FEBRILE SEIZURE; FEVER; This 1-year-old, female subject was participating in (Privacy) and experienced the medically significant solicited adverse reaction of fever and adverse event of special interest of febrile seizure.  The subject had no reported medical history. No concomitant medications were reported.   Prior to event onset, the subject received blinded mRNA-1273 or Placebo (Dose 1) on 18 Feb 2022, approximately 6 hours and 3 minutes before onset of the event, fever, and 1 day before onset of the event, febrile seizure.  On 18 Feb 2022 at 18:00, the subject experienced fever. Treatment for the event fever included oral paracetamol.   On 19 Feb 2022, the fever continued throughout the day and the subject experienced a febrile seizure. Around 23:00, the subject was heard crying. When the parents went into the room, the subject had a temperature of 104 degrees Fahrenheit. The subject's mother went to the change the child's diaper and she was limp, not making any purposeful movement, and alert but not tracking. Her parents immediately took her to the local emergency room (ER). After arrival to the ER around midnight, the subject was irritable but oriented and asking for water. Vitals revealed heart rate 170 beats per minute, temperature 39.1 degrees Celsius, respiratory rate 32 beats per minute, and oxygen saturation 97%. The ER physician diagnosed subject with febrile seizure. No seizure activity was witnessed by the parents or ED physician as the subject woke up crying and alert and was just reported as "not tracking" or having "purposeful movements." Subject stayed in ER for four hours for observation. Treatment for fever included oral ibuprofen. No other treatments were needed. The principal investigator safety unblinded the subject.   No action was taken with the study drug in response to the events.   The events, febrile seizure and fever, were reported as resolved on 20 Feb 2022.    The investigator assessed the events, febrile seizure and fever, as related to study drug and as not related to study procedure.  Company comment: This case concerns a 1-year-old female subject with no reported medical history who experienced the unlisted medically significant solicited adverse reaction of fever and adverse event of special interest of febrile seizure. The event fever occurred 6 hours after the first dose of mRNA-1273 or Placebo (Dose 1), and the event febrile seizure occurred the following day after the vaccine administration. As per the report, the subject was limp, not making any purposeful movement, and alert but not tracking, in the context of fever, and at after arrival to the ER, around midnight, the subject was irritable but oriented and asking for water; no seizure activity was witnessed by the parents or ED physician. The outcome of the events was reported as resolved. The event was considered related to IP in agreement with the Investigator based on the nature of the events and close temporal association between the study vaccine administration and the events. The principal investigator safety unblinded the subject.  On 24 Feb 2022, the safety database was searched for events similar to Febrile convulsion and Pyrexia using the following search criteria: PT: Febrile convulsion and PT: Pyrexia  As of 24 Feb 2022, under (Privacy) for mRNA-1273, 7 similar events were retrieved in 6 cases, including the current index case (see table below). Four cases were retrieved with PT: Pyrexia and 3 cases were retrieved with PT: Febrile convulsion.  Zero cases reporting the following events were previously submitted as (Privacy) Safety Reports: Febrile convulsion and Pyrexia.  There were 5 cases that were (Privacy) Safety Reports (expected and related or unexpected and unrelated or expected and unrelated to the (Privacy)) reporting the following events: Febrile convulsion (2) MCN# MOD-2021-261464 and MOD-2021-393204 and Pyrexia (3) MCN# MOD-2021-208296, MOD-2021-362859, and MOD-2021-406376.  In the index case, a 1-year-old female subject, with no reported medical history, experienced fever and a febrile seizure within 24 hours of the study vaccine administration; the subject was limp, not making any purposeful movement, and alert but not tracking, in the context of fever. At the local emergency room, she was found febrile, irritable but oriented. No neurological abnormality was reported, and no seizure activity was witnessed by the parents or ED physician. The subject stayed in ER for four hours for observation, was treated with antipyretics, and the events resolved. Risk factors for febrile seizures include viral illnesses, certain vaccinations, and genetic predisposition. Based on review of available data, the Sponsor believes there is a reasonable possibility of a cause-and-effect relationship between administration of IMP and the occurrence of Pyrexia and Febrile convulsion.  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 1-year-old female subject with no reported medical history who experienced the unlisted medically significant solicited adverse reaction of fever and adverse event of special interest of febrile seizure. The event fever occurred 6 hours after the first dose of mRNA-1273 or Placebo (Dose 1), and the event febrile seizure occurred the following day after the vaccine administration. As per the report, the subject was limp, not making any purposeful movement, and alert but not tracking, in the context of fever, and at after arrival to the ER, around midnight, the subject was irritable but oriented and asking for water; no seizure activity was witnessed by the parents or ED physician. The outcome of the events was reported as resolved. The event was considered related to IP in agreement with the Investigator based on the nature of the events and close temporal association between the study vaccine administration and the events. The principal investigator safety unblinded the subject.