Delta Variant (B.1.617.2) of SARS-CoV-2: Comparison
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Please note this is a comparison between Version 2 by Rita Xu and Version 1 by Seiya Yamayoshi.

Rapid antigen tests (RATs) for COVID-19 based on lateral flow immunoassays are useful for rapid diagnosis in a variety of settings. Although many kinds of RATs are available, their respective sensitivity has not been compared. 

  • SARS-CoV-2
  • COVID-19
  • rapid antigen test

1. Introduction

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19). The WHO reported that more than 230 million cases of COVID-19, including approximately 4.8 million deaths, have occurred as of 29 September 2021 (https://covid19.who.int/). To reduce the burden by SARS-CoV-2, nonpharmaceutical interventions, vaccination, and patient treatment are required. For mitigation of infectious diseases, early and accurate patient diagnosis is essential.
For COVID-19 diagnosis, reverse transcription-quantitative PCR (RT-qPCR) using upper respiratory swabs or saliva has become the gold standard [1] because it possesses high sensitivity and specificity against the target agent. RT-qPCR is usually not available in local clinics where patients who suspect they have COVID-19 go first. Therefore, the collected specimens are transported to sites with RT-qPCR capability, resulting in delayed test results. To obtain results at local clinics, rapid antigen tests (RATs) for COVID-19 have become popular because RATs require just 15–30 min to give results. RATs are also helpful as screening tests for asymptomatic individuals since model analyses showed that population screening tests should prioritize frequency and turnaround time over sensitivity [2,3][2][3]. Therefore, RATs might be useful to reduce COVID-19 clusters and spread if frequent self-testing using RATs was performed before mass gatherings, domestic travel, or dining at restaurants. Although the sensitivity of RATs is lower than that of RT-qPCR [4,5,6,7[4][5][6][7][8][9][10][11],8,9,10,11], it is essential to utilize RATs with superior sensitivity for better detection.

2. Comparison of Rapid Antigen Tests (RATs)

We evaluated 27 RATs that were available in Japan in September 2021 (Table 1). Of these 27 RATs (#1–17), 17 are approved for clinical diagnosis in Japan, whereas the other 10 RATs (#18–27) are not approved for such purpose in Japan. The 27 RATs are divided into three formats: the test strip format, the pen format, and the well format. In the test strip format, a test strip is soaked in lysis buffer containing the specimen or is dipped in the specimen and then soaked in the lysis buffer; the reaction occurs on the strip. In the pen format, the test strip is dipped into the specimen and the reaction occurs on the strip. This format allows saliva specimens to be loaded by holding the cartridge directly in the mouth. For the well format, lysis buffer containing the specimen is dropped into the well, and the reaction occurs inside a covered plastic body. The well format can be further subdivided into two groups based on how the result is evaluated; for tests #15, #16, and #17, a specific analyzer is required to evaluate the results, whereas the other well-format RATs are assessed by the human eye. Most RATs can process upper respiratory swabs including nasopharyngeal (NP), pharyngeal (P), oropharyngeal (O), or nasal (N) swabs, whereas saliva is the recommended sample for seven RATs (#19, #22, #23, #24, #25, #26, and #27) (Table 1). Tests #18, #20, and #21 can be used for both upper respiratory swabs and saliva. Since it is easy for individuals to collect nasal swabs and saliva, the RATs available for such specimens are suitable for self-testing.
Table 1. Characteristics of the rapid antigen tests for COVID-19 evaluated in this study.
No. Rapid Antigen Test Input Rate (%) a Detection Limit b Time to Result (min) c
Rapid Antigen Test Virus Titer Tested (PFU/Test)
75,000 7500 750 75 7.5
1 ESPLINE SARS-CoV-2 8.0 25 pg/mL 10–30
2 ImmunoAce SARS-CoV-2 13.3 35.6 TCID50/test 15
3 PanbioTM COVID-19 Ag Rapid Test Device 14.3 157.7 TCID50/mL 15–20
- RT-qPCR 17.1 a 20.9 24.5 27.6 31.0
4
1 ESPLINE SARS-CoV-2 + b + + – n.d. PRORAST SARS-CoV-2 Ag
2 ImmunoAce SARS-CoV-2 + – – n.d. n.d. 18.2 42 Pfu/mL 490 TCID50/mL 15–30
6
3 PanbioTM COVID-19 Ag Rapid Test Device +15
+ – n.d. n.d. 5 SARS-CoV-2 Rapid Antigen Test 14.3Fuji Dry-Chem IMMUNO AG Handy COVID-19 Ag
4 PRORAST SARS-CoV-2 Ag + – – n.d. n.d. 6.0 110 TCID50/mL
5 SARS-CoV-2 Rapid Antigen Test n.d. c10
+ – – n.d. 7 ALSONIC COVID-19 Ag 10.9 800 TCID50/mL 5
8 COVID-19 and Influenza A+B Antigen Combo Rapid Test
6 Fuji Dry-Chem IMMUNO AG Handy COVID-19 Ag n.d. + – – n.d. 28.6 100 pg/mL 15
7 ALSONIC COVID-19 Ag n.d. + ± – n.d. 9 ImmunoArrow SARS-CoV-2 22.2 25 pg/mL 15
8 COVID-19 and Influenza A+B Antigen Combo Rapid Test n.d. + + – n.d. 10 Check MR-COV19 21.9 100 TCID50/mL 15
9 ImmunoArrow SARS-CoV-2 n.d. + + – n.d. 11 RapidTesta SARS-CoV-2 21.8 110 TCID50/mL 10
10 Check MR-COV19 + – – n.d. n.d. 12 QuickNavi-Flu+COVID19 Ag 12.5 53 TCID50/mL 10
11 RapidTesta SARS-CoV-2 + + – n.d. n.d. 13 QuickNavi -COVID19 Ag 12.5 53 TCID50/mL 10
12 QuickNavi-Flu+COVID19 Ag + + – n.d. 14 KBM LineCheck nCoV 100 625 TCID50/mL 10
n.d.
13 QuickNavi -COVID19 Ag + + – – n.d. 15 BD Veritor System for Rapid Detection of SARS-CoV-2 26.7 140 TCID50/mL 15
14 KBM LineCheck nCoV + – – – n.d. 16 Sofia SARS Antigen FIA 34.3 113 TCID50/mL 15
15 BD Veritor System for Rapid Detection of SARS-CoV-2 + + – – n.d. 17 Fuji Dri-chem immuno AG cartridge COVID-19 Ag 23.1
16 Sofia SARS Antigen FIA10 pg/mL 15
+ + – – n.d. 18 COVID-19 NP rapid test kit 22.2 N.A. d 15
17 Fuji Dri-chem immuno AG cartridge COVID-19 Ag n.d. + + – n.d. 19 SARS-CoV-2 Antigen Rapid Test 8.6 N.A. +20
– – 20 2019-nCoV Ag rapid detection kit 2.0 N.A. 15
18 COVID-19 NP rapid test kit +21 Novel Coronavirus (SARS-CoV-2) Antigen Rapid Test Kit 11.1 100 TCID50/mL 15
22 Saliva SARS-CoV-2(2019-nCoV) Antigen Test Kit 100 N.A. 15
n.d.
19 SARS-CoV-2 Antigen Rapid Test + + – – n.d.
20 2019-nCoV Ag rapid detection kit + + ± – n.d.
21 Novel Coronavirus (SARS-CoV-2) Antigen Rapid Test Kit + + – – n.d. 23 Corona Virus (COVID-19) Antigen Rapid Test 25 N.A. 15
24 SARS-COV-2 Antigen Rapid Test Kit
22 Saliva SARS-CoV-2(2019-nCoV) Antigen Test Kit n.d. + + ± – 45.7 160 TCID
2350/mL 15–20
Corona Virus (COVID-19) Antigen Rapid Test + ± – – n.d. 25 Novel coronavirus (2019-nCoV) antigen testing kit 22.9 121 TCID50/mL
2415–20
SARS-CoV-2 Antigen Rapid Test Kit + + – – n.d. 26 COVID19 antigen rapid test device 100 N.A.
2515
Novel coronavirus (2019-nCoV) antigen testing kit + + – – n.d. 27 Rabliss SARS-CoV-2 antigen detection kit COVID19 AG 10.9 N.A. 8
26
COVID19 antigen rapid test device
+
+
–
–
n.d.
27
Rabliss SARS-CoV-2 antigen detection kit COVID19 AG
n.d.
+
±
–
–
NoRapid Antigen TestManufacturerCountry of OriginClinical Use in JapanFormat aRecommended Test Sample b
1ESPLINE SARS-CoV-2FujirebioJapanYesWellNP or N swab
2ImmunoAce SARS-CoV-2TAUNS LaboratoriesJapanYesWellNP or N swab
3PanbioTM COVID-19 Ag Rapid Test DeviceAbbott Diagnostics MedicalUSAYesWellN swab
4PRORAST SARS-CoV-2 AgADTEC/LSI MedienceJapanYesWellNP or N swab
5SARS-CoV-2 Rapid Antigen TestRoche DiagnosticsSwitzerlandYesWellNP or N swab
6Fuji Dry-Chem IMMUNO AG Handy COVID-19 AgFujifilmJapanYesWellNP or N swab
7ALSONIC COVID-19 AgAlfresa PharmaJapanYesWellNP or N swab
8COVID-19 and Influenza A+B Antigen Combo Rapid TestNichirei Bioscience/Hangzhou AllTest BiotechJapan/ChinaYesWellNP or N swab
9ImmunoArrow SARS-CoV-2ToyoboJapanYesWellNP or N swab
10Check MR-COV19Rohto PharmaceuticalJapanYesWellNP or N swab
11RapidTesta SARS-CoV-2Sekisui MedicalJapanYesWellNP or N swab
12QuickNavi-Flu+COVID19 AgDenkaJapanYesWellNP or N swab
13QuickNavi -COVID19 AgDenkaJapanYesWellNP or N swab
14KBM LineCheck nCoVKohjin BioJapanYesTest stripNP swab
15BD Veritor System for Rapid Detection of SARS-CoV-2Becton DickinsonUSAYesWell + AnalyzerN swab
16Sofia SARS Antigen FIAQuidelUSAYesWell + AnalyzerNP or N swab
17Fuji Dri-chem immuno AG cartridge COVID-19 AgFujifilm/Mizuho MedyJapanYesWell + AnalyzerNP or N swab
18COVID-19 NP rapid test kitShanghai Cagenbio ScienceChinaNoWellSaliva or P or O swab
19SARS-CoV-2 Antigen Rapid TestZhuhai Encode Medical EngineeringChinaNoWellSaliva
202019-nCoV Ag rapid detection kitGuangdong Longsee BiomedicalChinaNoWellSaliva or O or NP swab
21Novel Coronavirus (SARS-CoV-2) Antigen Rapid Test KitBeijing Jinwofu Bioengineering TechnologyChinaNoWellSaliva or O or NP swab
22Saliva SARS-CoV-2(2019-nCoV) Antigen Test KitJiaxing Wisetest Bio-techChinaNoPenSaliva
23Corona Virus (COVID-19) Antigen Rapid TestHoyotek BiomedicalChinaNoWellSaliva
24SARS-CoV-2 Antigen Rapid Test KitJOYSBIO (Tianjin) BiotechnologyChinaNoWellSaliva
25Novel coronavirus (2019-nCoV) antigen testing kitNanjing Norman Biological TechnologyChinaNoWellSaliva
26COVID19 antigen rapid test deviceToa IndustryJapanNoTest stripSaliva
27Rabliss SARS-CoV-2 antigen detection kit COVID19 AGUndisclosedChinaNoWellSaliva
a RATs were divided into three types based on their format: (i) well format, in which the lysed sample is dropped into the well and the reaction occurs inside a covered plastic body; (ii) test strip format, in which a test strip is soaked in lysis buffer containing the specimen or dipped in the specimen and then soaked in the lysis buffer, and the reaction occurs on the strip; or (iii) pen format, in which a test strip is dipped into the specimen and the reaction occurs on the strip. “+ Analyzer” means that these RATs need an analyzer to evaluate the result. b NP, nasopharyngeal; N, nasal; P, pharyngeal; O, oropharyngeal.
All of the RATs we tested are immunochromatographic tests, meaning that their sensitivity is dependent on the binding kinetics and epitopes of the monoclonal antibodies used in each RAT, the composition of the lysis buffer, the volume of specimen used for analysis, and the method to visualize the result. We cannot directly compare the performance of monoclonal antibodies because the manufacturers do not disclose the properties or amino acid sequence of monoclonal antibodies; however, most RATs likely use monoclonal antibodies against the nucleoprotein of SARS-CoV-2. Because the amino acid sequences of nucleoprotein are similar among human betacoronaviruses, especially the subgenera sarbecovirus, cross-detection is likely to occur against SARS-CoV or SARS-CoV-2-related viruses such as RaTG13 and bat SARS-like coronaviruses. Most of the RATs claim cross-detection of SARS-CoV, with three exceptions: the manufacturers of tests #11 and #15 state that their tests show no cross-reactivity against SARS-CoV, and test #6 cross-detects a high concentration of human coronavirus HKU1 as well as SARS-CoV. Therefore, RATs that show cross-reactivity against SARS-CoV are not able to differentiate patients infected with SARS-CoV-2 and other sarbecoviruses under conditions where these viruses are co-circulating.
The amount of specimen used for each test varied between the RATs (Table 2). The input ratio for three RATs with the pen and test strip formats (#14, #22, and #26) was 100% because of the mechanism. Among the well-format tests, the lowest input ratio was for test #20 at 2%, and the highest was for test #24 at 45.7%. According to the detection limits stated in the manufacturers’ product information, the RATs could detect SARS-CoV-2 at 35–800 TCID50/mL or target virus protein at 10–25 pg/mL (Table 2). The results are assessed 5–30 min after adding the analyte (Table 2).
Table 2. Rapid antigen tests for COVID-19.

3. Sensitivity of RATs for SARS-CoV-2 Delta Variant Detection

To compare the sensitivity of the 27 RATs, a delta variant (lineage B.1.617.2) of SARS-CoV-2 was diluted to the indicated PFU and then examined by RT-qPCR to determine the Cq value of each sample. The Cq values were 17.1, 20.9, 24.5, 27.6, and 31.0 at 75,000, 7500, 750, 75, and 7.5 PFU (Table 3). Test #22 detected 75 PFU of delta variant in one out of the two tests but failed to detect 7.5 PFU of virus (Table 3). Tests #1, #8, #9, and #17 detected 750 PFU of delta variant in both two tests, whereas tests #7, #20, and #27 detected 750 PFU of delta variant in one out of the two tests. Tests #2, #4, #1, and #14 detected 75,000 PFU of delta variant in both two tests but failed to detect 7500 PFU. The other RATs detected 7500 PFU of delta variant. Taken together with the RT-qPCR data, our findings show that the sensitivity for delta variants of tests #1, #7, #8, #9, #17, #20, #22, and #27 is relatively high but lower than that of RT-qPCR.
Table 3. Sensitivity of rapid antigen tests for the delta variant.
No.

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