Blood condition, MBL monoclonal B cell lymphocytosis, the low count type; Arm was swollen about double the size; Had a terrible rash; Allergies; Inflammation; This spontaneous case was reported by a consumer and describes the occurrence of MONOCLONAL B-CELL LYMPHOCYTOSIS (Blood condition, MBL monoclonal B cell lymphocytosis, the low count type) in an 80-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 036A21A and 030M20A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     Concurrent medical conditions included Drug allergy (Allergic to ibuprofen.).   On 04-Feb-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 04-Mar-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On an unknown date, the patient experienced MONOCLONAL B-CELL LYMPHOCYTOSIS (Blood condition, MBL monoclonal B cell lymphocytosis, the low count type) (seriousness criterion medically significant), PERIPHERAL SWELLING (Arm was swollen about double the size), INFLAMMATION (Inflammation), RASH (Had a terrible rash) and HYPERSENSITIVITY (Allergies). The patient was treated with ARNICA MONTANA EXTRACT at an unspecified dose and frequency; ASPIRIN [ACETYLSALICYLIC ACID] at an unspecified dose and frequency and PARACETAMOL (TYLENOL [PARACETAMOL]) at an unspecified dose and frequency. At the time of the report, MONOCLONAL B-CELL LYMPHOCYTOSIS (Blood condition, MBL monoclonal B cell lymphocytosis, the low count type) outcome was unknown and PERIPHERAL SWELLING (Arm was swollen about double the size), INFLAMMATION (Inflammation), RASH (Had a terrible rash) and HYPERSENSITIVITY (Allergies) had resolved.        For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter did not provide any causality assessments.   Treatment information included heat and ice were noted.  Company comment:  This case concerns an 80-year-old female patient, with no relevant medical history reported, who experienced the serious (medically significant) unexpected event of Monoclonal B-Cell lymphocytosis. The event occurred on an unknown date after receiving the second dose of mRNA-1273. The rechallenge is not applicable since the outcome of the event is unknown. The benefit-risk relationship of mRNA-1273 is not affected by this report. Event was considered serious as per the Important medical event list and a query has been sent to obtain further information for assessment.   This case was linked to MOD-2021-404282 (Link).; Sender's Comments: This case concerns an 80-year-old female patient, with no relevant medical history reported, who experienced the serious (medically significant) unexpected event of Monoclonal B-Cell lymphocytosis. The event occurred on an unknown date after receiving the second dose of mRNA-1273. The rechallenge is not applicable since the outcome of the event is unknown. The benefit-risk relationship of mRNA-1273 is not affected by this report. Event was considered serious as per the Important medical event list and a query has been sent to obtain further information for assessment.