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The potency of purified chick embryo cell rabies vaccine Rabipur was evaluated by four different in vivo test systems: (1) NIH test with homologous strain LEP challenge; (2) intramuscular challenge of vaccinated laboratory animals; (3) postexposure vaccine treatment trials in laboratory animals; and (4) antibody induction tests in laboratory animals. These different test systems were compared with the Standard NIH test and an in vitro test, the modified ABT. Each of the four in vivo methods chosen demonstrated potency values superior to that of the Standard NIH test in which challenge was with CVS strain. Potency relations were: Standard NIH test versus alternative methods versus modified ABT 1.0-2.2-1.85 respectively. There was good correlation between alternative in vivo methods and the ABT. In addition examples of antibody induction in man following administration of various postexposure treatment regimens are presented which lead to the conclusion that postexposure vaccination method and composition of the study group play a more important role in achieving optimum treatment results than vaccine potency, provided that it corresponds to WHO requirements (minimum 2.5 IU per dose).

A new inactivated rabies vaccine (purified chick embryo cell vaccine) has been developed using the Flury LEP-C 25 strain of rabies virus propagated in primary chick embryo cell cultures. The antigen was purified and concentrated by continuous density gradient centrifugation and inactivated by betapropiolactone. This vaccine was tested for innocuity, tolerability and protective capacity in a series of laboratory tests and compared with human diploid cell strain (HDC)-vaccines of similar antigenicity. The results indicated that this new vaccine was excellently tolerated and that its protective activity met the high standard of HDC-vaccine, conditions which were imposed on this vaccine before entering clinical trials in man.

A number of concentration and purification procedures for rabies virus antigen have been developed during the last decades but only ultrafiltration and density gradient centrifugation have been found suitable for large scale production. The antibody binding test developed by Arko, Wiktor and Sikes was modified and was found useful for evaluation of inactivated antigen-concentrates. This test was used as an 'in process' control. Three examples of practical experience are reported and discussed.

The modified antibody binding test, an in vitro method, was validated by a number of studies and was used as a reliable method for in process controls in the manufacture of rabies vaccines for man and animals over the last 6 years. A calculation was made showing the number of laboratory mice which were not needed for animal experiments because these were substituted by the in vitro method mentioned above.

Behringwerke has developed the new, safe and economical purified chick embryo cell (PCEC) rabies vaccine. Due to the purification by zonal centrifugation the compatibility of this vaccine is excellent. Among 933 vaccinations in 219 healthy volunteers the only side-effect was mild pain at the injection-site in 17 vaccinations (1.7 per cent). The sero-conversion of PCEC rabies vaccine was 100 per cent in the tested healthy volunteers. The kinetics of antibody induction after PCEC rabies vaccine is comparable to antibody induction after HDC rabies vaccine. PCEC rabies vaccine induces cellular immunity as measured in lymphocyte transformation test, but no interferon activity.

A new inactivated rabies vaccine (Purified Chick Embryo Cell vaccine) for use in humans has been developed. The FLURY-LEP C25 strain of rabies virus was adapted to propagate in primary chick embryo cell cultures. The antigen was purified and concentrated by continuous density gradient centrifugation and inactivated by Betapropiolactone. The PCEC-rabies vaccine was tested for innocuity, tolerability and protective capacity in a series of laboratory tests and compared with Human Diploid Cell Strain rabies vaccine (HDCSV) of similar antigenicity. The results of the laboratory tests indicated that this new vaccine was excellently tolerated and that its protective activity met the high standard of HDCS-vaccine, conditions which were imposed on the PCEC-vaccine before clinical trials in man were initiated.

63 NIH tests were analysed. A BPL inactivated tissue culture rabies vaccine No. 108 was compared with the national standard vaccine A 18 fourteen times in different tests. The results show that reproduction of results is unsatisfactory with the NIH test and that this test in its present traditional method with two injections is not very suitable for satisfactory potency evaluation of inactivated rabies vaccines. Proposals for modifications are made.

A critical review of the NIH test is given. The results of four studies are presented which show that in the case of purified chick embryo cell inactivated rabies vaccine there is a profound difference between the result after heterologous intracerebral challenge with the prescribed CVS strain and homologous challenge with the LEP-C 26 strain. CVS strains from various laboratories produced relatively uniform results following intracerebral administration. Two in vitro tests, the single radial immunodiffusion test and the modified antibody binding test were employed in the studies. The results are discussed and alternative procedures to the NIH test are recommended for future consideration.

Clinical reactions and neutralizing antibody responses to six pre-exposure regimens of purified chick embryo cell culture rabies vaccine (PCECV) and human diploid cell strain rabies vaccine (HDCSV) were studied in 177 volunteers. Antibody kinetics, height of the response and persistence of antibody over two years were virtually identical after PCECV and HDCSV. An antibody response was detected in all subjects on day 14 when the highest titres were found after two intramuscular (i.m.) 1.0 ml doses of a schedule of immunization on days 0, 7 and 21. In comparison, a schedule of immunization on days 0, 28 and 56 ultimately evoked the highest titres 21 days after the final injection, but antibody persisted equally well over two years with either schedule. Neutralizing antibody titres were lower after intradermal (i.d.) vaccination with 0.1 ml compared to 1.0 ml i.m. on days 0, 7 and 21, but when given on days 0, 28 and 56 the responses were comparable. Three subjects with a personal or family history of atopy developed urticarial lesions after PCECV. Both vaccines were otherwise well tolerated.