The reason for the swelling and pain in the arm after the nine-valent HPV shot is that after the vaccine is injected, the substances in the vaccine, as well as the medium that dissolves the vaccine, will cause a local immune response, leading to inflammatory cell infiltration, soft tissue hyperplasia and partial fibrosis, in the process of inflammation occurring, leading to increased permeability of blood vessels, fluid leakage from blood vessels, and localized oedema, and the inflammatory substances will stimulate the nerve endings resulting in Pain. Localized hot compresses can be applied to promote dissipation of the inflammation, and non-steroidal anti-inflammatory drugs, such as ibuprofen or diclofenac sodium, can be used to control symptoms and reduce inflammation.