Aseptic meningitis: Case definition and guidelines for collection, analysis and presentation of immunization safety data

Furthermore, in infants younger than two years of age (the age group that receives most immunizations) typical symptoms are commonly absent [40]. [...]at least one acute symptom or sign should be present to exclude persons who are examined or followed up for a chronic condition.  Level 2 of diagnostic certainty * Clinical evidence of acute meningitis such as fever, headache, vomiting, bulging fontanelle, nuchal rigidity or other signs of meningeal irritation,  AND * Pleocytosis in CSFa determined as: [composite function] >5 leukocytes/mm3 (μL) if patient is 2 months of ageb or older, [composite function] >15 leukocytes/mm3 (μL) in infants younger than 2 months,b  AND * Absence of any microorganism on Gram stain of CSF,  AND * No bacterial culture of CSF obtained, OR negative culture in the presence of antibiotic treatment before obtaining the first CSF sample.