| Summary: | Abstract Background and Aims Fever after coronavirus disease 2019 (COVID‐19) vaccination is generally a mild and benign event, but can cause excessive anxiety in younger adults. This study aimed to find key factors that include allergic diseases or physique that determine fever after vaccination. Methods We conducted an observational cohort study in our hospital to assess post‐COVID‐19 vaccination fever from April to June 2021. A total of 153 medical personnel aged 22–86 years of age were involved in the study to receive two doses, intramuscularly 21 days apart, of the Pfizer‐BioNTech COVID‐19 vaccine (30 μg per dose). Vaccination records were taken more than 72 h after vaccination. Clinical and laboratory variables (age, sex, allergy history, weight, height, serum hemoglobin concentration, and these derivatives) were examined by multivariable logistic regression analysis using the peak axillary temperature in the 4‐day period after the second vaccination as a dependent variable. Results No serious safety problems were detected. The incidence of a postsecond vaccination fever of 37.3°C or above was 29.4%. Logistic regression analysis found age, history of perennial allergic rhinitis, body surface area, body weight, percent overweight, and serum hemoglobin concentration as independent predictors of postvaccination fever. The characteristics of this individual were incorporated into the numerical model of human thermoregulation. The evaluation of this model had a sensitivity of 66.1% and a specificity of 90.7% in the detection of postvaccination fever. The multiple coefficient of determination (R2) was 0.410. Conclusion The COVID‐19 vaccine induced higher rates of fever during the 4‐day period after the second vaccination. Younger age, part of the allergy history, small and light body, and concentrated blood were associated with postvaccination fever.
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