Further observations on the stability of cholera vaccine at different temperatures.

The stability of cholera vaccine samples held at 4-8 and 20-25 degrees C for 2 1/2 years was studied with regard to the number of organisms, total nitrogen content and relative antigenicity. After this length of time, the percentage loss in the number of organisms averaged 10.7 at 4-8 degrees C and 22.7 at 20-25 degrees C. The maximum loss in number of organisms took place during the first six months and in the next two years there was very little loss in the number of organisms. The total nitrogen content after 2 1/2 years remained almost unaltered at these temperatures. All the ten samples had acceptable relative antigenicity for both the Inaba and Ogawa serotypes after exposure at 4-8 degrees C for 2 1/2 years, while at 20-25 degrees C only two samples for the Inaba serotype and one sample for the Ogawa serotype retained antigenicity within acceptable limits after the same time. The loss of antigenicity after 2 1/2 years at 20-25 degrees C in comparison with antigenicity at 4-8 degrees C ranged from 41.5 to 83.1% (mean 61.9%) for the Inaba serotype and from 47.3 to 80.3% (mean 65.1%) for the Ogawa serotype. From this study on the cholera vaccine manufactured at the Central Research Institute, Kasauli, it is concluded that the shelf life of cholera vaccine may be increased from 18 months to at least 24 months, when the vaccine is stored under refrigeration.