rationale: Many children develop acute vitamin A deficiency during measles, which can lead to keratomalacia and blindness from corneal scarring

Prevention

1. Achieve immunization rate >95%
2. On-going immunization through successive generations

Measles Vaccination

• Only live attenuated vaccines are recommended (safe and effective)
• Available as: monovalent measles; MR (measles-rubella); MMR (measles-mumps-rubella); MMRV (measles-mumps-rubella-varicella)
• Freeze-dried product; reconstituted with sterile diluent; each dose 0.5 mL with ≥1,000 viral infective units
• Contains sorbitol and hydrolyzed gelatin as stabilizers; small amount of neomycin; no thiomersal
• Sensitive to heat and sunlight; kept in colored glass vials
• Reconstituted vaccine loses ~50% potency in 1 hour at 20°C; nearly all potency at 37°C in 1 hour
• After reconstitution: store at 2-8°C in dark; use within 4 hours

• Pregnancy (theoretical grounds)
• Altered immunocompetence (except HIV-infected who are not severely immunocompromised)
• History of anaphylaxis to prior dose or vaccine component
• Note: patients with anaphylactic reactions to eggs can be vaccinated without prior skin testing

Immunoglobulin (Passive Immunization)

• Dose: 0.25 ml/kg body weight given within 3-4 days of exposure
• Live measles vaccine should be given 8-12 weeks later to establish active immunity
• Need is now much reduced due to availability of live attenuated vaccine

Outbreak Control Measures

1. Isolation for 7 days after onset of rash
2. Immunization of contacts within 2 days of exposure
3. If vaccine is contraindicated: immunoglobulin within 3-4 days of exposure
4. Prompt immunization at the start of an epidemic to limit spread

Eradication of Measles

• Humans are the only reservoir
• There is only one serotype
• Effective live attenuated vaccines are available

• ≥90% MCV1 coverage nationally; ≥80% in every district
• Annual incidence <5 cases per million population
• ≥95% reduction in measles mortality compared to 2000

1. 95% coverage with measles and rubella vaccination
2. Case-based MR surveillance with laboratory support
3. Linkage with other child health interventions
4. Increased public confidence and demand for immunization

Recent Evidence

• A 2024 meta-analysis (PMID 39574018) confirms high measles vaccine effectiveness in African children
• A 2025 systematic review (PMID 39799850) examined adverse events following measles vaccines in infants <12 months
• A 2026 systematic review (PMID 41619351) provides updated guidance on measles exposure periods for contact tracing

Measles - Cyclical (Epidemic) Trend Explained

The Core Concept

Why Cycles Occur - The Susceptible Pool Mechanism

Epidemic burns through susceptibles
       ↓
Susceptible pool depleted → epidemic dies out
       ↓
New births (unimmunized) + waning maternal antibodies
add susceptibles back to the population
       ↓
Susceptible pool builds up again over successive birth cohorts
       ↓
Threshold reached (~40% susceptible) → next epidemic ignites
       ↓
Repeat

• There are no animal reservoirs and no latent/persistent infections with measles virus
• One attack confers lifelong immunity - once infected, a person is removed from the susceptible pool permanently
• The primary source of new susceptibles is unimmunized newborns, who lose maternal antibody protection by 6-9 months of age

Park's Textbook - What It Says Directly

"Measles tends to occur in epidemics when the proportion of susceptible children reaches about 40 per cent."

• Park's Textbook of Preventive and Social Medicine

"Prior to the immunization programme, cyclical increase in the incidence of measles were recorded every third year in India. With the increase in immunization coverage levels, the intervals between cyclical peaks has increased and the intensity of the peak minimized."

• Park's Textbook of Preventive and Social Medicine

• Before the Universal Immunization Programme (UIP): epidemics every 3 years
• In 1987: ~2.47 lakh (247,000) cases reported
• After UIP implementation: cases fell to 20,895 with 34 deaths in 2018
• The cyclical pattern persisted in tribal and remote areas with low vaccine coverage

Two Overlapping Cycles (Harrison's, 22nd Ed.)

The Role of R₀ and Herd Immunity Threshold

• Measles has the highest R₀ of any known pathogen - estimated at 12-18 (Goldman-Cecil Medicine), meaning one case can infect 12-18 susceptible contacts
• This extreme transmissibility means:
  • Outbreaks can occur when even <10% of the population is susceptible (Harrison's)
  • The herd immunity threshold needed to prevent spread is very high: approximately 92-95% immune coverage required
  • Achieving >95% vaccination rate is required to prevent epidemic cycles (Park's)

How Vaccination Changes the Cycle

1. Lengthens the interval between epidemic peaks
2. Reduces the intensity (size) of each peak

Shift in Age Distribution with Vaccination

• Low coverage, high density (developing countries): disease hits infants and young children (peak <2 years)
• Moderate coverage: cases shift to school-age children
• High coverage: cases shift to adolescents and adults (who escaped natural infection and were not vaccinated)
• Some countries develop a bimodal distribution: cases in young infants (born to mothers with vaccine-derived - rather than naturally acquired - antibodies, whose maternal antibody levels are lower) AND adults

Summary Diagram of the Cycle

EPIDEMIC PEAK
 (susceptibles ~40% of population)
        |
        ↓ (epidemic burns through susceptibles)
INTER-EPIDEMIC TROUGH
 (susceptibles now <threshold; virus has nothing to spread to)
        |
        ↓ (new births accumulate unimmunized susceptibles)
        ↓ (waning maternal antibody in infants)
        ↓ (migration of susceptibles into area)
SUSCEPTIBLE POOL REBUILDS
        |
        ↓ (threshold ~40% reached again)
NEXT EPIDEMIC PEAK

The 95% Herd Immunity Threshold for Measles - Mathematical Explanation

Step 1: Start with R₀ (Basic Reproduction Number)

"Measles may be the most contagious pathogen known to humans... its basic reproduction number R₀ has been estimated to be 12 to 18."

• Goldman-Cecil Medicine

Step 2: The Effective Reproduction Number (Rₑ)

• Rₑ = effective reproduction number (actual number of new cases per index case)
• R₀ = basic reproduction number (12-18 for measles)
• p = proportion of the population that is immune

Step 3: The Condition for Epidemic Control

Step 4: Plug in Measles R₀

1. It covers the upper end of the R₀ range
2. It builds in a safety margin for vaccine efficacy (vaccines are ~95-99% effective, not 100%)
3. It accounts for geographic clustering of unvaccinated individuals

Step 5: Why Vaccine Efficacy Matters - Required Coverage ≠ HIT

Step 6: What Happens at Different Coverage Levels

Step 7: Intuitive Explanation

R₀ = 15: One case tries to infect 15 people

At 93% immune:     [ I I I I I I I I I I I I I S S ] → 2 new cases (Rₑ=2 → grows)
At 93.3% immune:   [ I I I I I I I I I I I I I I S ] → 1 new case (Rₑ=1 → stable)
At 95% immune:     [ I I I I I I I I I I I I I I I ] → <1 new case (Rₑ<1 → dies)

I = Immune, S = Susceptible

Step 8: Why Clustering Breaks the Model

Comparison with Other Vaccine-Preventable Diseases

"Studies have shown that it is neither possible nor necessary to achieve 100 per cent herd immunity in a population to halt an epidemic... The proportion of immune individuals above which a disease may no longer persist is the herd immunity threshold. Its value varies with the virulence of the disease, the efficacy of the vaccine and the contact parameter for the population."

• Park's Textbook of Preventive and Social Medicine

Summary