Left axillary low grade b cell lymphoma consistent with small lymphocytic lymphoma/chronic lymphocytic leukemia (SLL/CLL); Lymph nodes swelled up all over worse/They swelled under her left axillary, neck, and in the breast; They swelled under her left axillary; This spontaneous case was reported by a nurse (subsequently medically confirmed) and describes the occurrence of CHRONIC LYMPHOCYTIC LEUKAEMIA (Left axillary low grade b cell lymphoma consistent with small lymphocytic lymphoma/chronic lymphocytic leukemia (SLL/CLL)) in a 76-year-old female patient who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch nos. 058F21A, 002A21A and 010M20A) for COVID-19 vaccination. The occurrence of additional non-serious events is detailed below.     No Medical History information was reported.   On 08-Feb-2021, the patient received first dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 09-Mar-2021, received second dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. On 15-Nov-2021, received third dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) dosage was changed to 1 dosage form. In November 2021, the patient experienced CHRONIC LYMPHOCYTIC LEUKAEMIA (Left axillary low grade b cell lymphoma consistent with small lymphocytic lymphoma/chronic lymphocytic leukemia (SLL/CLL)) (seriousness criterion medically significant), LYMPHADENOPATHY (Lymph nodes swelled up all over worse/They swelled under her left axillary, neck, and in the breast) and VACCINATION SITE LYMPHADENOPATHY (They swelled under her left axillary). At the time of the report, CHRONIC LYMPHOCYTIC LEUKAEMIA (Left axillary low grade b cell lymphoma consistent with small lymphocytic lymphoma/chronic lymphocytic leukemia (SLL/CLL)), LYMPHADENOPATHY (Lymph nodes swelled up all over worse/They swelled under her left axillary, neck, and in the breast) and VACCINATION SITE LYMPHADENOPATHY (They swelled under her left axillary) had not resolved.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): On an unknown date, Biopsy lymph gland: abnormal (abnormal) Diagnosed with leukemia last week, Left axillary low grade b cell lymphoma consistent with small lymphocytic lymphoma/chronic lymphocytic leukemia (SLL/CLL).     For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter did not provide any causality assessments.   Concomitant medication were not provided  Treatment medication were not reported.  Nurse practitioner had reported her experience after receiving the booster dose. Patient was diagnosed with leukemia last week and read from the report of the lymph node biopsy.  This case concerns a 76-year-old female patient with no relevant medical history, who experienced the unexpected serious event of Chronic Lymphocytic Leukemia and expected events of Lymphadenopathy and Vaccination Site Lymphadenopathy. The event of Chronic Lymphocytic Leukemia, which was medically significant, and events of Lymphadenopathy and Vaccination Site Lymphadenopathy occurred approximately the same month after receiving the third dose of mRNA-1273 Vaccine. The rechallenge was not applicable since event Lymphadenopathy occurred on second dose however did not resolved. The events, Lymphadenopathy and Vaccination Site Lymphadenopathy are consistent with the current understanding of the mechanism of action of the vaccine. The benefit-risk relationship of mRNA-1273 Vaccine is not affected by this report.   This case was linked to MOD-2021-404994 (Patient Link).; Sender's Comments: This case concerns a 76-year-old female patient with no relevant medical history, who experienced the unexpected serious event of Chronic Lymphocytic Leukemia and expected events of Lymphadenopathy and Vaccination Site Lymphadenopathy. The event of Chronic Lymphocytic Leukemia, which was medically significant, and events of Lymphadenopathy and Vaccination Site Lymphadenopathy occurred approximately the same month after receiving the third dose of mRNA-1273 Vaccine. The rechallenge was not applicable since event Lymphadenopathy occurred on second dose however did not resolved. The events, Lymphadenopathy and Vaccination Site Lymphadenopathy are consistent with the current understanding of the mechanism of action of the vaccine. The benefit-risk relationship of mRNA-1273 Vaccine is not affected by this report.