At the International Society for Pharmacoepidemiology (ISPE) 2025 Annual Conference in Providence, Rhode Island, Dr. Eileen C. Farnon, Director of Research for the Safety Platform for Emergency vACcines (SPEAC) project, presented on a critical issue in global vaccine safety: how to prepare for adverse events of special interest-X (AESI-X) that may emerge with novel vaccines.
Her talk, “Preparing for
Vaccine Adverse Events of Special Interest-X (AESI-X): a standardized approach applied to novel vaccines,” highlighted SPEAC’s leadership in developing proactive, standardized methods for vaccine safety preparedness.
Why AESI-X Matters
Real-time harmonization of case definitions (CDs) for unexpected or novel AESIs is essential for vaccine safety surveillance. During the COVID-19 vaccine rollout, for example, the emergence of Thrombotic Thrombocytopenia Syndrome (TTS) underscored the challenge: different groups developed different case definitions, leading to fragmented data and reduced comparability across surveillance systems.
With initiatives like CEPI’s 100 Days Mission aiming to make new vaccines available within 100 days of a pandemic, the ability to rapidly align on standardized safety definitions will be indispensable.
The AESI-X Preparedness Plan
To ensure readiness, the SPEAC project, which builds on the Brighton Collaboration’s foundational work, has developed a consensus AESI-X Case Definition Preparedness Plan. This structured process enables vaccine safety stakeholders to respond quickly and consistently when signals of an AESI-X arise.
Key elements include:
- Outreach to countries introducing new vaccines to offer assistance with AESI-X case definition development.
- Signal detection and evaluation in collaboration with regulators, academia, and health authorities.
- Formation of a dedicated Case Definition Working Group.
- Rapid reviews and publication of draft case definitions.
- Implementation of AESI-X CDs in surveillance systems and safety studies.
- Continuous revision of CDs as new information emerges.
- Assessment of lessons learned after the response.
- Updates to the preparedness plan to strengthen future readiness.
This plan addresses three possible AESI-X categories:
- Known-knowns: syndromes already linked to other vaccines.
- Known-unknowns: syndromes recognized in medicine but not previously tied to vaccines.
- Unknown-unknowns: novel or variant syndromes never before observed in association with vaccination.
Moving Forward
By developing this consensus process ahead of time, SPEAC and its partners aim to ensure that novel vaccines — whether for epidemic-prone pathogens or other targets — can be monitored effectively and safely from the start.
Next steps include:
- Broadening stakeholder consultation, especially with regulators and health agencies in low- and middle-income countries.
- Increasing awareness of the AESI-X Preparedness Plan among pharmacovigilance and real-world data systems worldwide.
SPEAC’s Commitment
Dr. Farnon’s presentation at ISPE 2025 emphasized not only the technical innovation behind AESI-X but also the collaborative approach that defines SPEAC’s work. By engaging regulators, researchers, and global health partners, SPEAC continues to ensure that vaccine safety preparedness keeps pace with vaccine innovation.
