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Many reports have suggested that vaccination in the deltoid muscle can trigger subdeltoid or subacromial bursitis. However, shoulder bursitis is so common among adults that a precise cause-and-effect relation has been difficult to establish.
Researchers probed the CDC's Vaccine Safety Datalink database to correlate cases of shoulder bursitis with receipt of routine influenza vaccination during the 2016–2017 flu season. A random subset of cases was abstracted manually with unusual precision: Bursitis cases were sorted by laterality and classified as definite, probable, or possible, depending on the quality of confirmatory data and the expertise of diagnosing physicians. Laterality of the vaccination and credentials of the vaccinator were tabu…