Can leaky vaccines favor nastier viruses?
Imperfect Marek’s disease vaccination keeps infected chickens alive longer, boosting transmission of strains too lethal to persist without vaccines.
Source
Imperfect Vaccination Can Enhance the Transmission of Highly Virulent Pathogens
Study at a glance
- Design
- Animal / in-vitro — Experimental MDV infection of vaccinated vs unvaccinated chicks with shedding and sentinel transmission assays
- N
- Multiple experiments (e.g., n=200, 120, 60 vaccinated + 60 maternal-Ab groups) — no single primary N
- Population
- One-day-old chicks infected with Marek's disease virus strains
- Outcome
- Infectious period, shedding, and sentinel infection under imperfect vaccination
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Inspect source passages
Can leaky vaccines favor nastier viruses?
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
Vaccination prolonged infectious periods by about 2 weeks.
Vaccination protected sentinel birds from death (), prolonging infectious periods by about 2 wk (; standard error of the difference ±3.2 d, F 1,36 = 19.9, p < 0.0001).
Sentinels became positive a median 9 days earlier with vaccination.
Nonetheless, even for strain 595, vaccination led to more rapid infection of sentinels (; median time to positivity 9 d earlier than in unvaccinated birds, p < 0.05), thus increasing the rate at which secondary cases were generated, a critical determinant of both viral fitness and case incidence in a rising epidemic.
What they did
Authors experimentally infected vaccinated and unvaccinated chickens with MDV strains of different virulence and measured shedding, sentinel infection, and infectious periods.
What they found
Vaccination prolongs infectious periods (~2 weeks), speeds sentinel infection (~9 days earlier), and can maintain hyperpathogenic strains that kill unvaccinated birds at up to 100%.
The limits
What it doesn't show
Experiments show vaccination can maintain hypervirulent strains, not that vaccines alone historically caused MDV evolution.
Key terms
- Imperfect vaccine
- Vaccine that reduces disease but does not fully block infection/transmission.
- Virulence
- Harm to the host, here mortality rate/speed in unvaccinated birds.
- Marek’s disease virus (MDV)
- Poultry herpesvirus that evolved hyperpathogenic strains.
- Infectious period
- Time an infected host can shed/transmit pathogen.
- Sentinel bird
- Contact animal used to measure onward transmission.
Flashcards
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Quiz yourself
Leaky vaccines can favor virulent strains by:
Common questions
How do leaky vaccines help virulent strains?
They keep hosts alive, extending shedding/transmission.
How much longer was infectious period?
About 2 weeks with vaccination.
Did vaccination speed secondary infection?
Yes—sentinels positive ~9 days earlier.
Did this prove vaccines caused historical evolution?
No—authors say vaccination is sufficient to maintain hyperpathogens now.
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