A collaborative study to assess the proficiency of laboratory estimates of potency of live measles vaccines.

A collaborative study was undertaken to assess the variability in estimates of the potency of measles vaccines. Overall a median variation of 2.0 log10 between estimates was observed. This was reduced to a median of 1.0 log10 when the potencies were expressed relative to a reference vaccine. A difference in the sensitivity between plaque assays and TCID50 assays was also reduced when relative potencies were used. The benefit of including a common reference preparation in vaccine assays was therefore demonstrated. For the vaccines assayed in this study, it was not necessary to use a measles reference of the same strain as the vaccines tested. We therefore recommend that measles vaccines be assayed against a single international reference preparation.