Background
The single radial immunodiffusion (SRID) assay is used to determine the potency of inactivated influenza vaccines. Nevertheless, development of alternative influenza vaccine potency assays with greater sensitivity and less reliance on large quantities of reference reagents is needed. Many candidate alternative potency assays use monoclonal antibodies (mAbs) to quantify the relevant immunogenic form of HA in vaccines. Although the feasibility of such assays has been demonstrated, concerns remain as to whether the necessary antibody reagents can be generated in the timeframes of vaccine manufacture, particularly for pandemic vaccines.

Methods
We describe the development and evaluation of a mAb-based ELISA assay to measure the potency of several candidate H5 clade 2.3.4.4b A/Astrakhan/3212/2020 influenza vaccines prepared recently in response to the ongoing H5N1 outbreak in the United States. The assay uses H5 mAbs that were generated and characterized several years prior to the emergence of these H5N1 viruses.

Results
Correlation of the ELISA potency results with traditional SRID potency values was excellent for a cell-based vaccine and one egg-based vaccine. The ELISA and SRID potency values for the second egg-based vaccine were not as well aligned initially, but alignment was improved by use of an internal standard.

Conclusions
The results demonstrated that the ELISA potency assay is suitable for determining the potency of inactivated H5 A/Astrakhan/3212/2020 influenza vaccines. Moreover, the results demonstrate the importance of preparing libraries of antibody reagents for influenza subtypes with pandemic potential so that suitable mAbs are available for development of alternative mAb vaccine potency assays when needed.