SAR HS 325 Lecture Notes - Lecture 3: Case Fatality Rate, Smallpox Vaccine, Herd Immunity
1 May 2018
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SMALLPOX & POLIO
Herd Immunity
• Herd Immunity – the idea that a sufficient number of immune individuals can prevent a
new epidemic
o Essentially a secure group of people who cannot contract an infection
o Immunity can be through vaccination or previous infection
o A good way to prevent an epidemic
• Waning Herd Immunity – when herd immunity drops below the threshold
o Essentially means an epidemic can start spreading in a herd
• Epidemic – the number of infections in a population increases above the baseline
• Baseline – the normal number of cases in a population in a given time
Why would you not receive a vaccine?
• Allergies
• Immunocompromised
• Passive Immunity – already having the disease
• Religious/Cultural Reasons
Smallpox
• Biology & Symptomology of Smallpox
o Two clinical forms
▪ Variola Major is the most common form
o Transmission – face-to-face contact, direct contact, bodily fluid contact,
contaminated objects (less likely but still possible)
o CFP (case fatality rate) = 30-33%
▪ Can vary between countries
o R0 = 5-7
▪ *R0 = reproductive rate (how many people can single person likely infect)
o Threshold for Herd Immunity = 80-85%
o Prevention – pre-exposure vaccination
o Treatment – none
• Summary: Smallpox spreads quickly and has a high death toll
o Question: Why does America keep vaccines of Smallpox?
o Answer: Bioterrorism – select countries keep samples of Smallpox and an
outbreak could occur
Smallpox v. Chickenpox
• Chickenpox - can only be contracted once, has prevention (vaccine) and treatment,
development of spots on mostly the torso
• Smallpox – can be contracted repeatedly, has prevention (vaccine) but no treatment,
develops pustules on mostly hands/feet/face
Stages of Infection
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• Incubation Period (7-17 hours) – Smallpox is not contagious at this stage and there are no
symptoms showing
• Initial Symptoms (Prodrome) (2-4 days) – Smallpox is sometimes contagious at this stage
and symptoms showing include a fever of 101-104, malaise, head/body aches, vomiting
• Early Rash (4 days) – Smallpox is most contagious at the stage and there are symptoms
specific to the day of infection
o Day 1: Small red spots develop in the tongue and mouth. These spots develop into
sores in the mouth and throat indicating the height of an individual’s contagion. A
rash appears on the face and eventually spreads to the arms/legs/hands/feet.
o Day 2: The rash spreads to all other parts of the body within this 24-hour period.
The initial fever falls.
o Day 3: The rash develops into raised bumps.
o Day 4: The raised bumps fill with a thick/opaque fluid and have a depression in
the center (this is the distinguishing characteristic of smallpox). The fever spikes
once again.
Stage Identifiers
• Pustular Rash (5 days) – Smallpox is contagious during this stage and symptoms include
the bumps turning to pustules.
• Pustules and Scabs (5 days) – Smallpox is contagious during this stage and symptoms
include the pustules forming a crust and eventually scabbing. 14 days after the rash has
appeared most of the sores will have scabbed over.
• Resolving Scabs (6 days) – Smallpox is contagious during this stage while scabs are still
present on the body and symptoms include the scabs falling off and leaving pitted scars.
When all scabs have fallen off Smallpox is no longer contagious during this stage.
• Scabs Resolved – Smallpox is no longer contagious at this stage.
Smallpox Timeline
• 1798 – Jenner develops Smallpox vaccine
• 1920s – Smallpox vaccine is improved leading to mass vaccinations
o Improvements: no need for cold storage, leading to easy shipments and no
refrigeration (ideal for places with hot climates and no electricity)
• 1953 – World Health Assembly (WHA) rejects Smallpox eradication proposal
• 1958 – Zhdanov proposes the 10-year eradication plan and the Soviet Union (USSR)
pledges 25M vax doses
• 1959 – WHO report projects possible eradication of Smallpox in 4-5 years, USSR
proposal passed
o At this point, Smallpox is epidemic in 59 countries
• BETWEEN 1959-1965: countries are lackadaisical about pledging money, Smallpox is
not receiving necessary attention
• 1965 – Smallpox eradication unit of WHO is founded
• 1966 – 10-15 million cases and 1.5-2 million deaths are reported annually in 50 countries
• 1973 – Smallpox remains in only 5 countries due to increased surveillance and
containment in other countries
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Immunocompromised: allergies, passive immunity already having the disease, religious/cultural reasons. Incubation period (7-17 hours) smallpox is not contagious at this stage and there are no symptoms showing. These spots develop into sores in the mouth and throat indicating the height of an individual"s contagion. A rash appears on the face and eventually spreads to the arms/legs/hands/feet: day 2: the rash spreads to all other parts of the body within this 24-hour period. The initial fever falls: day 3: the rash develops into raised bumps, day 4: the raised bumps fill with a thick/opaque fluid and have a depression in the center (this is the distinguishing characteristic of smallpox). When all scabs have fallen off smallpox is no longer contagious during this stage: scabs resolved smallpox is no longer contagious at this stage. How did smallpox become eradicated: strong cdc involvement with who"s smallpox eradication unit. Impacts and lessons learned: four major impacts/lessons, 1.
