Measles epidemiology from parks textbook

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Measles Epidemiology - Park's Textbook of Preventive and Social Medicine

Definition

Measles (Rubeola) is an acute, highly infectious disease of childhood caused by a specific virus of the group myxoviruses. It is characterized by fever, catarrhal symptoms of the upper respiratory tract (coryza, cough), followed by a typical rash. Measles occurs only in humans - there is no animal reservoir.

Problem Statement / Global Burden

Measles is endemic virtually in all parts of the world
Epidemics occur when the proportion of susceptible children reaches ~40%
When introduced into a virgin community, >90% of that community will be infected
In 1980, before widespread vaccine use: estimated 2.6 million measles deaths occurred worldwide
By 2018: ~9.7 million cases and >140,000 measles-related deaths globally
Measles deaths declined 73% and cases by 76% from 2000-2018 following vaccination programs
Responsible for approximately 2% of under-five mortality worldwide

WHO 2015 Targets (milestones toward eradication):

Routine MCV1 coverage ≥90% nationally, ≥80% in every district
Annual measles incidence < 5 cases per million
Reduce measles mortality by ≥95% compared to year 2000

Challenges for measles elimination:

Weak immunization systems
High infectious nature of measles
Populations inaccessible due to conflict
Increasing refusal of immunization
Changing epidemiology - increased transmission among adolescents and adults
Need to provide catch-up vaccination to >130 million children in India
Gaps in human and financial resources

Agent Factors (Epidemiological Triad)

Factor	Detail
Agent	Measles virus - RNA myxovirus; only one antigenic type
Survival	Cannot survive outside the human body for any length of time; retains infectivity at sub-zero temperatures
Source of infection	Only a case of measles; carriers are not known to occur
Infective material	Secretions of nose, throat and respiratory tract during the prodromal period and early rash stages
Communicability	~4 days before to 4 days after appearance of rash
Isolation	For 1 week from onset of rash (covers the communicability period)
Second attack	Only one antigenic type; infection confers lifelong immunity. Most "second attacks" are diagnostic errors

Host Factors

(a) Age: Affects virtually everyone in infancy or childhood

Developing countries: 6 months to 3 years (poor environmental conditions)
Developed countries: Older children, usually >5 years
After vaccine use: disease now seen in somewhat older age groups

(b) Sex: Incidence is equal in both sexes

(c) Immunity:

No age is immune without prior immunity
One attack generally confers lifelong immunity; second attacks are rare
Infants protected by maternal antibodies up to 6 months (may persist beyond 9 months in some)
Immunity after vaccination is solid and long-lasting

(d) Nutrition:

Measles is very severe in malnourished children
Mortality up to 400 times higher in malnourished vs. well-nourished children
Related to poor cell-mediated immunity secondary to malnutrition
Severely malnourished children excrete measles virus for longer periods - prolonged spread risk
Even in a healthy child, severe measles may precipitate malnutrition

Environmental Factors

The virus can spread in any season
Tropical zones: Most cases during the dry season
Temperate climates: A winter disease (people crowd indoors)
India: Epidemics common in winter and early spring (January to April)
Population density and movement affect epidemicity
Less favourable socio-economic conditions → lower average age of attack

Transmission

Mode: Directly person-to-person via droplet infection and droplet nuclei
Period: 4 days before onset of rash to 4 days thereafter
Portal of entry: Respiratory tract (infection through the conjunctiva is also considered likely)
The measles virus is one of the most communicable infectious agents known - secondary attack rates in susceptible household contacts approach 90%

Incubation Period

Event	Timing
Exposure to onset of fever	Commonly 10 days
Exposure to appearance of rash	14 days
Artificially induced (vaccine injection)	Average 7 days

Clinical Features - The Three Stages

1. Prodromal (Pre-eruptive) Stage

Begins 10 days after infection, lasts until day 14
Fever, coryza, sneezing, nasal discharge, cough, red eyes (lacrimation), photophobia
May have vomiting or diarrhoea
Koplik's spots: Appear 1-2 days before the rash on buccal mucosa opposite the 1st and 2nd lower molars - small bluish-white spots on a red base. Pathognomonic of measles

2. Eruptive Phase

Dusky-red, macular or maculo-papular rash beginning behind the ears
Spreads rapidly to face and neck, then downwards - takes 2-3 days to reach lower extremities
Rash may be discrete or confluent/blotchy
Fever disappears in 3-4 days after rash onset
Rash fades in same order of appearance, leaving brownish discoloration (persists up to 2 months)
Virus present in tears, nasal/throat secretions, urine, and blood during prodrome and first 2-5 days of rash
In defective cell-mediated immunity: no rash develops

3. Post-Measles Stage

Child will have lost weight, become anaemic and debilitated
A period of immunosuppression follows measles infection

Diagnosis: Typical rash + Koplik's spots. In developed countries: IgM antibodies by ELISA or RT-PCR of throat swabs/oral fluid/urine. (Note: diagnosis is likely incorrect if red eyes and cough are absent)

Prevention

1. Measles Vaccination (Primary Prevention)

Live attenuated vaccine - given at 9-12 months (developing countries) or 12-15 months (developed countries)
Schedule in India: Two doses - first at 9 completed months, second at 15-18 months (as MMR or MR)
Reconstituted vaccine given subcutaneously (also effective intramuscularly)
Reactions: Mild measles illness (fever + rash) in 15-20% vaccinees, 5-10 days post-immunization; fever lasts 1-2 days, rash 1-3 days

Contraindications:

Severe immunosuppression (congenital immunodeficiency, HIV with severe immunosuppression, leukaemia, lymphoma, alkylating agents, large-dose corticosteroids)
Known pregnancy (avoid pregnancy for 4 weeks post-MMR)
Moderate/severe acute illness (minor illness is NOT a contraindication)
Blood product recipients (delay 3 months after IG; 7-11 months after IV immunoglobulin)

TST and MMR: Do TST at same visit OR delay TST by at least 4 weeks if MMR given first. Measles-containing vaccines may transiently suppress TST response.

2. Herd Immunity

High vaccination coverage (≥95%) creates herd immunity that prevents outbreaks

Eradication Prospects

Measles is a candidate for eradication: only one antigenic type, no animal reservoir, effective vaccine available
Global strategic plans (2012-2016, and continuing) aim for measles elimination region by region as a step toward eradication

Source: Park's Textbook of Preventive and Social Medicine, pp. 166-170

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