No definitive evidence for L-Zagreb mumps strain associated aseptic meningitis: a review with special reference to the da Cunha study

The study by da Cunha et al. published in 2002 reported that MMR vaccine containing L-Zagreb mumps strain manufactured by Serum Institute of India Ltd. caused a high incidence of aseptic meningitis (AM) from routine surveillance during two mass immunization campaigns (MIC) conducted in 1998 in two states in Brazil. Since the results were contrary to those in India, Egypt and Bahamas, a critical analysis of the study was done. Several inconsistencies were found in the study, which undermined the conclusions drawn. Two similar studies from Brazil reported similar results. Review of these studies and those done on the vaccine from Zagreb, Croatia showed that in no study the L-Zagreb mumps virus has been isolated from cerebrospinal fluid (CSF) of an AM case. Isolation of the vaccine virus is necessary for definite causal association of AM with the vaccine. There is no such evidence to causally link MMR vaccine containing L-Zagreb mumps strain with AM.