Determining the reliability of rapid SARS-CoV-2 antigen detection in fully vaccinated individuals

Background: Rapid antigen detection tests (RADT) are commonly used as SARS-CoV-2 diagnostic tests both by medical professionals and laypeople. However, the performance of RADT in vaccinated individuals has not been fully investigated. Objectives: RT-qPCR and rapid antigen detection testing were performed to evaluate the performance of the Standard Q COVID-19 Ag Test in detecting SARS-CoV-2 breakthrough infections in vaccinated individuals.Study design: Two swab specimens, one for RT-qPCR and one for RADT, were collected from vaccinated individuals in an outpatient clinic. For comparison of RADT performance in vaccinated and unvaccinated individuals, a dataset already published by this group was used as reference. Results: During the delta wave, a total of 696 samples were tested with both RT-qPCR and RADT that included 692 (99.4%) samples from vaccinated individuals. Of these, 76 (11.0%) samples were detected SARS-CoV-2 positive by RT-qPCR and 45 (6.5%) samples by the Standard Q COVID-19 Ag test. Stratified by Ct values, sensitivity of the RADT was 100.0%, 94.4% and 81.1% for Ct <= 20 (n=18), Ct <= 25 (n=36) and Ct <= 30 (n=53), respectively. Samples with Ct values >= 30 (n=23) were not detected. Overall RADT specificity was 99.7% and symptom status did not affect RADT performance. Notably, RADT detected 4 out of 4 samples of probable Omicron variant infection based on single nucleotide polymorphism analysis. Conclusion: Our results show that RADT testing remains a valuable tool in detecting breakthrough infections with high viral RNA loads.