Questions: Disease Elimination and Eradication: Feasibility and Requirements

Question 1

A global health organization proposes eradicating Plasmodium falciparum malaria through an intensive vaccination and treatment campaign targeting all human cases. What is the most fundamental biological obstacle to this goal?

• A. Malaria has an R₀ too high to achieve herd immunity with any available vaccine
• B. Malaria parasites develop drug resistance too rapidly to sustain control over a multi-decade campaign
• C. Malaria is maintained in non-human primate reservoir hosts, so eliminating human cases cannot prevent reintroduction from animal sources
• D. The malaria vaccine is not thermostable enough for use in the tropical regions where the disease is endemic

Answer: (C)

The presence of an animal reservoir is the most fundamental barrier to eradication — not vaccine efficacy, drug resistance, or logistics. Even if every human case were eliminated, zoonotic reintroduction from reservoir hosts would restart transmission. This is why zoonotic diseases cannot be eradicated regardless of how effective the intervention is against human cases: the pathogen has a survival route that bypasses human immunity entirely.

Question 2

The 'ring vaccination' strategy that helped eradicate smallpox was specifically feasible because:

• A. The smallpox vaccine was more effective than any vaccine before or since, achieving near-perfect protection
• B. Smallpox had a very low R₀, requiring only modest population coverage to suppress transmission
• C. The disease produced a clinically distinctive rash that made cases easy to identify, enabling rapid contact tracing and targeted vaccination
• D. Smallpox had no asymptomatic carriers, so every infectious case was automatically detected

Answer: (C)

Ring vaccination targeted all contacts of identified cases rather than attempting universal coverage. This was feasible precisely because smallpox produced an obvious, distinctive rash that allowed rapid identification of cases and their contacts. Without this clinical visibility, contact tracing at the required speed would have been impossible. The strategy cut off transmission chains without needing to vaccinate entire populations.

Question 3

If a disease has been eliminated from a country, it has been eradicated.

• T. True
• F. False

Answer: False

Elimination means zero ongoing transmission in a defined geographic region; eradication means zero cases globally, permanently, with no further intervention required. An eliminated disease can re-emerge through international importation or reintroduction from animal reservoirs — eradication closes all such pathways globally. The distinction matters enormously for planning: elimination requires sustained surveillance, while eradication allows complete cessation of control efforts.

Question 4

Vaccine-derived poliovirus (VDPV) outbreaks can occur when the live attenuated oral polio vaccine reverts to virulence in under-immunized populations.

• T. True
• F. False

Answer: True

OPV uses attenuated live poliovirus that can accumulate mutations during replication and occasionally revert to a virulent form. In populations with low vaccination coverage, this vaccine-derived virus can circulate and cause outbreaks — a biological complication that has prolonged the final stage of polio eradication and necessitated the transition to inactivated polio vaccine (IPV), which cannot revert but provides weaker mucosal immunity.

Question 5

Why does the distinction between 'elimination' and 'eradication' matter practically when planning disease control campaigns?

Model answer: Elimination (no ongoing regional transmission) and eradication (no global cases, ever) require different biological prerequisites and resources. A disease with an animal reservoir can be locally eliminated but not globally eradicated, because the reservoir will reintroduce it. Conflating the terms risks wasting resources on biologically impossible eradication targets, or prematurely halting surveillance after elimination as though eradication had been achieved.

The practical consequence is that target-setting must precede resource allocation. Setting an eradication target for a zoonotic disease like malaria misdirects effort that could be better spent on control or regional elimination. Conversely, treating elimination as equivalent to eradication and relaxing vaccination campaigns (as happened with measles in some regions) can allow resurgence. Biological constraints should drive realistic goal-setting.