Nidoviruses are positive-sense single-stranded RNA viruses that infect a large number of vertebrates. Within these is the family of coronaviruses, which has four groups, that caused three epidemic outbreaks in recent decades [1]. Coronaviruses were described as causing common respiratory symptoms in the 1960’s [2]. They may be responsible for between 7% and 15% of uncomplicated upper respiratory infections [3]. SARS (severe acute respiratory syndrome) in 2002, from China, was the first report of a coronavirus outbreak with a mortality around 10%. The virus was presumably transmitted to humans by a mammal (civet cat), probably derived from bats. The second outbreak was MERS (Middle East respiratory syndrome), originated in Saudi Arabia, transmitted by camels, but also probably derived from bats; with a mortality close to 40% [1]. Now, we have a third epidemic, the coronavirus (CoV) SARS-CoV-2, which produces COVID-19 (coronavirus disease 2019). The outbreak began in the Wuhan province in China, but has now turned into a pandemic. The sequence of the virus genome isolated from patients is similar to a bat virus [4]. In China, the infection produced mild respiratory symptoms in about 80% of those infected, however, 5% were admitted to the Intensive Care Unit (ICU), 2.3% received mechanical ventilation and the mortality rate was 1.3% [5]. The rapid case growth around the world means that in a short time the health systems could saturate rapidly [6]. The current standard assay for COVID-19 diagnosis is the detection of viral RNA in nasopharyngeal swabs using reverse-transcriptase polymerase chain reaction (RT-PCR) [7]. Rapid and simple immunoassay tests have been developed to detect viral antigen or antibodies against the SARS-CoV-2 virus in human blood even within 15 minutes. Antibody response can be detected as early as 5 days post-infection [8] and the antibody-secreting cells peak around day 7 - 8 post-infection [9] [10]. One of the first peer reviewed studies of this kind of assays showed a sensitivity of 88.66% and a specificity of 90.63% in 397 patients with SARS-CoV-2 confirmed by PCR [11]. There are currently more than 200 immunoassays for SARS-CoV-2 to detect antigens or specific antibodies [12]. The goal of this study is to carry out a comprehensive review of the wide offer of serological kits to detect SARS-CoV-2 antigen or antibodies, in order to help institutions and policymakers define the best option for massive testing. There is an urgent need for rapid serological assays for SARS-CoV-2 that will be a useful tool for public health in the upcoming days.

The current standard for COVID-19 diagnosis is the amplification of viral RNA by RT-PCR. However, this technique requires special equipment and trained individuals [7]. Also, detection of the virus is dependent on the sample origin and time of sampling [15] [16]. Detection of virus specific SARS-CoV-2 antibodies could help determine the exposure of a large population to the virus [5] [8] [9] [10]. In infected individuals, antibody detection by ELISA using nucleocapsid protein as antigen was identified at day 5 for IgM and at day 14 for IgG [17]. IgM antibodies are known to be produced early during a viral infection, followed by the presence of IgG antibodies, which have a longer lifespan and are responsible for the memory response [18]. Besides diagnosing the disease, it is important to determine the immune status of the individual against the SARS-CoV-2 using detection of specific antibodies. Now, there is an offer of more than 200 diagnostic assays that detect viral antigen or specific antibodies, all of them in different stages of validation.

Initial reports of the new SARS-CoV-2 causing acute respiratory distress syndrome came up in Wuhan, China. Since then, several assays have been developed in order to improve the diagnosis, most of them from China [4] [19]. As expected, most of the available assays detect antibodies using blood-derived samples. Although RT-PCR is considered the most sensitive detection method in respiratory fluid samples, it increases the risk of contamination of healthcare workers [20]. Blood-derived samples are easier to obtain, and compared to RT-PCR, serological assays are faster, require less training and less equipment, so they can be used in almost any setting [11]. The most common method behind is immunochromatography [21]. These assays have a long shelf-life, do not require refrigeration and visual results exclude the need for additional equipment compared with other methods such as ELISA [22]. Antigens used for detection are very important; the genome of SARS-CoV-2 codifies for several structural proteins, including the spike (S), membrane (M), envelope (E), and nucleocapsid (N) proteins [23]. The most common antigens used for indirect assays are the recombinant spike and nucleocapsid proteins [16] [17]. The S protein contains the domain that allows attachment to the human host cells [24], and the nucleocapsid protein is one of the major structural components involved in many processes of the virus, including viral replication, transcription, and assembly [18]. Interestingly, there is a 90.5% homology among nucleocapsid proteins of SARS-CoV-1 and SARS-CoV-2 [17]. Also SARS-CoV-2 showed a homology of about 85% with a coronavirus isolated from bats [4] [16].

Assay specificity and sensitivity are key for determining the role of these tests in diagnosis and public health programs [25]. Unfortunately, only a minority of the assays present this information, maybe due to the short development time. However, most of them present a sensitivity and specificity over 90%, but with a low number of infected individuals. Nonetheless, some have sensibility as low as 45%. Patients with RT-PCR confirmed disease began developing specific viral antibodies around 7 days from disease onset [8] [9]. Some studies showed that IgM seroconversion occurs earlier in the course of illness, followed by IgG seroconversion [8] [9] [11], while others describe a simultaneous seroconversion [26]. All studies agree that the seroconversion rate for IgG and IgM increases with time [8] [9] [26]. The detection rate of molecular based methods decreased to as low as 45% in the first 2 weeks [9] [15] [27], while antibodies were detected in 100% of the patients after a month of disease onset [8] [9] [26]. The latter shows that the sensitivity of each immunoassay is variable depending on the time of onset, with more positive results given in a later time of the disease [17]. Most tests evaluate the presence of IgG and IgM simultaneously. The dual detection of IgG-IgM improves the sensitivity in comparison with individual IgG or IgM antibody assays [11], suggesting a possible improvement in infection detection. Additionally, the different samples that can be used for serological diagnosis offer more consistent results. No significant differences were found when using samples from whole blood, serum or plasma [11]. This is opposed to the great variability from samples used in viral RNA detection (blood, sputum, naso/oropharyngeal swabs, anal swabs and bronchoalveolar lavage) [8] [15] [27] [28]. In terms of efficiency, serological assays have a shorter procedure time compared to molecular diagnostic methods, which means greater testing capacity. RT-PCR and antibody assays have their own advantages, the combination of both can provide more accuracy to the initial diagnosis of SARS-CoV-2 infection [11] [20]. Apart from this, current label requirements showed that most of the offered immunoassays (50%) are intended for in-vitro diagnosis and this application represents the usefulness in a clinical environment. Limitations to our study include the lack of support by literature and/or certification by government regulation agencies of the majority of the tests that we found. It is worth mentioning that at the time of this review only few months had passed since the spread of the virus worldwide so, fewer studies and tests were available compared to a more advanced pandemic. Nevertheless, we consider the information found was the best obtainable evidence at the time of the review.

At this point of the pandemic, it would be difficult to suggest which assays are the best for clinical application. However, the information here presented sheds light into the large number of assays available, and the number increases day by day, so it has to be analyzed carefully. We suggest that researchers and policymakers focus on the tests with the most information available, such as a rigorous internal validation data and well-defined TDS. More research is needed, especially studies that compare between different assays; which provide more accurate information than studies testing assays individually [29]. Yet, the initial data looks promising and immunoassays could help screen larger populations in less time, increasing the detection rate and increasing the testing capacity; which is needed to decrease the SARS-CoV-2 spread. Recent studies show that convalescent patients have high levels of SARS-CoV-2 neutralizing antibodies (NAbs), which increased with patients’ age [30]. Interestingly, transfusion of convalescent plasma obtained from COVID-19 cases, improved clinical outcomes of patients with severe disease [31]; thus, suggesting that the antibodies produced by COVID-19 patients during the infection have a posterior protective effect. Large serological studies to detect virus-specific antibodies will be needed to determine the infected asymptomatic population and also could help to suspend social isolation in seropositive individuals.

The authors want to thank Manuel Franco MD, PhD from Pontificia Universidad Javeriana, Bogotá DC, Colombia for critical revision of the manuscript.

The authors declare no conflicts of interest regarding the publication of this paper.

González, J.M., Shelton, W.J., Díaz-Vallejo, M., Rodriguez- Castellanos, V.E., Zuluaga, J.D.H., Chamorro, D.F. and Arroyo-Ariza, D. (2020) Analysis of commercial assays for the detection of SARS-CoV-2 antibodies or antigens. Open Journal of Immunology, 10, 21-35. https://doi.org/10.4236/oji.2020.102003