Peste des Petits Ruminants Vaccine: Criteria for Assessing Its Thermotolerance

The Peste des Petits Ruminants (PPR) live attenuated vaccines, the PPR virus (PPRV) Nigeria 75/1 strain (lineage II) and PPRV India Sungry 96 strain (lineage IV), currently used for control and eradication programme are very efficient vaccines as they provide the host, sheep and goats, a lifelong immunity after a single minimum recommended dose of 10^2.5 TCID₅₀/mL. Unfortunately, both live attenuated vaccines are thermolabile and their use requires maintaining the cold chain from the manufactory premises to the field as most PPR-infected regions are facing of hot climate, with poor infrastructure, and the maintenance of an effective cold chain remains a challenge. To address this challenge, efforts have focused on developing thermotolerant (ThT) PPR vaccines using different stabilisers and improving the freeze-drying process. This study aimed to define the criteria for the evaluation of the stability of ThT PPR vaccines. A total of 37 batches of freeze-dried PPR vaccines using the PPRV Nigeria 75/1 strain, including eight (8) and twenty-nine (29) vaccines labelled as ThT and conventional formulations, respectively, were tested to evaluate the stability at temperatures of 40 °C to simulate the field conditions in some hot climate regions. All the vaccine batches included in this study initially showed acceptable levels of residual moisture, below 3%, and titres above the minimum WOAH standard requirement of 10^2.5 TCID₅₀/mL. Following the incubation at 40 °C, 56.7% and 46% of the 37 vaccine batches tested retained titres above 10^2.5 TCID₅₀/mL on day 3 and day 5, respectively. These vaccines use stabilisers such as skimmed milk, lactalbumin-sucrose, trehalose and one unnamed product (which may be protected for patent). The mean of titre loss among the PPR vaccines maintaining titres above 10^2.5 TCID₅₀/mL was 0.78 log10 at day 3 and 0.99 log10 at day 5, suggesting a significant early degradation during the first 3 days. Based on these data, it is proposed that thermotolerant PPR vaccines should maintain a minimum titre of 10^2.5 TCID₅₀/mL for vaccine dose on day 5 post-incubation at 40 °C with a titre loss below 1 log10 per mL. Preliminary immunogenicity test results showed that the PPR ThT vaccine meeting this criterion could be used in the field without maintaining a cold chain for up to 3 weeks, offering a practical solution for vaccination in remote areas.