• Chengyi Zheng, Jonathan Duffy, In-Lu Amy Liu, Lina S Sy, Wansu Chen, Lei Qian, Ronald A Navarro, Denison S Ryan, Sunhea S Kim, Cheryl Mercado, and Steven J Jacobsen.
• Department of Research & Evaluation, Kaiser Permanente Southern California, Pasadena, California (C.Z., I.A.L., L.S.S., W.C., L.Q., D.S.R., S.S.K., C.M., S.J.J.).
• Ann. Intern. Med. 2022 May 1; 175 (5): 634-643.

BackgroundAlthough shoulder conditions have been reported as an adverse event after intramuscular vaccination in the deltoid muscle, epidemiologic data on shoulder conditions after vaccination are limited.ObjectiveTo estimate the risk for shoulder conditions after vaccination and assess possible risk factors.DesignRetrospective cohort study.SettingKaiser Permanente Southern California, a large integrated health care organization.ParticipantsKaiser Permanente Southern California members aged 3 years or older who had an intramuscular vaccination administered in the deltoid muscle between 1 April 2016 and 31 December 2017.MeasurementsA natural language processing (NLP) algorithm was used to identify potential shoulder conditions among vaccinated persons with shoulder disorder diagnosis codes. All NLP-identified cases were manually chart confirmed on the basis of our case definition. The characteristics of vaccinated persons with and without shoulder conditions were compared.ResultsAmong 3 758 764 administered vaccinations, 371 cases of shoulder condition were identified, with an estimated incidence of 0.99 (95% CI, 0.89 to 1.09) per 10 000 vaccinations. The incidence was 1.22 (CI, 1.10 to 1.35) for the adult (aged ≥18 years) and 0.05 (CI, 0.02 to 0.14) for the pediatric (aged 3 to 17 years) vaccinated populations. In the adult vaccinated population, advanced age, female sex, an increased number of outpatient visits in the 6 months before vaccination, lower Charlson Comorbidity Index, and pneumococcal conjugate vaccine were associated with a higher risk for shoulder conditions. Among influenza vaccines, quadrivalent vaccines were associated with an increased risk for shoulder conditions. Simultaneous administration of vaccines was associated with a higher risk for shoulder conditions among elderly persons.LimitationGeneralizability to other health care settings, use of administrative data, and residual confounding.ConclusionThese population-based data suggest a small absolute risk for shoulder conditions after vaccination. Given the high burden of shoulder conditions, clinicians should pay attention to any factors that may further increase risks.Primary Funding SourceCenters for Disease Control and Prevention.

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