Overview of TAG-CO-VAC and December 2025 Preparations
The World Health Organization’s Technical Advisory Group on COVID-19 Vaccine Composition (TAG-CO-VAC) continues to monitor the genetic and antigenic evolution of SARS-CoV-2, immune responses to natural infection and vaccination, and how vaccines perform against circulating variants. In anticipation of its December 2025 meeting, TAG-CO-VAC outlines the data it considers essential for guiding future updates to COVID-19 vaccine antigen composition. The forthcoming statement and an accompanying data annex will be published on the WHO website after the decision-making session. These steps balance the latest epidemiological, immunological, and virological insights with the kinetics of vaccine-induced protection and the lead time manufacturers need to update antigen designs.
What data are being sought for the December 2025 deliberations?
To inform decisions on antigen composition, TAG-CO-VAC assesses data on the genetic evolution of SARS-CoV-2 and the antigenic properties of both previously circulating and current variants. This includes analyses of animal antisera following primary infection or vaccination in one-way and two-way neutralization tests, as well as immunogenicity data that gauge the breadth and durability of immune responses using sera from sequentially immunized or infected animals and pre- and post-vaccination human samples.
The group also considers vaccine effectiveness (VE) estimates—particularly those that adjust for time since vaccination and provide variant-specific protection across different vaccine platforms for any infection, symptomatic disease, and severe disease. Additional examples of data reviewed by TAG-CO-VAC and used to inform antigen composition decisions are documented in annexes accompanying earlier statements.
Data categories and sources
TAG-CO-VAC reviews available data from vaccine manufacturers, including non-clinical and clinical studies that demonstrate the breadth and durability of immune responses elicited by currently authorized vaccines as well as candidates in development. For vaccine candidates in development, the utility of non-clinical and clinical immunogenicity data in informing antigen composition decisions is emphasized. Comparable immunogenicity data against the same variants from prior vaccine compositions are particularly valuable in enabling consistent cross-comparisons.
Genetic evolution and antigenic characterization
Data on the evolving genetic changes in SARS-CoV-2 and how these changes affect antigen recognition are critical. TAG-CO-VAC considers how newer variants may influence vaccine protection and whether updates to antigen composition are warranted.
Immunogenicity and neutralization data
Immunogenicity data—including breadth and durability of neutralizing antibody responses—are evaluated using sera from sequential infections or vaccinations and from different vaccination regimens. This information helps gauge the potential protection offered by current vaccines against emerging variants.
Vaccine effectiveness and real-world data
Observational VE data, particularly those that account for time since vaccination and provide variant-specific estimates across platforms, are weighed to understand real-world performance in preventing infection, disease, and severe outcomes.
Data from manufacturers and candidates in development
Manufacturers are asked to provide data from both non-clinical and clinical studies that illustrate the breadth and durability of immune responses for currently authorized vaccines and any candidates in development. For candidates, immunogenicity data against the same target variants as previous compositions are especially useful for comparative assessment.
How these data inform decisions and timing
The December 2025 decision-making cycle is designed to align the latest epidemiological and immunological evidence with the kinetics of vaccine-induced protection and the lead time required for antigen updates. The resulting statement and annex will guide scientists and manufacturers on the specific data to generate ahead of the deliberations and on how to present it for evaluation.
Guidance for researchers and manufacturers
Beyond immunogenicity and VE data, TAG-CO-VAC stresses data quality, comparability, and relevance to circulating variants. The guidance serves as an update to the March 2025 statement, clarifying priority data types and reporting standards to inform the next antigen update. This ensures that vaccine antigen compositions stay aligned with evolving virus characteristics and population immunity.
Conclusion
As SARS-CoV-2 continues to evolve, TAG-CO-VAC’s data requests aim to support timely, evidence-based decisions on COVID-19 vaccine antigen composition, helping to sustain protection against circulating variants and safeguard global vaccination programs.
