Real Time Quantitative PCR Vs Serological Detection

On December 31, 2019, the first case of pneumonia with unknown causes was reported, in Wuhan City, of China. By January 30, 2020, the disease had spread across China which forced the World Health Organization(WHO) to declare the outbreak a public health emergency of international concern due to its rapid spread and mortality rate.
The disease had no known name until February 11, 2020, when WHO, announced the name of the viral disease as the new coronavirus disease (COVID-19) which causes severe acute respiratory syndrome in patients. By this time, the global number of confirmed cases had exceeded 42,500 with over 1,000 deaths. The virus spread from one individual to another through contact with droplets that contain the SARS-CoV-2 either by inhalation or physical contact. The pathogenesis of the disease had three major phase characteristics that include; Window phase, the Decline phase, and a Convalescence phase.

 

During the window period, the patient goes through an asymptomatic stage that lasts about 5days from the time of contact. The period between exposure and onset of symptoms, also known as the incubation period of COVID-19 estimated to be between 5 to 6 days with some cases that may go up to 14 days. Symptoms may include fever, myalgia, anosmia, cough, shortness of breath, sore throat, headache and fatigue. Studies have shown that there are cases of individuals that acquire the virus but exhibit no symptoms, however, can actively spread the infections to other people that they get in contact with. RT-qPCR techniques are able to detect the virus in this phase as compared to the serological
assays that can only detect antigens or antibodies after the window period. Between days 7 and 14, somatic cells infected by the virus trigger a cell-mediated immune response by producing different types of cytokines that activate other immune cells to clear the virus. Matter of fact, the massive production of inflammatory cytokines, a phenomenon referred to as the cytokines storm, has been suggested by some studies to lead to the damaging of host tissues which accelerates the progression of the disease and eventual death. Studies on COVID-19 patients have found that viral proteins trigger nucleocapsid protein (NP)-specific antibody response and S-Protein specific antibody response. The first antibody to be serologically detectable after the 7th day is IgM that is responsible for marking pathogens so they can be destroyed by phagocytes. These antibodies are partly responsible for the body’s reaction and transition into the next phase—the decline phase.

The decline phase starts about 12-13 days into the infection and during this period, the patient starts to recover. Serological assays can start detecting the presence of IgG antibodies around the same time that this period starts. The production of these antibodies continues and reaches its peak during the Convalescence phase when the patient is fully recovering, however, IgM antibodies are almost non-detectable during this phase. IgG antibodies remain in circulation even after the 28th day when the patient has recovered and are responsible for the long term immunity of the body against the virus.

It is estimated that there are currently 112 molecular assays for nucleic acid detection of SARS-CoV-2 of which 90% utilize Reverse Transcription Polymerase Chain Reaction (RT-PCR) technologies, 6% utilize Isothermal Nucleic Acid Amplification Technologies and the rest utilize other technologies for example hybridization technologies and CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) technologies. Some of the serological/immunological assays available for the COVID-19 supply chain include Enzyme-Linked Immunosorbent Assay (ELISA), immuno-chromatographic lateral flow assay, luminescent immunoassays, neutralization bioassays, specific chemosensors and rapid antigen tests. However much PCR is the gold standard for COVID-19 laboratory diagnosis, it is not suitable for large scale screening of the infection in resource-constrained countries due to its high-performance costs and competence needs. It is therefore important to be supplemented by point of care serological testing technologies.

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