1∣INTRODUCTION
The coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has become a worldwide pandemic and rapidly spread. The disease causes a series of non-specific symptoms such as fever, sleepiness and cough in infected patients. However, subsequent studies have found that patients with SARS-CoV-2 exhibit high heterogeneity in their symptoms. Mild cases can be asymptomatic, while severe cases can have acute kidney injury, liver dysfunction, acute respiratory distress syndrome and other systemic organ dysfunction and eventually lead to death [1-2]. As of April 2022, SARS-CoV-2 has been spreading worldwide for 29 months, infecting 508 million people cumulatively. This pandemic poses an unprecedented challenge to the global health system and the epidemiological statistics. In order to control further spread of the disease, early or asymptomatic infections must be screened and isolated in a timely manner. Rapid, highly sensitive and inexpensive laboratory tests play an important role in screening the infected patients. Currently, there are two main SARS-CoV-2 detection methods applied in clinical practice. The first is immunological diagnostic techniques represented by immunochromatography assay (ICA) [3,4] and chemiluminescence immunoassay (CLIA) [5]. The second is molecular diagnostic technology represented by reverse transcription-polymerase chain reaction (RT-PCR). The products based on ICA are portable and fast, but its specificity and sensitivity are not satisfactory. Compared with ICA, CLIA has improved specificity and sensitivity and can achieve quantitative detection. However, the time lag of antibody production in infected persons makes it difficult to screen the positive ones in the early stage. Due to its unparalleled analytic accuracy, RT-PCR remains the gold standard for molecular diagnosis of the SARS-COV-2 [6]. However, RT-PCR is usually restricted to professional laboratories with specialized equipment and well-trained personnel. This limits its application in some remote areas, especially in areas with inadequate medical infrastructure, as well as in secondary medical institutions in developed areas. Therefore, there is an urgent need to develop a convenient assay for the fast and early detection of SARS-CoV-2 in resource-limited conditions. Compared with RT-PCR, recombinase polymerase amplification (RPA) does not require a PCR machine, being more suitable for point-of-care testing (POCT). It is known as a nucleic acid detection technique that can replace PCR and has the advantages of rapid, high sensitivity and isothermal [7, 8]. However, since there is no thermal cycling to avoid binding between primers, it is difficult to avoid non-specific amplification in RPA amplification. CRISPR/Cas systems were reported in the bacterial genome for the first time and utilized to detect nucleic acids [9-11]. Cas12 protein, also known as Cpf1, specifically binds and cleaves single and double stranded DNA under the mediation of guiding CRISPR RNA (crRNA). It is worth mentioning that crRNAs recognize target sequences of amplicons, thereby improving the specificity of amplification reactions. Here, we develop the V isualD etection of RPA A mplified P roducts (VDAP) to improve the sensitivity and specificity of SARS-COV-2 detection (Figure 1). Briefly, RT-RPA amplification was first performed on nucleic acids extracted from nasopharyngeal swabs and then visualized by the VDAP. In this work, the quenched fluorescent single-stranded DNA (ssDNA) reporter was introduced, which would be cleaved by Cas12a when target DNAs were detected in the system, and the green fluorescence signal could be detected under the blue light. It provides a rapid, inexpensive, stable and convenient assay for SARS-CoV-2 nucleic acid detection, which has great potential for diagnosing COVID-19 and curbing disease outbreaks.