Coronavirus disease 2019 (COVID-19) is a novel coronavirus pneumonia caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)1,2. Emergence of new infectious diseases poses serious clinical issues3–9, this new disease was first encountered in December 2019 in Wuhan, Hubei Province, China, and then spread worldwide taking on the appearance of health emergency of international concern. Starting from February 2020, the COVID-19 outbreak spreaded in Europe, particularly interesting northern Italy and Spain 10–12. World Health Organization (WHO), on 11th March 2020 declared COVID-19 disease a global world pandemic. SARS-COV-2 belongs to the beta coronavirus family along with other human pathogens known as SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-Cov) 13. As COVID-19 was identified as a health emergency by WHO, large-scale population testing proved to be of crucially importance to identify and isolate symptomatic and asymptomatic case, in the global efforts to contain its expansion.
In December 2019, SARS-COV-2 was firstly transmitted to humans through human-animal contact at live animals market in Wuhan (China) 14.
SARS-CoV-2 belongs to the subfamily of the Coronavirinae, which is part of the order Nidoviralescoronaviruses. It is a single-stranded RNA-enveloped virus, containing 4 structural proteins (from the 3’end open reading frames (ORF)) and 16 accessory proteins (nsp 1 to nsp 16) from the 5’end ORFs. The viral envelop contains structural proteins E and M, while the N protein nucleocapsid binds the viral RNA. The S glycoprotein is the key player for the interaction with ACE2 on the host cells (Fig. 1)15. The interaction between angiotensin-converting enzyme 2 (ACE2) and the S glycoprotein was conserved also in the SARS-CoV, the virus responsible of the SARS outbreak of 2002–2003. The S protein binds to the receptor to target host organism cells. The virus uses also other host cell receptors such as the type 2 transmembrane serine, protease, TMPRSS2, to trigger the endocytotic process employed to access the cells16. Viral polyproteins are expressed in the host cell, RNA can be synthetized via its RNA-dependent RNA polymerase and new viral particles can be produced and released.
Cleavage at the S1/S2 and the S2’ site of the S protein by the proteases of the host cell is necessary for membrane fusion17 (Fig. 2). Cleaved S protein is therefore the activated form ready to enter the cell. This proteolytic step can also occur in the constitutive secretory pathway of infected cells by endosomal cathepsins B and L and furin18. When on the viral membrane the S protein is cleaved (primed) in two segments (Fig. 2). The N-terminal S1 segment is responsible for the interaction with the host cell receptor, as it contains a signal peptide and the receptor binding domain (RBD). The S2 segment anchors the S protein to the viral membrane, contains the fusion peptide which mediates the fusion of the viral membrane with the plasma membrane of the target cell. The proteases responsible for the S protein activation represents promising drug targets for the treatment of the disease, following failure of first attempts, such as hydroxychloroquine19.
Many mutations in the SARS-CoV-2 virus have been observed. One among the most prevalent is the D614G, at the C terminal region of subunit S1 of the Spike protein, which is the region in which subunit S1associates with S2 (Fig. 2b). How and from where this mutation spread is not clear, however it appears to give the virus a decisive transmission advantage over the non-mutated variant 20.
SARS-COV-2 infection displays a broad spectrum of symptoms ranging from asymptomatic forms, mild to moderate symptoms, up to severe respiratory symptoms and lung abnormalities which require intensive care including assisted oxygenation 10,21. The most frequently symptoms are: fever, dry cough, upper tract respiratory symptoms, myalgia, anosmia, ageusia and headache 22,23. Other fearsome complications are represented by Acute Respiratory Distress Syndrome (ARDS), respiratory failure and liver injury, acute myocardial injury and acute kidney injury, septic shock and multiple organ failure 24. Recently, the alteration of the intestinal microbiota has been described in patients with COVID-19, as occurs in chronic non-communicable diseases (CNCDs) 25,26. In the future, the possible understanding of the mechanisms underlying the alterations of the intestinal microbiota following SARS-CoV-2 infection could represent a new diagnostic biomarker and therapeutic target for the fight against COVID-19. The incubation period ranges from 0 to 24 days 27.
SARS-COV-2 infection mainly affects the geriatric population (subjects aged over 65 years) and subjects with altered immune systems or with chronic diseases (such arterial hypertension, chronic kidney disease, chronic obstructive bronchopathy, etc.) 28,29.
Serological tests, for the determination of IgG and IgM are one of the most important components of the public health response to COVID-19, along with viral diagnostic tests, for the contact tracing and the lockdown. However, given the simplicity of the method of serological tests, especially those performed through a point of care test (POCT) method, able to detect simultaneously the presence of IgM and IgG, their use could probably reduce the extent of the shielding required to obtain a better reduction of COVID-19 transmission, in order to allow a considerable number of individuals to return to social and economic interactions 30.
Accurate and rapid diagnostic tests will be critical for achieving control of COVID-19. OMICs approaches and data integration have facilitated identification of biomarkers for many diseases31–35. Similarly, production models have been proven as useful tools36,37, however serology represents a critical step in the COVID-19 control. Diagnostic tests for COVID-19 fall into two main categories: molecular tests that detect viral RNA, and serological tests that detect anti-SARS-CoV-2 immunoglobulins. Reverse transcriptase polymerase chain reaction (RT-PCR), a molecular test, is widely used as the reference standard for diagnosis of COVID-19; however, its limitations include potential false negative results 38,39 that affect diagnostic accuracy over the disease course 40, and precarious availability of test materials 41. Serological tests have generated substantial interest as they represent an alternative or complement to RT-PCR in the diagnosis of acute infection. Serological tests might be cheaper and easier to implement in the POCT. A clear advantage of these tests over RT-PCR is that they can identify individuals previously infected by SARS-CoV-2, even if they never underwent testing while acutely ill. Serological tests could be deployed as surveillance tools to better understand the epidemiology of SARS-CoV-2 and potentially inform individual risk of future disease. Many serological tests for COVID-19 have become available in a short period, including some marketed for use as rapid (POCT).
Aim of this study is to compare two different diagnostic laboratory methods, rapid lateral flow immunoassay (FIA) vs automated chemiluminescent immunoassay (CLIA), in order to assess their specificity and sensibility against COVID-19 antibodies detection. For the assessment of COVID-19 and evaluation of its spread, it should be advisable to develop a rapid laboratory test for its serological early-diagnosis.