Statistical analysis
Statistical analysis was performed using IBM SPSS Statistics for Windows, version 27.0 (Armonk, NY: IBM Corp).As for phase 1 (laboratory testing), the sensitivity (Se) and specificity (Sp) of a dog's indication of samples compared with the true diagnosis confirmed by RT-PCR were calculated. The sensitivity refers to the conditional probability of the dog indicating COVID-19 when the condition was present, and specificity refers to the conditional probability of the dog ignoring a sample from a healthy donor. Both sensitivity and specificity were expressed as proportions. Point estimates were calculated with 95% confidence intervals. The probability of a perfect test trial (finding the right sample and ignoring the controls) by chance was 1/6 (17%). Similarly to a previous studyPercentage of agreementAs for phase 2 (field work), Se and Sp were calculated compared with the RAD result. In each trial, the probability of success or failure was 50%. Thus, we calculated 95% CI of the Se and Sp and considered statistically different from a random choice those that did not overlap 50%, which is the randomness region.Cohen’s Kappa was used to measure the agreement of the two methods (sniffer dog and RAD) to screen people. Results were interpreted as follows: values ≤ 0 as indicating no agreement, 0.01–0.20 as none to slight, 0.21–0.40 as fair, 0.41–0.60 as moderate, 0.61–0.80 as substantial, and 0.81–1.00 as almost perfect agreementFor all the analyses, a two-sided p < 0.05 was considered statistically significant.