Preparation of a national determination reference of virus titer for live oral rotavirus vaccines

Objective To prepare a national evaluation reference of virus titer for live oral rotavirus (RV) vaccines. Methods A bulk of qualified live attenuated RV vaccine was mixed with preservative, packed into ampoules (1.0 ml/vial) and lyophilized. The virus titer of the reference preparation was determined by a micro-cytopathic method and 50% cell culture infective dose (CCID50) was calculated with Karber method. The reference was collaboratively evaluated in 3 different laboratories and results were analyzed by one way ANOVA and least-significant difference test. In addition, the virus titers were determined after the reference was stored at -20 ℃ for 1 year, 4 ℃ for 1 year, 37 ℃ for 14 d and -60 ℃ for 2 years, respectively. Results There was no significant difference among results from the 3 laboratories by one way ANOVA (F=0.379, P=0.686) or by pairwise comparison with least-significant difference test (=0.078 2, all P values>0.05). The average titer of the reference was 6.5 lgCCID50/ml. The reduction of titer was no more than 1.0 lgCCID50/ml after the preparation was stored at different temperatures for different periods. The coefficient of variation in long-time stability test was 4.9%. Conclusion The reference prepared can be used for virus titer evaluation of live oral RV vaccines. Key words: Rotavirus vaccine; Virus titer; Reference standards