Review and evaluation of data defining FMD vaccine potency standards for endemic countries

FMD vaccine is a complex conventional veterinary vaccine. Animal experiments within BSL3 facilities pose additional challenges. Therefore, alternative to cattle challenge for vaccine potency is always needed. The SN₅₀ antibody titre seems a good option as an alternative to cattle challenge. Both SN₅₀ and cattle challenge are subjected to variations. However, the variations in SN₅₀ could be reduced using a narrower acceptable range of virus inoculum (56-133 TCID₅₀/well). Unlike the SN₅₀ thresholds log10, 1.75 as observed by reference laboratories in FMD-free countries, the data for a similar level of protection (75%-80%) from endemic countries is about log10, 1.5. Looking into the possible variations, in potency assessment, vaccine manufacturers have to prepare a vaccine with double the potency (at least 6 PD₅₀) to avoid failure during stringent quality testing for potency assessment. In general, log10, SN₅₀ from 1.26 to 1.64 were considered to correlate with 75-80% protection, while values between 1.50 and 2.12 were expected to achieve protection between 95% and 100% for FMD endemic countries. The protective SN₅₀ values for other viral vaccines inducing virus neutralizing antibodies may be in a similar range as FMD. For the endemic countries like India, the standards seem to be more appropriate as defined in disease-endemic countries like Brazil, Turkey, Egypt, Argentina, and the Republic of Korea, etc.