There has been continuous and substantial evolution of SARS-CoV-2 since the virus emerged in the human population, posing challenges to the ongoing public health response, including ensuring that vaccines continue to provide protection.

In September 2021, the World Health Organization (WHO) established the Technical Advisory Group on COVID-19 Vaccine Composition (TAG-CO-VAC). This is an independent and multidisciplinary group of experts that assesses the public health implications of emerging SARS-CoV-2 variants on the performance of COVID-19 vaccines and issues timely recommendations on proposed modifications to vaccine antigen composition. Specifically, the TAG-CO-VAC considers the genetic and antigenic evolution of SARS-CoV-2 variants, immune responses to SARS-CoV-2 infection and COVID-19 vaccination, and the performance of COVID-19 vaccine products against circulating SARS-CoV-2 variants. Based on these assessments, the TAG-CO-VAC evaluates the implications for COVID-19 vaccine antigen composition and issues recommendations to WHO to either maintain current COVID-19 vaccine antigen composition or to consider updates.

The work of the TAG-CO-VAC supports a larger risk monitoring and assessment framework for SARS-CoV-2 variants which is coordinated by WHO. As such, the work of the TAG-CO-VAC is closely aligned with the Technical Advisory Group on Virus Evolution (TAG-VE) and the Strategic Advisory Group of Experts on Immunization (SAGE).

As an advisory body to WHO, the main functions of TAG-CO-VAC are to:

1. Make recommendations to WHO on the methods to assess the impact of SARS-CoV-2 variants on COVID-19 vaccines;
2. Provide interpretation of available evidence on the effect of SARS-CoV-2 variants on COVID-19 vaccines, including but not limited to vaccine effectiveness; and
   
3. Recommend to WHO, for each COVID-19 vaccine platform, adaptations (if any) needed so that vaccines continue to safely provide WHO-recommended levels of protection against SARS-CoV-2 variants.