possible case of cellulitis; arm feeling heavy; hot to touch; swelling; rash developed that was 3-4 inches diagonal that appear to spread to upper arm in couple of days; redness; shower with itchiness at the site of injection that was round in shape; thrust from antibiotics bactrim and Amo/Clav; on and off adverse event with feeling left ear on fire; A spontaneous report was received from a healthcare facility staff member concerning a 45-years-old, female patient who experienced possible case of cellulitis, on and off adverse event with feeling left ear on fire, hot to touch, redness, swelling, arm feeling heavy, thrust from antibiotics, rash developed that was 3-4 inches diagonal that appear to spread to upper arm in couple of days and shower with itchiness at the site of injection that was round in shape.   The patient's medical history was not provided. Concomitant product use was not provided by the reporter.    On 06-Jan-2021, prior to the onset of the symptoms, the patient received their first of two planned doses of mRNA-1273 [Lot number 012L20A] intramuscularly in the right upper arm for prophylaxis of COVID-19 infection.    On 06 Jan 2021, the patient started experiencing on and off adverse event with "feeling left ear on fire". Eight days later the patient noticed itchiness at the site of the injection that was round in shape. Rash developed that was 3-4 inches diagonal that appeared to spread to upper arm. The injection site was hot to touch, redness, swelling and arm feeling heavy. On 13 Jan 2021 or 14 Jan 2021, the patient was recommended Bactrim (sulfamethoxazole, trimethoprim), amox/Clav (amoxicillin, clavulanic acid) and clotrimazole lozenges because the primary care physician (PCP) was believing she possible has a case of cellulitis. The patient went to the emergency room (ER) on 17 Jan 2021, in which an ultrasound was performed and did not suspect any blood clot or cellulitis. Upon discharge, the patient experienced thrust from antibiotics. Treatment for the event included Bactrim (sulfamethoxazole, trimethoprim), amox/clav (amoxicillin, clavulanic acid), clotrimazole lozenges and Benadryl (diphenhydramine hydrochloride). Patient informed that the symptoms are on-going where the swelling and redness comes and goes.   Action taken with mRNA-1273 in response to the events was not provided.  The outcome of the events possible case of cellulitis, on and off adverse event with feeling left ear on fire, hot to touch, arm feeling heavy, thrust from antibiotics, rash developed that was 3-4 inches diagonal that appear to spread to upper arm in couple of days and shower with itchiness at the site of injection that was round in shape was considered as not recovered and for redness and swelling outcome is considered as unknown.; Reporter's Comments: This case concerns a 45 year old female subject with a serious unexpected cellulitis, with NS unexpected burning sensation, feeling hot, limb discomfort, thrush, and NS expected Vaccination site events of erythema, swelling, pruritus, and rash. Event onset same day as first dose mRNA-1273. US without blood clot or cellulitis. Based on the current available information and temporal association between the use of mRNA-1273 and the start date of the events, a causal relationship cannot be excluded.