Uses of Epidemiology

"A means of learning, or asking questions... and getting answers that lead to further questions."

1. To Study the Historical Rise and Fall of Disease

• The first contribution to the study of coronary heart disease (CHD) was recognizing it as an "epidemic."
• Old diseases like smallpox are conquered, while new ones like Legionnaires' disease, Lassa fever, and AIDS emerge.
• By studying time trends, useful projections into the future can be made and emerging health problems identified.

2. Community Diagnosis

• Priority setting - quantifying problems helps decide which diseases to tackle first.
• Benchmark creation - morbidity/mortality data serves as a baseline for evaluating health services later.
• New knowledge - reveals patterns of disease distribution, causation, and prevention.

3. Planning and Evaluation of Health Services

• Determining the number of hospital beds required for specific diseases
• Health manpower planning
• Planning immunization campaigns and screening programs
• Provision of sanitary services

4. Evaluation of Individual Risks and Chances

• Absolute risk - incidence rate, specific rates
• Relative risk - comparison between exposed and non-exposed groups
• Attributable risk - the portion of risk attributable to a specific exposure

• Risk of bearing a child with Down syndrome (trisomy 21)
• Risk assessment for smokers vs. non-smokers for cancer and CHD

5. Syndrome Identification

• Differentiation of gastric vs. duodenal ulcer - a "poverty gradient" in gastric ulcer certification but not in duodenal ulcer led to recognition of two distinct entities (before 1920 this distinction was poorly understood).
• Testing the Paterson-Kelly syndrome (dysphagia + iron-deficiency anaemia) epidemiologically showed the association was not supported.
• Distinct subtypes of essential hypertension identified via plasma renin levels.

6. Completing the Natural History of Disease

• The epidemiologist studies disease in the community in relation to agent, host, and environmental factors.
• Example: Epidemiology revealed that one-third to two-thirds of deaths from ischaemic heart disease are sudden (within less than one hour). Most victims never reach the hospital. This finding gave tremendous impetus to the development of intensive coronary care units (ICCUs).

7. Searching for Causes and Risk Factors

• Rubella as a cause of congenital defects in the newborn
• Thalidomide as a teratogenic agent
• Cigarette smoking as a cause of lung cancer
• Excess oxygen exposure in premature babies as the cause of retrolental fibroplasia

Uses of Descriptive Epidemiology (additionally)

1. Provide data on the magnitude of disease load (morbidity and mortality rates)
2. Provide clues to disease aetiology and help formulate aetiological hypotheses
3. Provide background data for planning, organising, and evaluating preventive and curative services
4. Contribute to research by describing variations in disease occurrence by time, place, and person

The Carrier: Role in Disease Transmission

Definition

"An infected person or animal that harbours a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection for others."

Elements of a Carrier State

1. Presence of the disease agent in the body
2. Absence of recognizable symptoms and signs of disease
3. Shedding of the disease agent in discharges or excretions, thus acting as a source of infection for others

Why Carriers Are Epidemiologically Dangerous

• They escape recognition - neither they nor others know they are infected
• They continue living a normal life in the community
• They can infect susceptible individuals over a wider area and longer period of time under favourable conditions

Classification of Carriers

A. By Type

• Shed the infectious agent during the incubation period, before clinical illness begins
• Capable of infecting others before the onset of their own symptoms
• Usually occurs during the last few days of the incubation period
• Examples: measles, mumps, polio, pertussis, influenza, diphtheria, hepatitis B

• Continue to shed the disease agent during the period of convalescence (after clinical recovery)
• Clinical recovery does NOT coincide with bacteriological recovery
• Example: A typhoid fever patient may excrete the bacilli for 6-8 weeks after recovery - posing a serious threat to household members
• Other examples: dysentery (bacillary and amoebic), cholera, diphtheria, whooping cough
• Highlights the importance of bacteriological surveillance after clinical recovery

• Emerge from subclinical (inapparent) infection - they never had overt disease but shed the agent
• Examples: poliomyelitis, cholera, meningococcal meningitis, salmonellosis, diphtheria
• Important note: not all subclinical infections lead to carrier state. In polio, a subclinical case may act as a temporary carrier; in tuberculosis, most persons with a positive tuberculin test do NOT actively disseminate bacilli and are not labelled carriers.

B. By Duration

• Shed the infectious agent for short periods of time
• Includes incubatory, convalescent, and healthy carriers

• Excrete the infectious agent for indefinite periods
• Diseases: typhoid fever, hepatitis B, dysentery, cerebrospinal meningitis, malaria, gonorrhoea
• Far more important sources of infection than cases - the longer the carrier state, the greater the risk to the community
• Some excrete intermittently, some continuously
• Duration varies by disease: in typhoid fever and hepatitis B, may last several years; in chronic dysentery, a year or longer
• In diphtheria, carrier state is associated with infected tonsils; in typhoid fever with gall bladder disease
• Chronic carriers can reintroduce disease into areas otherwise free of infection (e.g., malaria)
• Early detection and treatment are essential to limit the spread

C. By Portal of Exit

Special Note: Pseudo-Carriers

• Carriers of avirulent (non-disease-causing) organisms are called pseudo-carriers
• Pseudo-carriers are not important epidemiologically as they cannot cause disease in others

Adverse Events Following Immunization (AEFI)

Definition

An AEFI is any untoward medical occurrence which follows immunization and which does not necessarily have a causal relationship with the usage of the vaccine. The adverse event may be any unfavourable or unintended sign, abnormal laboratory finding, symptom or disease.

• True adverse events - genuinely a result of the vaccine or immunization process
• Coincidental events - not due to the vaccine but temporally associated with immunization

CIOMS/WHO 2012 Cause-Specific Classification of AEFIs

"Immunization" includes all processes after a vaccine leaves the manufacturing site: handling, prescribing and administration.

1. Common Minor Vaccine Reactions

• Occur in about 10% of vaccinees
• DTP (whole cell) or tetanus boosters: up to 50% affected
• BCG produces a specific reaction: papule 2+ weeks post-immunization → ulcerates → heals in months → leaves a scar. Keloid formation is more common among Asian and African populations

• Fever in ~10% or less, except DTP (~50%)
• Measles vaccine: fever, rash, conjunctivitis in 5-15% (mild compared to wild measles; can be severe in immunocompromised)
• Mumps (swollen parotid) and rubella (joint pains, lymph nodes): <1% in children; rubella causes joint pains in up to 15% of adults
• OPV: systemic reactions <1%

2. Rare but Serious Vaccine Reactions

Most rare and serious reactions (seizures, thrombocytopenia, HHE, persistent screaming) do not lead to long-term problems. Anaphylaxis, while potentially fatal, is treatable without long-term effects.

3. Serious vs. Severe: An Important Distinction

• Serious: An AEFI that results in death, is life-threatening, requires hospitalization, causes persistent disability/incapacity, is a congenital anomaly, or requires intervention to prevent permanent impairment
• Severe: Describes the intensity of an event (mild/moderate/severe) - the event itself may be of minor medical significance (e.g., mild fever vs. moderate fever). Anaphylaxis is always both serious AND severe.

4. Case Definitions and Treatment of Specific AEFIs

5. Fainting vs. Anaphylaxis - Key Differences

6. Events That Should Be Reported

Minor common reactions such as local reactions, fever, and self-limiting symptoms need not be reported.

7. Investigating AEFI Clusters

• If all cases from one facility (same lot) → Programme error
• If all got the same vaccine/lot and it's a known reaction occurring at expected rate → Vaccine product-related reaction
• If the event also occurred in unimmunized people → Coincidental event
• If it's a known reaction occurring at an increased rate → Programme error or vaccine quality defect

8. Steps in an AEFI Investigation

1. Confirm information - Obtain medical records, identify other cases
2. Investigate and collect data - Patient history, immunization history, prior allergies, clinical description, vaccine storage conditions
3. Assess the service - Vaccine storage, reconstitution process, syringe/needle sterilization, training of vaccinators
4. Formulate a working hypothesis on the likely cause
5. Test the hypothesis - Does case distribution match? Are laboratory tests needed?
6. Conclude investigation - Reach a conclusion, complete AEFI Investigation Form, take corrective action

Health Advice to International Travelers

Why Travel Medicine Matters

Pre-Travel Consultation: Key Elements

The CDC Travelers' Health website provides comprehensive, up-to-date country-specific recommendations on vaccinations, malaria chemoprophylaxis, and travel health notices.

1. Immunizations

• Routine vaccines - all travelers should be up to date: influenza, COVID-19, tetanus-diphtheria-pertussis, MMR, varicella, pneumococcal (if indicated)
• Recommended vaccines based on destination and activities:
  • Hepatitis A - most common vaccine-preventable illness in travelers
  • Typhoid - recommended for travel to South Asia, sub-Saharan Africa, South America
  • Hepatitis B - if not already immunized
  • Yellow fever - required for entry to certain countries in Africa and South America; certificate needed
  • Meningococcal - required for Hajj pilgrims; recommended for sub-Saharan Africa meningitis belt
  • Japanese encephalitis - for rural travel to Asia
  • Rabies - for remote destinations, outdoor activities, prolonged stays
  • Cholera - for aid workers or high-risk areas
  • Chikungunya - a live vaccine approved in the US (2023) for adults ≥18 years at increased risk; single dose; contraindicated in immunocompromised
• Required vaccines: Yellow fever certificate (International Certificate of Vaccination or Prophylaxis - ICVP) is legally required for entry into some countries

2. Malaria Prevention

Mosquito Avoidance (Anopheles mosquitoes bite dusk to dawn)

• Wear long-sleeved shirts and long pants
• Apply DEET 20-50% insect repellent to exposed skin (preferred by CDC and WHO)
• Consider permethrin-treated clothing
• Sleep under insecticide-treated bed nets if not in well-screened accommodation
• Limit outdoor activity between dusk and dawn

Malaria Chemoprophylaxis

• Significant hepatic or renal impairment affects the choice of prophylaxis
• Drug-drug interactions with regular medications must be checked
• Prophylaxis reduces risk but does not eliminate it - any fever during or after travel should prompt urgent evaluation for malaria

3. Other Arthropod-borne Infections

• Dengue fever - leading cause of fever in travelers returning from most tropical destinations outside sub-Saharan Africa. No FDA-approved dengue vaccine for US travelers (Dengvaxia limited to residents with prior infection; Qdenga approved in EU for age ≥4). Personal mosquito protection (Aedes mosquitoes bite day and night) is the main strategy.
• Chikungunya - causes severe arthralgias that can become chronic; vaccine available for high-risk adults ≥18 years
• Zika - major concern for pregnant women; DEET and barrier contraception essential
• Yellow fever - use personal protection + vaccination for endemic areas

4. Travelers' Diarrhea

• Affects an estimated 25-50% of travelers to developing countries
• 90% are brief and self-limited (1-5 days)
• 5-10% develop dysentery; 1-2% have persistent diarrhea >2 weeks
• 3-17% develop post-infection IBS

High-Risk Destinations

Common Causes

• Bacterial (60-80% of cases): ETEC and EAEC account for 50-75% overall; regional variation - Campylobacter and Salmonella common in Southeast Asia; Shigella in Africa and South Asia
• Viral: Norovirus ~15% of cases
• Parasitic (Giardia, Entamoeba histolytica, Cryptosporidium): more common in prolonged diarrhea (>14 days)

Prevention

• Eat only thoroughly cooked hot foods
• Avoid raw vegetables, unpeeled fruits, shellfish, street food
• Drink bottled or boiled water; avoid ice in low-resource settings
• "Boil it, cook it, peel it, or forget it"

Treatment

Antibiotic prophylaxis is not recommended routinely - reserved for highly selected high-risk individuals only (immunocompromised, high-stakes travel with no access to care).

5. Food and Water Safety Summary

• Boil water for 1 minute (3 minutes at altitude >2000 m) OR use iodine/chlorine tablets OR use a filtration device
• Safe foods: hot cooked foods, fruits you peel yourself, pasteurized dairy
• Avoid: raw/undercooked meat and seafood, unpasteurized dairy, tap water, ice, raw salads in high-risk areas

6. Other Important Health Topics

Altitude Illness

• Risk at >2500 m; symptoms include headache, nausea, fatigue, dizziness
• Prevention: ascend gradually (≤300-500 m/day above 3000 m), rest, stay hydrated
• Acetazolamide (Diamox) 125-250 mg BD starting 1-2 days before ascent is recommended prophylaxis for those ascending rapidly
• Dexamethasone for treatment of high-altitude cerebral oedema (HACE)

Venous Thromboembolism (VTE)

• Long-haul flights (>4 hours) increase DVT risk, especially in high-risk individuals
• Advice: stay hydrated, move around the cabin, perform leg exercises, wear graduated compression stockings
• Anticoagulant prophylaxis for very high-risk travelers (prior DVT/PE, known thrombophilia, recent surgery)

Jet Lag

• Due to disruption of circadian rhythm crossing multiple time zones
• Short-acting hypnotics (e.g., zolpidem) and melatonin (0.5-5 mg at destination bedtime) can help adaptation

Sexually Transmitted Infections

• Incidence of STIs increases significantly during travel
• Carry and use condoms; pre-exposure prophylaxis (PrEP) for HIV-negative travelers at risk
• Hepatitis B vaccination if not already immune

Animal Bites and Rabies

• Avoid petting or approaching stray animals, dogs, bats, monkeys
• Pre-exposure rabies vaccination for travelers to remote destinations, children, and those with occupational exposure
• Any animal bite: wash wound immediately with soap and water for at least 15 minutes; seek post-exposure prophylaxis (PEP) urgently

Injury and Crime Prevention

• Avoid motorcycles/motorbikes - major cause of injury deaths
• Use seatbelts; avoid night driving in low-resource settings
• Be aware of personal security; avoid ostentatious displays of wealth

Mental Health

• Travel-related stress can exacerbate or unmask psychiatric disorders
• Depression and anxiety are common reasons for medical evacuation

7. Returning Traveler - Post-Travel Screening

• Any fever within 3 months of return from a malaria-endemic area = malaria until proven otherwise - urgent blood films required
• Persistent diarrhea >14 days: stool microscopy and culture for parasites
• Jaundice: consider hepatitis A/B/E, leptospirosis, malaria
• Skin lesions: consider cutaneous leishmaniasis, myiasis, larva migrans
• Unexplained eosinophilia: helminth infections (schistosomiasis, strongyloides, filariasis)

Special Traveler Groups