LEFT UPPER ARM ABSCESS; CELLULITIS OF RIGHT LOWER EXTREMITY; This 57-year-old, female subject was participating in A Phase 3, Randomized, Stratified, Observer-Blind, Placebo-Controlled Study to Evaluate the Efficacy, Safety, and Immunogenicity of mRNA-1273 SARS-CoV-2 Vaccine in Adults Aged 18 Years and Older (mRNA-1273-P301) and experienced left upper arm abscess and cellulitis of right lower extremity.  The subject's medical history, as provided by the investigator, included depression, chronic back pain, fusion back surgery, and hysterectomy. Concomitant medications reported included trazodone, docusate sodium, cefalexin, furosemide, azithromycin, buprenorphine hydrochloride/naloxone hydrochloride, risperidone, vitamins NOS, and fenofibrate. Medical history from medical records included intravenous drug user (IVDU), major depressive disorder with psychotic features, and rectal prolapse.  Prior to event onset, the subject received intramuscular mRNA-1273 (Dose 3, booster) for SARS-CoV-2 vaccination on 13 Oct 2021, approximately 4 months before onset of the event.  On 12 Feb 2022, the subject experienced left upper arm abscess. She presented to emergency department (ED) for left arm pain and swelling significant for large erythematous fluctuant mass near her left biceps most likely consistent with left upper extremity abscess at site of skin popping for several days in the setting of recent IVDU. On the same day, she was admitted to the hospital for further management. She also had worsening chronic low back pain most likely from L3-L4 disc bulge in 2013 which caused severe spinal canal narrowing. Laboratory results were remarkable for leukocytosis and white blood cell count (WBC) was 13.4. Complete metabolic panel (CMP) was unremarkable and lactic acid was within normal limits 0.8. She was tested negative for COVID-19 via a polymerase chain reaction test (PCR). Blood cultures showed no growth to date and bacteremia ruled out. Computed tomography attenuation correction (CTACC) team performed incision and drainage of left upper arm abscess. She had increased right forearm pain and area of erythema. Wound cultures showed Enterobacter cloacae and methicillin-resistant staphylococcus aureus (MRSA). Treatment included intraveous (IV) vancomycin, IV piperacillin-tazobactam, IV meropenem, oral sulfamethoxazole-trimethoprim, and oral clindamycin.  On 13 Feb 2022, laboratory results included urine squamous epithelial cells 6/HPF (high), blood urine small (abnormal). Ultrasound of left upper extremity showed a 5cm fluid collection within the antecubital fossa without evidence of a pseudoaneurysm which concerned for abscess.  On 14 Feb 2022, laboratory results included C-reactive protein (CRP) 137 mg/dl (high), sedimentation rate 76 mm/hr (high). Computed tomography (CT) of right upper extremity with contrast showed multiple air and fluid containing collections in the subcutaneous fat; the air might be from injections or gas-forming infection, diffuse subcutaneous edema and stranding, might be cellulitis or lymphedema, a small pool of contrast  was  seen in the dorsal aspect of the hand and wrist, compatible with contrast extravasation. The subject left the department at the time of Image Interpretation, ill-defined hypodense crescentlc fluid seen In the anterior aspect of the mid forearm, could be a developing collection, focus of subcutaneous ephysema seen ln the posterior aspect of the upper  arm, with no adjacent Inflammatory stranding, which might be representative of penetrating trauma.  On 15 Feb 2022, laboratory results included antibody screen (ABSC)-SP3 negative, ABO+Rh positive, auto monocytes 10% (high), immature granulocytes 1.4% (high), auto immature granulocytes  0.12 x 10 e3/mcl (high)  On 20 Feb 2022, vital signs included systolic blood pressure (SBP) 144, diastolic blood pressure (DBP) 57. Laboratory results included chloride 106 mmol/l (high), creatinine 0.5 mg/dl (low), potassium 5.1 mmol/l (high), blood urea nitrogen (BUN) 19 mg/dl (high), hemoglobin 11.3 g/dl (ow), mean corpuscular hemoglobin concentration (MCHC) 31.6 g/dl (low), red blood cell count (RBC) 3.73 X 10e6/mcl (low), hematocrit 35.8 % (low), red cell distribution- coefficient of variation (RDW-CV) 15.0 % (high),. On the same day, the subject was discharged from the hospital on oral antibiotics ciprofloxacin and trimethoprim/sulfamethoxazole. She was advised to follow up with infectious disease in 5 to 7 days and internal medicine within 1-2 weeks.  Discharge diagnosis included abscess of left arm, cellulitis of right arm, chronic obstructive pulmonary disease (COPD) with emphysema, opiate dependence, abscess of muscle of upper arm, abscess of tendon sheath of muscle of right forearm, MRSA infection, infection due to Enterobacter cloacae, arm abscess, back pain, cellulitis, heroin abuse and IVDU.  On 20 Feb 2022, the subject was discharged from the hospital.  On 24 Feb 2022, the subject experienced cellulitis of right lower extremity and was admitted to the hospital.   At unknown time during hospital stay, the laboratory results for the subject included a WBC count of 14.73 (high) with left shift. Prothrombin time, partial thromboplastin time and internationalized normalization ratio were all normal, lactic acid was normal (1.4). Urinalysis and comprehensive metabolic panel were both unremarkable.The subject tested negative for COVID-19 via a PCR test. Blood cultures were taken but the results were negative. A x-ray of the subject's right tibia and fibia was performed and the results were unremarkable for acute processes. An ultrasound of the right lower extremity was performed and results did not show any deep vein thrombosis. A CT scan of right lower extremity was performed and the results showed asymmetrical mild subcutaneous fat stranding in right leg which could represent venous insufficiency, cellulitis, or asymmetric dependent edema. Treatment for the event included IV clindamycin and IV ciprofloxacin.   On 25 Feb 2022, the subject was discharged from the hospital. The cellulitis improved and the subject's WBC count improved to 6.67. The subject was discharged with oral sulfamethoxazole-trimethoprim and oral clindamycin.   The subject received all scheduled doses of study drug prior to the events; therefore, action taken with study drug in response to the events was not applicable.   The event, left upper arm abscess, was reported as resolved on 27 Feb 2022.   The event, cellulitis of right lower extremity, was reported as resolved on 02 Mar 2022.   The investigator assessed the events, left upper arm abscess and cellulitis of right lower extremity, as not related to study drug and not related to study procedure.  Follow-up received on 08 Mar 2022 included updated event severity grade (previously grade 3).   Follow-up received on 08 Apr 2022 and 12 Apr 2022 included event end date, final event outcome, additional event, course of event details, treatment information, laboratory results, diagnostic details, and COVID-19 testing results.   Company Comment: This case concerns a 57-year-old, female subject with a history of intravenous drug user (IVDU) and major depressive disorder with psychotic features among others, who experienced the unexpected events of left upper arm abscess and cellulitis of right lower extremity. The event left upper arm abscess occurred approximately 1 year 4 months after the first dose of mRNA-1273 and 4 months subsequent to third and most recent dose of mRNA-1273.The event cellulitis of right lower extremity occurred approximately 1 year 4 months after the first dose of mRNA-1273 and 4 months 12 days subsequent to third and most recent dose of mRNA-1273. The events were reported as resolved and considered unrelated to the study drug per the sponsor and reporter's assessment. The event left upper arm abscess might be explained by intravenous drug administration. The benefit-risk relationship of drug mRNA-1273 is not affected by this report.; Reporter's Comments: Company Comment: This case concerns a 57-year-old female subject with a history of intravenous drug user (IVDU) and major depressive disorder with psychotic features among others, who experienced the unexpected events of left upper arm abscess and cellulitis of right lower extremity. The event left upper arm abscess occurred approximately 1 year 4 months after the first dose of mRNA-1273 and 4 months subsequent to third and most recent dose of mRNA-1273.The event cellulitis of right lower extremity occurred approximately 1 year 4 months after the first dose of mRNA-1273 and 4 months 12 days subsequent to third and most recent dose of mRNA-1273. The events were reported as resolved and considered unrelated to the study drug per the sponsor and reporter's assessment. The event left upper arm abscess might be explained by intravenous drug administration. The benefit-risk relationship of drug mRNA-1273 is not affected by this report.