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The PRISMA flow diagram of the meta‐analysis. We excluded 2719 studies after abstract evaluation for any of the following reasons: (1) original article was not accessible; (2) main outcome of the study was disease severity but not mortality; (3) total proportions of survivors/non‐survivors among hospitalized patients were not available; or (4) the number of patients with headache symptom was not reported adequately in any group (survivors or non‐survivors).
