Viability evaluation of freeze dried and suspension anthrax spore vaccine formulations stored at different temperatures
Anthrax is endemic in Ethiopia with sporadic outbreaks despite the regular vaccination of domestic livestock. This has raised concerns on the effectiveness of the vaccination strategy which may be associated with breaches in the vaccine cold chain maintenance. This study was aimed at demonstrating the tolerance of anthrax vaccine to cold chain breaches through evaluation of viable spore counts expressed as colony forming units per mL (CFU/mL) of freeze-dried and suspension anthrax vaccines stored at 5 °C, 20 °C and 37 °C for up to 6 months. Both vaccine formulations maintained above the recommended minimum required titre (2 × 106 culturable spores per dose for cattle, buffaloes and horses, and not <1 × 106 for sheep and goats) for up to 6 months at 5 °C storage. In storage at 20 °C, the viability of freeze-dried anthrax vaccine maintained the minimum required titre up to 6 months while up to 90 days in case of the suspension formulation. Both types of vaccine formulations maintained the minimum titre per dose for up to 30 days at 37 °C storage. Generally, both vaccine formulations showed similar trends in titre fall in all of the three storage temperatures (5 °C, 20 °C and 37 °C) as observed in the almost linearly overlapping 95% confidence intervals (CI) up to day 90 at 5 °C and 20 °C storages while up to day 30 at 37 °C storage. However, a significant (P < 0.05) drop in titre was observed after day 90 for storages at 5 °C and 20 °C, and after day 30 for 37 °C storage as observed in the non overlapping 95% CI from the average titres of previous time points. This study showed that if temperature excursion occurs above the recommended temperature range (4-8 °C) during storage or transport, the vaccine should remain effective and can still be used in vaccination programs.