Statistical analysis
Data were entered into Excel and analysed using SAS version 9.4 (SAS Institute Inc). The results of the three tests were compared using cross-tabulations. ELISA indeterminate results (OD 0.9−0.99 nm) were considered “not positive”. Sensitivity (positive result in a true positive case (TP)), specificity (negative result in a true negative case (TN)), positive and negative predictive values (PPV, NPV) (the probability of positive or negative results in true positive or negative cases, respectively) and positive and negative likelihood ratios (LRP and LRN) were calculated for the rapid test and Platelia ELISA using the Wantai ELISA as the reference test, which has shown superior performance in previous evaluations against RT-PCR confirmed samples [3,4]. Standard formulae were used: sensitivity (TP)/(TP + FN) (FN = false negative); specificity (TN)/(TN + FP) (FP = false positive; positive predictive value (TP/TP + FP); negative predictive value (TN/TN + FN); and positive and negative likelihood ratios (LR+, LR-) (probability that a person with or without the disease tested positive respectively; and probability that a person with or without the disease tested negative respectively). Results were calculated within 95% confidence intervals. The Kappa coefficient of agreement between the rapid test and the Wantai ELISA, and between the Platelia ELISA and the Wantai ELISA, were interpreted according to the criteria of Cohen (values ≤0 indicating no agreement; 0.01–0.20 as none to slight agreement; 0.21–0.40 as fair agreement; 0.41–0.60 as moderate agreement; 0.61–0.80 as substantial agreement; 0.81–1.00 as almost perfect agreement) [5]. The results of the total population were compared with those of the non-vaccinated population.