BIO220H1 Lecture 11: Final Topics in Evoluntionary Medicine
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Final Topics in Evolutionary Medicine
• Pathogen evolution in response to vaccination
• Myopia: a consequence of mismatch with the current environment?
• Fever: disease or defense?
• Vaccines are biological treatments meant to improve immune
responses to future exposures to specific diseases.
In essence, antigens in the vaccine prime the immune system so that it responds
quickly to future assaults by those same (or very similar) antigens.
• Making vaccines is an evolutionary process.
• Smallpox is an orthopox virus that was once widespread and led to
sometimes fatal infectious disease.
• Spread by inhalation of airborne virus particles or direct contact
with contaminated material.
• A worldwide vaccination program eliminated the disease.
• Last case in Canada was in 1946, and in the world 1977 (Somalia).
The smallpox vaccine
• In the 1700’s it was common knowledge that milkmaids did not get
• Milkmaids did get a much less virulent disease, cowpox (a related
• In 1796 Jenner began exposing patients to the puss from blisters of
• He subsequently determined those patients were immune to smallpox.
Vaccine preventable diseases
Haemophilus influenzae b
Human Papillomavirus (HPV)
Influenza (Seasonal Flu)
Japanese Encephalitis (JE)
Pertussis (Whooping Cough)
Rubella (German Measles)
Shingles (Herpes Zoster)
Deaths in children under 5 from vaccine preventable diseases
Access to, and delivery of, vaccines remain public health priorities.
Evolution in a vaccinated world
• Antibiotics are failing with increasing frequency, due to
evolutionary responses. What about vaccines?
• Immune systems are sources of massive selection on pathogens (e.g., influenza).
• By activating an immune response, vaccination can be
viewed as a potential source of selection on pathogens.
• Given widespread vaccination, should we be worried about
its evolutionary effects?
Evolution in a vaccinated world?
Evolutionary response in the pathogen→ resistant
Two case studies
Hepatitis B virus
• Globally significant cause of hepatitis, liver cirrhosis, and liver cancer.
• Vaccine targets the ‘a’ determinant (a major surface antigen).
• In the 90s, there was concern of vaccine resistance: a large clinical
trial found a number of vaccinated individuals ultimately acquired infections.
• These infections were caused by a vaccine resistant mutant.
Key mutation in the viruses → single nucleotide mutation
Could not be seen by the immune system
• Mutant allele increased in frequency in vaccinated people.
• Thus, the HBV populations are evolving in response to vaccine.
• BUT! These ‘vaccine resistant’ strains have not become a major public
health problem. Health benefits of vaccination have continued.
• Mutant strains are outcompeted in unvaccinated hosts?
(i.e., there is a cost to resistance)
• Vaccine still protects against severe disease?