Recently, several cases were identified where neutralization was considerably more potent or only detected in the older PBMC assay compared to the newer assay technology [43, 46, 47] . This raises important questions about current plans to employ a single assay for routine use, and it points to the need for a better understanding of the mechanisms of neutralization. Thus, it may be necessary to use more than one assay to assure that all neutralizing antibodies are detected. There is a need to standardize and compare neutralizing antibody assays and to decide which assay or combination of assays should be used for standardized assessments of vaccine-elicited neutralizing antibody responses. A major priority is to strengthen the standardization of the PBMC assay, given that it is the only assay that has been at least partially validated in passive antibody experiments in animal challenge models.