A case of subacute thyroiditis after influenza vaccination.

Summary: A 40-year-old Japanese woman presented to the outpatient clinic with fever and palpitations 2 days after receiving the influenza vaccine (Influenza HA Vaccine 'KMB'®) following the second dose of coronavirus disease 2019 (COVID-19) vaccine (COVID-19 vaccine Moderna intramuscular injection®). At the first visit, the patient presented with a swollen thyroid gland with mild tenderness, and she was diagnosed with subacute thyroiditis (SAT) based on the presence of thyrotoxicosis (free T3: 5.42 pg/mL; free T4: 2.34 ng/dL; and thyroid-stimulating hormone (TSH): <0.01 μIU/mL), a high C-reactive protein level (5.77 mg/dL), a negative TSH receptor antibody, and characteristic ultrasound findings. The patient's human leukocyte antigen types were A2, A11, B35, B51, DR4, and DR1403. Prednisolone (15 mg/day) was given as an initial dose, after which the fever subsided, and the dose was tapered and discontinued after 6 weeks. The patient was thought to have developed SAT due to influenza vaccination. SAT after influenza vaccination may be overlooked. For patients with SAT, it is necessary to obtain information regarding their vaccination history.

Learning Points: After influenza vaccination, subacute thyroiditis (SAT) may develop. If persistent fever, anterior neck pain, swelling, tenderness of the thyroid gland, and symptoms of thyrotoxicosis are observed immediately after vaccination for several viruses, including influenza, an examination to rule out the onset of SAT is recommended. Human leukocyte antigen type A2 (HLA-A2) and HLA-B35 may be linked to the development of SAT following influenza vaccination. The two doses of the coronavirus disease 2019 (COVID-19) vaccine given before the influenza vaccine may affect the onset of SAT.