Optimizing Vaccine Strategy with Fractional Doses During Shortages

Fractional-dose vaccines extend limited antigen supplies, enabling broader and faster population coverage during epidemic shortages and providing a practical tool to reduce transmission when vaccine availability constrains response efforts.

When stockpiles are constrained or deliveries are delayed, supply becomes the primary limiter of population immunity and reduces the share of people reachable before cases accelerate. An analytic framework that compares stockpile limits with achievable coverage helps identify scenarios where dose-sparing strategies offer the greatest net benefit; this approach is most useful in early epidemic phases or settings with limited supply and slow delivery.

Modeling and case analyses show measurable population gains from dose fractionation. In one historical modeling study, using fractional doses during the 2004–05 influenza season could have averted 64% more infections than a full-dose strategy—the estimate came from a transmission-model simulation that incorporated age-stratified contact patterns and reduced per-dose efficacy.

Trade-offs include a modest per-person reduction in efficacy counterbalanced by vaccinating larger absolute numbers, ethical considerations (equity and informed consent), and operational burdens such as ensuring dosing consistency, adapting cold-chain logistics, and scaling enhanced post-dose monitoring.

Small individual-level efficacy losses are offset only when coverage gains are sufficient to reduce transmission at the population level. These risks are manageable but require preplanned policies, transparent explanations, and clear eligibility plus tracking systems to operationalize the strategy.

Communications should state the rationale clearly, delineate population benefits versus individual trade-offs, and commit to predefined safety and effectiveness monitoring to preserve trust. Operational steps include defining fractional-dose thresholds in advance, aligning allocation algorithms with equity objectives, and setting monitoring end points tied to both coverage and effectiveness metrics.

With this planning, uptake and public confidence can be sustained during constrained-supply responses.

Key Takeaways:

• Fractional-dose vaccines can extend limited supplies and increase population-level infections averted when vaccine supply or delivery speed constrains coverage.
• Historical and modeled evidence—including a 64% greater infections-averted estimate for the 2004–05 influenza season—supports using fractionation when supply is the limiting factor.
• Successful deployment depends on clear eligibility criteria, allocation aligned to equity goals, and robust monitoring to maintain uptake and public trust.