Characteristics of study participants
Of the positive cases, 239 (57.7%) were asymptomatic patients, and the remaining 42.3% were symptomatic. Among the symptomatic patients, symptoms reportedly included fever (n = 76, 43.4%), cough (n = 66, 37.7%), pharyngitis (n = 45, 25.7%), rhinitis (n = 21, 12%), fatigue (n = 16, 9.1%), diarrhea (n = 2, 1.1%) and chest discomfort (n = 1, 0.6%) (Table 1). Among the 414 cases and 828 controls, 255 (61.6%) and 394 (47.6%) individuals were female, respectively. The mean “time since the last dose of vaccination” was 6.8 months (3.0–8.0 months) for cases and 3.8 months (2.9–7.5 months) for controls. There was a significant difference between the two groups in terms of “gender” and “time since the last dose of vaccination” (P < 0.001,P = 0.01). The median age was 38 years (18–55 years) in the case group, and 40 years (25–56 years) in the control group, and no statistical difference was observed (P = 0.353) ( Table 2).