heavy weight was pressuring their chest/experienced like a heavy thing was moving across their chest; hard time breathing as if earing several masks over their head/They felt their breathing was inhibited; oxygen level dropped to 80%; pulse was at 115; heart was really fastthrough the days, beating really fast and feeling it was moving up the throat given a choking feeling; A spontaneous report was received from a consumer who was a 73-year-old, female patient who received Moderna's COVID-19 vaccine (mRNA-1273) and experienced a heavy weight was pressuring their chest from the inside, and also felt like a heavy pressure coming from outside their body/Chest discomfort, like a heavy thing was moving across their chest, they had a hard time breathing as if earing several masks over their head felt their breathing was inhibited/dyspnea, oxygen level dropped to 80%/oxygen saturation decreased,  pulse was at 115/ heart rate increased, felt their heart was fast trough the day regularly, for 5 days, beating really fast and feeling like it was moving up their throat, giving them a choking feeling/choking sensation.   The patient's medical history included high blood pressure, and thyroid disorder.  Concomitant medications included enalapril hydrochlorothiazide, simvastatin for "cholesterol" and a "thyroid medication."  On 29-Jan-2021, at 0931, prior to the onset of the events, the patient received their first of two planned doses of mRNA-1273 (Lot number: 004M20A ) intramuscularly in the left arm for prophylaxis of COVID-19 infection.  On 29-Jan-2021, 46 minutes after vaccine administration, patient reported that she started feeling like a heavy weight was pressuring her chest from the inside and also felt like a heavy pressure was coming from outside her body. At 1100, she stated that she experienced a feeling like a heavy thing was moving across her chest and she reportedly had a hard time breathing. Her oxygen level dropped to 80%, but by breathing intensely she managed to bring it to 84-87%. She reported her pulse to be 115. All of these symptoms lasted for 20 minutes. Later, she reported that her oxygen level returned to 90% and then eventually to 94%.  However, she reported that for the next few days she felt like her heart rate was fast through the day, for about 5 days.  She said that she felt like it was beating really fast and felt like it was moving up her throat, giving her a choking feeling. Since last Friday, 05-Feb-2021, she stated that she has had no more problems. Patient reached out to her HCP who denied her any post-vaccine related attention.    Treatment activities for event included taking ibuprofen 600 mg after the vaccination.  Action taken with mRNA-1273 in response to the events was unknown.   The outcome of event, experienced a heavy weight was pressuring their chest from the inside, and also felt like a heavy pressure coming from outside their body, like a heavy thing was moving across their chest, they had a hard time breathing as if earing several masks over their head felt their breathing was inhibited, oxygen level dropped to 80%,  pulse was at 115, was considered  recovered/resolved.; Reporter's Comments: This case concerns a 73-year-old female who experienced serious unexpected events of Chest discomfort, dyspnea, oxygen saturation decreased along with a nonserious unexpected events of heart rate increased and choking sensation.  Event onset 46 minutes after the first dose of mRNA-1273. Treatment with ibuprofen. Events resolved. Based on current available information and temporal association between the use of the product and the start date of the event, a causal relationship cannot be excluded.