Impact of the Number of Vaccinations on the Duration of Fever and Symptoms in Adult Patients with Mild COVID-19 during the Period of Spread of the Omicron Variant of SARS-CoV-2 in Japan

<p>While mRNA vaccines against COVID-19 have shown progressively reduced effectiveness in preventing infection due to a mismatch between the antigen strain of the vaccine and the circulating strains, it has been reported that they maintain a certain level of effectiveness in preventing hospitalization and severe outcomes. There are few reports on the impact of times a subject is vaccinated (hereinafter, number of vaccinations) on the duration of fever and symptoms in patients with mild COVID-19. In this study, we distributed questionnaires to patients diagnosed as having COVID-19 and their families at outpatient clinics in three medical institutions in Ishikawa Prefecture during the period of spread of the Omicron variant of SARS-CoV-2 and conducted a retrospective analysis of the background information of the patients and the pattern of progression of their symptoms after the diagnosis. We conducted a multivariate analysis of the data of 501 adults with mild symptoms of COVID-19 to determine the factors affecting the highest body temperature during the course of infection, the duration of fever, and the rates of symptom persistence beyond 7, 14, and 30 days. Background factors and vaccination history were used as the explanatory variables. Patients who developed COVID-19 after receiving the vaccine three or more times showed a significantly lower maximum body temperature during the illness and a shorter duration of fever than those who developed the infection after receiving the vaccine once or twice. Furthermore, a trend towards milder and shorter duration of fever was observed as the number of vaccinations increased. Although a trend toward lower rates of symptom persistence other than fever was observed as the number of vaccinations increased, no significant differences were found according to the number of vaccinations. The data support booster vaccinations as being useful for reducing the degree and duration of fever in adult patients with mild symptoms of COVID-19 and suggest the possibility of higher effectiveness of a higher number of vaccinations.</p>