The risk of aseptic meningitis associated with the Leningrad-Zagreb mumps vaccine strain following mass vaccination with measles-mumps-rubella vaccine, Rio Grande do Sul, Brazil, 1997

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Abstract
Background Few data are available on the risk of aseptic meningitis following vaccination with the Leningrad-Zagreb (L-Z) strain of mumps vaccine. In 1997 the mumps vaccine was introduced into the state of Rio Grande do Sul in Brazil through mass vaccination with mumps-measles-rubella (MMR), targeting children aged 1–11 years. Five municipalities used exclusively MMR vaccine containing the L-Z strain of mumps. An outbreak of aseptic meningitis was observed shortly after the mass campaign. Methods To estimate the risk of aseptic meningitis associated with this strain, we analysed vaccination and meningitis case surveillance data from the selected municipalities. A case of vaccine-associated aseptic meningitis was defined as one with a pleocytosis of 10–1500 leukocytes/ml and occurring within 15–35 days after vaccine receipt. Results We estimated a risk of 2.9 cases per 10 000 doses of L-Z administered, equivalent to 1 case per 3390 doses administered. The overall risk of aseptic meningitis following the campaign was increased 12.2-fold (95% CI: 6.0–24.7) compared with the same period in 1995–1996. Following the mass campaign, the incidence of mumps declined 93% during 1998–2000. Conclusions Vaccination with the L-Z strain of mumps vaccine as part of a mass campaign was associated with a significantly increased risk of aseptic meningitis. Decisions about type of mumps vaccine and mumps vaccination strategies must consider vaccine safety issues in addition to other criteria.