The use of reference preparations in virus content testing of vaccines

A series of virus content tests on Newcastle disease, avian infectious bronchitis and Marek's disease vaccines was carried out over a period of several years on commercial products together with a standard preparation of each virus. Analysis of the results of Marek's disease vaccine titrations revealed a good correlation between plaque count and the ratio obtained for the standard vaccine tested in parallel. Expression in relation to a reference preparation is therefore of value to vaccine manufacturers and quality control authorities as a method of reducing variation among assays carried out on different occasions or by different laboratories. For infectious bronchitis vaccines, expression in relation to a standard was of less value and the method was of little help for evaluating Newcastle disease vaccines. However, in each case, the standard preparation was of value in judging the acceptability of assays.