Aseptic meningitis after vaccination with L-Zagreb mumps strain-virologically confirmed cases. Faulty calculation of rates.

In a Letter to the Editor published in Vaccine recently (Vaccine 24 (2006) 6371-6373), the authors describe clinical features of vaccine associated aseptic meningitis (AM) of patients hospitalized in one hospital, proceeding to calculate incidence rates of AM based on hospital admission data. Since the denominator for hospital-based cases is unknown, the incidence rates calculated in such a way are methodologically incorrect and should not be considered relevant.