This study investigates the effect of daily consumption of a concentrated garlic and onion extract on COVID-19 symptoms among elderly nursing home residents. Volunteers consumed a daily capsule of the concentrated powder rich in organosulfur compounds over 36 weeks during lunch. The incidence and severity of COVID-19 symptoms between the treatment and control groups were compared, along with monitoring the safety of consumption, incidence of other diseases, and medicine usage. The treatment group showed a significant reduction in both the number and severity of COVID-19 symptoms compared to the control group, with no significant adverse effects observed. No significant reduction in symptom duration was detected. This study provides preliminary evidence that concentrated garlic and onion extract may aid in the treatment of COVID-19 among older adults. These findings suggest potential public health benefits, emphasizing the need for further research to explore the immunomodulatory properties of these natural compounds.