It was painful to walk, especially down steps.; weakness in both knees; severe stiffness in both knees; flu-like symptoms; both knees were swollen; pain in both knees; chills; Soreness at the injection site; This spontaneous case was reported by a consumer and describes the occurrence of GAIT DISTURBANCE (It was painful to walk, especially down steps.) in a 56-year-old male patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 037A21B and 040A21A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     No Medical History information was reported.   On 20-Mar-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 17-Apr-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 17-Apr-2021,  after starting mRNA-1273 (Moderna COVID-19 Vaccine), the patient experienced VACCINATION SITE PAIN (Soreness at the injection site). On 18-Apr-2021, the patient experienced ASTHENIA (weakness in both knees), JOINT STIFFNESS (severe stiffness in both knees), INFLUENZA LIKE ILLNESS (flu-like symptoms), JOINT SWELLING (both knees were swollen), ARTHRALGIA (pain in both knees) and CHILLS (chills). On 19-Apr-2021, the patient experienced GAIT DISTURBANCE (It was painful to walk, especially down steps.) (seriousness criteria disability and medically significant). At the time of the report, GAIT DISTURBANCE (It was painful to walk, especially down steps.), ASTHENIA (weakness in both knees), JOINT STIFFNESS (severe stiffness in both knees), INFLUENZA LIKE ILLNESS (flu-like symptoms), JOINT SWELLING (both knees were swollen), ARTHRALGIA (pain in both knees), CHILLS (chills) and VACCINATION SITE PAIN (Soreness at the injection site) outcome was unknown.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On an unknown date, Computerised tomogram: normal (normal) Had a CAT scan of the legs where they did not find anything..     For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter did not provide any causality assessments.   Patient had scan of the legs using a hand-held device. The right leg had more issues than the left where they thought to have blood clots in the legs and could not find anything. No relevant concomitant medication information provided. No relevant treatment medication information provided.  Company comment: This case concerns a 56-year-old male patient with no relevant medical history, who experienced the serious (due to disability) unexpected event of gait disturbance, the non serious unexpected AESI of Joint Stiffness and Joint Swelling. The events occurred approximately 1-2 days after receiving the second dose of mRNA-1273 Vaccine. The rechallenge was not applicable, as the events were reported after the second dose. The benefit-risk relationship of mRNA-1273 Vaccine is not affected by this report.   This case was linked to MOD-2021-378974 (Patient Link).; Sender's Comments: This case concerns a 56-year-old male patient with no relevant medical history, who experienced the serious (due to disability) unexpected event of gait disturbance, the non serious unexpected AESI of Joint Stiffness and Joint Swelling. The events occurred approximately 1-2 days after receiving the second dose of mRNA-1273 Vaccine. The rechallenge was not applicable, as the events were reported after the second dose. The benefit-risk relationship of mRNA-1273 Vaccine is not affected by this report.