Interchange of vaccine products; Patient experienced complete remission of symptoms by day 10 from start of illness all symptoms resolved. Worsening symptoms on day 15 and 16, with new positive antigen test.; This spontaneous case was reported by a physician and describes the occurrence of COVID-19 (Patient experienced complete remission of symptoms by day 10 from start of illness all symptoms resolved. Worsening symptoms on day 15 and 16, with new positive antigen test.) and INTERCHANGE OF VACCINE PRODUCTS (Interchange of vaccine products) in a  patient of an unknown age and gender who received mRNA-1273 (Moderna COVID-19 Vaccine) (batch no. 035C21A) for COVID-19 vaccination.     Previously administered products included for Drug use for unknown indication: Pfizer / BioNTech (Dose Number: 1 Batch/Lot No: ER8735) on 02-Apr-2021, Pfizer / BioNTech (Dose Number: 2 Batch/Lot No: ER8735) on 23-Apr-2021 and clarithromycin (clarithromycin). Past adverse reactions to the above products included Allergy with clarithromycin; and No adverse event with Pfizer / BioNTech and Pfizer / BioNTech. Concurrent medical conditions included Asthma, Obesity and Hypothyroidism.    On 26-Oct-2021, the patient received third dose of mRNA-1273 (Moderna COVID-19 Vaccine) (unknown route) 1 dosage form. On 17-May-2022, the patient experienced COVID-19 (Patient experienced complete remission of symptoms by day 10 from start of illness all symptoms resolved. Worsening symptoms on day 15 and 16, with new positive antigen test.). On an unknown date, the patient experienced INTERCHANGE OF VACCINE PRODUCTS (Interchange of vaccine products). The patient was treated with NIRMATRELVIR, RITONAVIR (PAXLOVID) (oral) from 02-May-2022 to 06-May-2022 for COVID-19 treatment, at an unspecified dose and frequency. At the time of the report, COVID-19 (Patient experienced complete remission of symptoms by day 10 from start of illness all symptoms resolved. Worsening symptoms on day 15 and 16, with new positive antigen test.) outcome was unknown and INTERCHANGE OF VACCINE PRODUCTS (Interchange of vaccine products) had resolved.      DIAGNOSTIC RESULTS (normal ranges are provided in parenthesis if available): In 2022, SARS-CoV-2 test: positive (Positive) Worsening symptoms on day 15 and 16, with newpositive antigen test, negative (Negative) tested negative on day 10 and negative (Negative) Tested negative again on day 11.     For mRNA-1273 (Moderna COVID-19 Vaccine) (Unknown), the reporter did not provide any causality assessments.   No concomitant medications information was reported. Patient did not take any other medications/products within 2 weeks of starting COVID-19 treatment. It was reported that the worsening symptoms on day 15 and 16, with new positive antigen test (without any exposures, was isolated still). Device Date  was reported as 18-May-2022.  Company Comment: This is a Spontaneous case concerning a patient of unknown age and gender, with no relevant medical history, who experienced the unexpected, non-serious AESI of COVID-19, approximately 6 months and 20 days after the third dose of COVID-19 vaccination schedule. Primary vaccination completed with Pfizer vaccine; hence, Interchange of vaccine products is noted. Patient experienced complete remission of symptoms on treatment with Nirmatrelvir, Ritonavir (Paxlovid). By day 10 from start of illness all symptoms resolved and tested negative on rapid antigen testing. Tested negative again on day 11. Worsening symptoms on days 15 and 16, with new positive antigen test (without any exposures, was isolated still). At the time of the report, COVID-19 outcome was unknown. The benefit-risk relationship of mRNA-1273 vaccine is not affected by this report.   Most recent FOLLOW-UP information incorporated above includes: On 20-May-2022: Follow up received contains no new information.; Sender's Comments: This is a Spontaneous case concerning a patient of unknown age and gender, with no relevant medical history, who experienced the unexpected, non-serious AESI of COVID-19, approximately 6 months and 20 days after the third dose of COVID-19 vaccination schedule. Primary vaccination completed with Pfizer vaccine; hence, Interchange of vaccine products is noted. Patient experienced complete remission of symptoms on treatment with Nirmatrelvir, Ritonavir (Paxlovid). By day 10 from start of illness all symptoms resolved and tested negative on rapid antigen testing. Tested negative again on day 11. Worsening symptoms on days 15 and 16, with new positive antigen test (without any exposures, was isolated still). At the time of the report, COVID-19 outcome was unknown. The benefit-risk relationship of mRNA-1273 vaccine is not affected by this report.