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CM7.2 | CM7.2 | Disease Transmission and Control Principles — SDL Guide (Part 2)
Principles of Prevention and Control
Prevention and control strategies are catalogued by which link in the chain of infection they target, and by level of prevention (primary, secondary, tertiary). Park's classic framework aligns these two axes.
Targeting the reservoir/source:
- Treatment of cases: curative treatment (antibiotics for TB, antiretrovirals for HIV) reduces the infectious period and lowers the source's infectivity. Mass drug administration for lymphatic filariasis reduces the reservoir at population scale.
- Isolation: physical separation of infectious cases from susceptible persons during the communicable period (respiratory isolation for TB, enteric precautions for typhoid).
- Quarantine: separation of exposed but not-yet-symptomatic persons during the incubation period to prevent transmission if they become infectious.
- Surveillance and contact tracing: rapidly identifying cases and their contacts so the chain can be broken before secondary transmission occurs (core strategy in TB RNTCP, HIV, COVID-19).
- Environmental management (reservoir elimination): draining mosquito breeding sites for malaria/dengue, rodent control for plague/leptospirosis.
Interrupting the mode of transmission:
- Safe water supply and sanitation (breaks water-borne and food-borne chains)
- Food hygiene and quality control
- Vector control: insecticide-treated bed nets, indoor residual spraying (IRS), larval source management
- Universal precautions and healthcare infection control (gloves, gowns, sterile procedures)
- Harm reduction for blood-borne transmission (needle exchange programmes)
Protecting the susceptible host:
- Active immunisation (vaccination): induces protective immunity before exposure; primary prevention. Herd immunity occurs when enough of the population is immune that transmission chains cannot sustain: the herd immunity threshold = 1 − 1/R₀. For measles (R₀ ≈ 12–18), herd immunity requires ≥92–95% coverage; for polio (R₀ ≈ 5–7), ≈80–85% is sufficient.
- Passive immunisation: administration of pre-formed antibodies (immunoglobulins) for immediate but short-term protection (post-exposure prophylaxis for hepatitis B, rabies, varicella in immunocompromised).
- Chemoprophylaxis: antimicrobial agents given to susceptible persons to prevent infection (isoniazid preventive therapy for TB contacts, cotrimoxazole prophylaxis in HIV)
- Personal protective equipment and behaviour modification: condoms for STI/HIV prevention, mosquito repellents, hygiene promotion
For non-communicable diseases, the analogous framework targets: the source (tobacco companies, food industry — via regulation and taxation), the exposure pathway (smoke-free environments, healthy food labelling), and the host (cessation programmes, dietary counselling, pharmacological prevention of cardiovascular disease in high-risk individuals).
CLINICAL PEARL
Quarantine and isolation are not the same. Isolation applies to confirmed infectious cases during their communicable period — they are separated because they can transmit now. Quarantine applies to exposed contacts during the incubation period — they may transmit if they become infectious. The duration of quarantine equals the maximum incubation period of the disease (e.g. 10 days for COVID-19, 21 days for Ebola). Confusing the two can be catastrophic: releasing a quarantined contact before the incubation period expires allows a pre-symptomatic infectious person to seed new chains of transmission. In COVID-19 the concept of pre-symptomatic transmission (up to 2 days before symptom onset) meant that self-isolation at symptom onset was insufficient — hence the policy shift toward quarantine of contacts regardless of symptoms.
SELF-CHECK
The herd immunity threshold for measles is approximately 92–95%. A district achieves 85% measles vaccination coverage. Which statement best describes the epidemiological consequence?
A. Measles cannot spread because most individuals are immune
B. Herd immunity is achieved because the majority are protected
C. Unvaccinated individuals are protected because the threshold is exceeded
D. Epidemic measles can still occur because the threshold is not reached
Reveal Answer
Answer: D. Epidemic measles can still occur because the threshold is not reached
At 85% coverage, the measles herd immunity threshold (≥92–95%) is NOT reached. The R₀ of measles (12–18) means each case can infect up to 18 susceptibles in a fully naive population; only when ≥92–95% are immune does the chain reliably break. With 15% susceptibles and a high R₀, sustained transmission and epidemic spread remain possible, especially in spatially clustered pockets of unvaccinated individuals. This is why measles can cause large outbreaks even in countries with apparently high national coverage if the coverage is uneven.
Applying Control Principles: Communicable and Non-Communicable Disease
Applying the chain-of-infection framework to two contrasting diseases illustrates its practical utility.
Tuberculosis (a communicable disease with airborne transmission):
The chain for TB runs: reservoir = humans with active pulmonary TB; portal of exit = respiratory secretions during coughing/sneezing; mode = airborne (droplet nuclei, <5 µm, may survive in poorly ventilated spaces); portal of entry = respiratory tract; host = immunocompromised, malnourished, HIV-positive individuals, household contacts.
Control strategies organised by chain link:
- Source: DOTS (Directly Observed Treatment, Short-course) under RNTCP/NTEP — treat and cure infectious cases, reducing communicable period. Case notification and contact tracing identifies secondary cases.
- Transmission: ventilation of healthcare settings (negative pressure isolation rooms), N95 respirators for healthcare workers, patient mask use.
- Host: BCG vaccination (reduces severe childhood TB and miliary/meningeal TB); isoniazid preventive therapy (IPT) for TB-HIV co-infected persons and close contacts.
Hypertension (a non-communicable disease with multi-factorial 'transmission'):
While hypertension has no infectious agent, the web of risk factors propagates through shared environments and behaviours:
- Source reduction: policy interventions — tobacco taxation and smoke-free laws (reduce smoking prevalence), salt reduction in processed food (reduces dietary sodium at population level), trans-fat regulation.
- Pathway interruption: healthy urban design (walkable cities, cycling infrastructure, reducing sedentary behaviour), workplace wellness programmes, healthy school meals.
- Host protection: individual-level counselling (dietary modification, physical activity, smoking cessation), pharmacological prevention (antihypertensives in individuals with persistent hypertension ≥140/90 mmHg or lower with organ damage/diabetes per current Indian guidelines).
The parallel structure reveals that the prevention framework is more general than the communicable disease context in which it was originally developed. Both communicable and non-communicable disease control ultimately require interventions at all three levels — source, pathway, and host — because no single intervention is sufficient alone.