Citrate-mediated disaggregation of rotavirus particles in RotaTeq vaccine

For the routine manufacture of live virus vaccines, virus is diluted into a formulation buffer to stabilize it for long-term storage, and to facilitate vaccine administration. The characteristics of this buffer are dependent on the storage temperature of the vaccine, as well as the desired characteristics of the product. The formulation buffer for RotaTeq, Merck's live, pentavalent, oral rotavirus vaccine to prevent rotavirus gastroenteritis was developed as a fully liquid solution that requires no pre-feeding prior to administration, and is stable for 24 months at refrigerated temperatures. In studying the effects of the formulation buffer on the live virus contained within RotaTeq, we observed that the formulation buffer also directly impacts the state of rotavirus aggregation. This observation, termed "the matrix effect," was first noted as an approximately 50% increase in measured in vitro infectivity, following dilution of the virus into the buffer. Subsequent experiments confirmed that citrate in the formulation buffer facilitates the disaggregation of viral particles, likely through a carboxylic-acid mediated interaction. For vaccine manufacture, bulk virus is titered and subsequently diluted to a target concentration for dosing. Aggregation of the virus and subsequent inaccurate measurement of the amount of virus contained in either the bulk sample or in the final dosing container could lead to an inability to accurately predict final vaccine concentrations. Thus, discerning the nature and extent of the matrix effect was key principally for providing an accurate prediction of final virus concentration upon dilution, to ensure a robust manufacturing process. In addition, understanding potential contributions of the formulation buffer to clinical efficacy of the vaccine was critical. Clinical data have confirmed that the citrate-mediated disaggregation had no measurable impact on vaccine safety, immunogenicity, or efficacy.