Why Vaccine Success Varies: 8 Determinants of Immunization Impact

1. Vaccine Intrinsic Effectiveness

Measles vaccine: 97% effective (2 doses). Influenza: 40–60% (strain match dependent).

2. Pathogen Biology

Stable antigens (measles, smallpox, polio) → durable immunity. High mutation (flu, HIV, SARS-CoV-2) → immune escape.

3. Herd Immunity Threshold

R₀ determines coverage needed: Measles (R₀~12–18) → 95%+. Polio (R₀~5–7) → 80–86%. Sub-threshold → outbreaks.

4. Antigenic Variation

Flu requires annual reformulation; universal vaccines remain elusive. HIV diversity thwarted vaccines for decades.

5. Immune Complexity

Malaria (complex life cycle, antigenic variation), TB (intracellular, latency) resist simple vaccine solutions.

6. Cold Chain Integrity

mRNA vaccines (-70°C to -20°C) challenge low-resource settings. Thermostable formulations are a priority.

7. Vaccine Hesitancy

Misinformation, distrust, complacency reduce uptake. Measles resurgence in high-income countries is largely hesitancy-driven.

8. Health System Capacity

Funding, workforce, supply chain, data systems. Gavi, COVAX aim to close equity gaps.

No single factor dominates; success requires alignment across all.