LEFT ARM PARESTHESIA; This 42-year-old, white, female subject (US3142236) 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 arm paresthesia.  The subject's medical history, as provided by the investigator, included nearsighted bilateral, astigmatism bilateral, nose bleeds, seasonal allergies, kidney stones, hypothyroidism, migraines, obsessive-compulsive disorder, acne and simplex herpes on lip. Concomitant medications included levocetirizine dihydrochloride, botulinum toxin type A, spironolactone, amfetamine aspartate/amfetamine sulfate/dexamphetamine saccharate/dexamphetamine sulfate, liothyronine sodium, levothyroxine, cyanocobalamin, and medroxyprogesterone acetate.  The subject was randomized to receive blinded intramuscular Placebo (dosed on 19 Aug 2020 and 16 Sep 2020) and was unblinded on 12 Jan 2021. The subject consented to receive mRNA-1273 after being unblinded and received their first dose of mRNA-1273 on 12 Jan 2021.  On 20 Jan 2021, the subject experienced a medically significant event of left arm paresthesia. The subject presented for an unscheduled visit due to swelling, redness, and tenderness at the injection site. Possible treatment with steroids or antibiotics was discussed; however, the subject declined starting treatment at that time. No treatment was given for the event.  On 21 Jan 2021, the subject went to the emergency room due to left side pain and paresthesia. Disposition at the emergency department was not reported by the investigator. The subject was not hospitalized overnight and was discharged from the hospital the same day.  Study drug dose was delayed in response to the event.  The outcome of the event, left arm paresthesia, was reported as resolved on 22 Jan 2021.   On 18 Feb 2021, the subject received the second dose of mRNA-1273 and was treated prophylactically with antihistamines prior to dosing.   The investigator assessed the event, left arm paresthesia, as related to study drug and related to study procedure. The investigator's rationale for assessing the event as related was because event occurred within 24 hours of receiving first dose of investigational product, same laterality of dose, and it was assessed by the emergency department as a histamine response.   Follow-up information received on 28 Jan 2021 included updated medical history.  Follow-up information received on 09 Mar 2021 included updated subject's gender, event start date, event outcome, discharge date, and action taken with study drug.  Follow-up information received on 01 Apr 2021 included updated event end date and seriousness criteria; investigator rationale for study drug and procedure relatedness; and confirmation subject was not hospitalized overnight, no treatment given for event, and no known COVID testing or results.  Analysis of Similar Events: On 25-Jan-2021, the safety database was searched for events similar to paraesthesia using the following search criteria: PT: Immune-mediated neuropathy and Paraesthesia.   As of 25-Jan-2021, under IND 019745 for mRNA-1273, two events were retrieved from the safety database, including the current index case. There have been no similar events submitted as IND Safety Reports. There were two additional events retrieved from the post-authorization period that were previously submitted to RA. All cases retrieved reported the event of Paraesthesia: MOD-2020-002352, MOD-2021-002735, and MOD-2020-000323; however, none were considered similar to the index case.  In MOD-2021-002352 and MOD-2021-002735, the event Paraesthesia occurred within 2-5 minutes after mRNA-1273 vaccine administration in association with circulatory effects and without reported unilaterality. In case MOD-2020-002352, an event of tingling was reported in a 34-year-old female patient within 3-4 min after first dose of mRNA-1273 vaccine administration, together with events of numbness of body, face, and neck, tachycardia at 144, palpitation, elevated blood pressure at 140/90, chest tightness, shortness of breath, dizziness, and flushing of face. In case MOD-2021-002735, an event of tingling sensation inside the mouth occurred after 5 mins of mRNA-1273 vaccine administration, together with events of tingling sensation side of jaw, tingling sensation inside tongue, blood pressure increased, and heart rate increased.   In case MOD-2020-000323, an event of tingling on right side of face was reported in a subject with a recent dental history of root canal, together with an event of swelling to the right side of the face, 7 days after blinded study vaccine administration and 20 days after root canal.   In the index case, a 42-year-old male subject developed an event of left arm paresthesia 9 days after the administration of their first dose of mRNA-1273. One day before, the subject was evaluated by a clinical trial investigator for swelling, redness, and tenderness at the injection site.   After review of the clinical details and investigator comments pertaining to this adverse event, and based upon experience to date, the Sponsor does not believe that changes to the conduct of this clinical trial are warranted. The Company will continue to monitor these and other serious adverse events reported in association with the RA and will communicate any relevant changes to the protocol, Informed Consent Form, Investigator's Brochure, and/or Core Safety Information.; Reporter's Comments: This case concerns a 42-year-old, white, male subject with medical history of seasonal allergies, kidney stones, hypothyroidism, migraines, and obsessive-compulsive disorder, who experienced a serious unexpected event of left arm paresthesia. The event of left arm paresthesia occurred 9 days after the administration of their first dose of mRNA-1273. One day before the event occurred, the subject experienced swelling, redness, and tenderness at the injection site. There is no additional medical information to inform causality assessment.  Because left arm paresthesia was associated temporally with swelling, redness, and tenderness at the injection site which occurred one day prior to the event, it was considered related to mRNA-1273 and to study procedure, in agreement with the Investigator.  Potential confounders include obsessive-compulsive disorder.  This case will be reassessed once further information is obtained.