Background: Subdeltoid bursitis has been reported as an adverse event after intramuscular vaccination in the deltoid muscle. Most published case reports involved influenza vaccine.

Objective: To estimate the risk for subdeltoid bursitis after influenza vaccination.

Design: Retrospective cohort study.

Setting: The Vaccine Safety Datalink, which contains health encounter data for 10.2 million members of 7 U.S. health care organizations.

Patients: Persons who received an inactivated influenza vaccine during the 2016-2017 influenza season.

Measurements: Potential incident cases were identified by searching administrative data for persons with a shoulder bursitis diagnostic code within 180 days after receiving an injectable influenza vaccine in the same arm. The date of reported bursitis symptom onset was abstracted from the medical record. A self-controlled risk interval analysis was used to calculate the incidence rate ratio of bursitis in a risk interval of 0 to 2 days after vaccination versus a control interval of 30 to 60 days, which represents the background rate. The attributable risk was also estimated.

Results: The cohort included 2 943 493 vaccinated persons. Sixteen cases of symptom onset in the risk interval and 51 cases of symptom onset in the control interval were identified. The median age of persons in the risk interval was 57.5 years (range, 24 to 98 years), and 69% were women. The incidence rate ratio was 3.24 (95% CI, 1.85 to 5.68). The attributable risk was 7.78 (CI, 2.19 to 13.38) additional cases of bursitis per 1 million persons vaccinated.

Limitation: The results may not be generalizable to vaccinations done in other types of health care settings.

Conclusion: Although an increased risk for bursitis after vaccination was present, the absolute risk was small.

Primary funding source: Centers for Disease Control and Prevention.