CONNECTIVE TISSUE DISORDER; This 35-year-ol male subject was participating in a study and experienced medically important event and adverse event of special interest of connective tissue disorder.  The subject's medical history, as provided by the investigator, included rosacea, seborrhea, back pain and hyperlipidemia. Concomitant medications included tamsulosin, ivermectin, phenylephrine hydrochloride,  diphenhydramine, brimonidine tartrate,  influenza vaccine, hydrocodone bitartrate/paracetamol, ciclopirox, ketoconazole, rosuvastatin, and vitamin D.    The subject previously experienced the serious adverse event of kidney stone (MOD-2021-280546) while participating in this study.   The subject was randomized to receive intramuscular Placebo (dosed on 03 Sep 2020 and 30 Sep 2020) and was unblinded on 08 Jan 2021. The subject consented to receive mRNA-1273 for SARS-CoV-2 vaccination after being unblinded and, prior to event onset, the subject received mRNA-1273 (Dose 2) on 03 Feb 2021, approximately 8 months and 8 days before onset of the event.  On 12 Oct 2021, the subject experienced the medically important event of connective tissue disorder. During a site call, the subject noted stiffness in his left hand joints which he had been experiencing for the previous month, starting 'around' 12 Oct 2021.   On 08 Nov 2021, the subject received his mRNA-1273, dose 3 (booster). He reported that stiffness in his left hand joints was a little worse since his booster dose. He had planned to follow up with his primary care physician.   On 29 Nov 2021, the subject reported that his primary care physician (PCP) did labwork and found his rheumatoid factors were elevated. Per the subject, PCP suspected rheumatoid arthritis.   On 03 Dec 2021, the subject was seen by rheumatologist and was diagnosed with connective tissue disorder by rheumatologist and was started on meloxicam and hydroxychloroquine.   The subject received all scheduled doses of study drug prior to the event; therefore, action taken with study drug in response to the event was not applicable.  The event, connective tissue disorder, was reported as resolving.  The investigator assessed the event, connective tissue disorder, as related to study drug and not related to study procedure.  Company comment:  This case concerns a 35-year-old male subject with a medical history of rosacea, seborrhea, back pain and hyperlipidemia who experienced medically important event and adverse event of special interest of connective tissue disorder. The event occurred 8 months and 8 days after second dose of mRNA-1273.  The subject experienced joint stiffness with elevated rheumatoid factor and was diagnosed with connective tissue disorder by a rheumatologist.  The subject received the third dose of mRNA-1273 approximately 26 days after start of symptoms. The subject started treatment and the event was reported as resolving. The event was assessed as related to mRNA-1273 in agreement with the Investigator's assessment.   Analysis of similar events: On 14-Dec-2021, the safety database was searched for events similar to Connective Tissue Disorder using the following search criteria: PT: Connective Tissue Disorder ; HLT: Connective tissue disorders NEC; HLGT Connective tissue disorders (excl congenital).   As of 14-Dec-2021 under IND 019745 for mRNA-1273, 1 case of Connective Tissue Disorder was retrieved which is the current index case. No other cases were retrieved.   The index case concerns a 35-year-old male subject with a medical history of rosacea, seborrhea, back pain and hyperlipidemia who experienced the medically important event and adverse event of special interest of connective tissue disorder. The subject experienced stiffness in his left hand joints starting 8 months and 8 days after the second dose of mRNA-1273. The subject received the third dose of mRNA-1273 approximately 26 days after the start of symptoms. He was seen by a PCP and by a rheumatologist, he was found to have elevated rheumatoid factor and was diagnosed with connective tissue disorder by the rheumatologist.  The subject started treatment with meloxicam and hydroxychloroquine, and the event was reported as resolving. The Investigator assessed the event as related to mRNA-1273.   Based on review of available data, the Sponsor cannot rule out a possible cause and effect relationship between administration of mRNA-1273 and the occurrence of connective tissue disorder.  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 IMP 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 35-year-old male subject with a medical history of rosacea, seborrhea, back pain and hyperlipidemia who experienced medically important event and adverse event of special interest of connective tissue disorder. The event occurred 8 months and 8 days after second dose of mRNA-1273.  The subject experienced joint stiffness with elevated rheumatoid factor and was diagnosed with connective tissue disorder by a rheumatologist.  The subject received the third dose of mRNA-1273 approximately 26 days after start of symptoms. The subject started treatment and the event was reported as resolving. The event was assessed as related to mRNA-1273 in agreement with the Investigator's assessment.