A single negative serological test may reflect a false-negative result, so it does not exclude SARS-CoV-2 infection [66,70]
, particularly in highly exposed persons; this could be due to the low antibody concentrations if the test is performed at the beginning of the infection [68,71]
, the type of a specimen, or due to the decrease in the number of antibodies after the clearance of the infection. In this case, repeating the test is the best advice. The relatively low negative predictive value of many COVID-19 antibody tests indicates the missing data of many acute infections based on seronegative results 
. Negative results also allow clinicians to suspect other diseases, because COVID-19 symptoms can resemble those of many other diseases.
By contrast, a positive antigen-based detection test is considered very accurate for identifying acute or early infection 
and can indicate that a person is likely infected with SARS-CoV-2. Nevertheless, the test positivity may be due to cross-reactivity with other infections, including other human coronaviruses 
. Therefore, a full-panel test, including other CoVs, SARS-CoV-2, bacterial bronchitis, and influenza, is recommended if applicable 
4.4. Population Serological Testing
Serological testing can help identify who is infected or exposed and who is immune by assuming protective immunity. Accordingly, population-based serological information will be helpful for officials in making decisions about lifting or enforcing any control measures. The potential of this test in determining the accurate number of infected people in a large population has been tested [83,84]. Population seropositivity indicates that the number of people positive for anti-SARS-CoV-2 antibodies is much higher than that of the reported cases [84,85]. Population-based serological surveillance has been carried out around the world, including countries in Europe [86,87], America [88,89], Asia [90,91], and Africa [92,93]. Indeed, healthcare workers are the population most targeted for serological surveillance due to their high risk of SARS-CoV-2 infection, with ELISA being the most commonly used diagnostic tool for detecting anti-SARS-CoV-2 antibodies.
5. Antibody Tests and Seroprotection
5.1. Antibodies in SARS-CoV-2 Vaccine Development
Immunity against SARS-CoV-2 infection is normally acquired in one of two ways. Contracting the virus typically ends in natural immunity for a certain period, and vaccination is another way to become resistant. The entry of SARS-CoV-2 into the human cell is the first step of the infection and one of the most crucial processes in the virus’ life cycle. As a result, it is a prime target for vaccinations and therapeutics.
The virus enters the cells of the lung, gastrointestinal tract (GI tract), kidneys, liver, heart, and other organs through the binding of the S protein’s receptor-binding domain (RBD) to its target host receptor, the angiotensin-converting enzyme 2 (ACE2), and a host protease known as transmembrane serine protease 2 (TMPRSS2), which facilitates the cleavage of the S glycoprotein, allowing viral access into the host cells [99,100]
. Therefore, one of the main goals of SARS-CoV-2 vaccine development is to generate neutralizing antibodies that block virus entry or prevent membrane fusion.
Despite the fact that the post-vaccination immune response has several components, including innate, humoral, cellular, and cytokine responses, immunological surveillance that measures antibody response is fundamental for assessing the efficacy of all SARS-CoV-2 vaccines. In particular, measuring the level of circulating anti-S-RBD antibodies could provide important information on SARS-CoV-2 acquired immunity 
5.2. Mechanism of Antibody-Mediated Protection
The role of antibodies in resistance to SARS-CoV-2 infection was explored. Understanding the properties and mechanisms by which antibodies provide protection is essential to defining immunity. Infection or vaccination history may have a role in providing protection against the subsequent infection, and several studies have provided evidence for such protective associations 
. Upon infection, pre-existing antibodies bind to the surface of SARS-CoV-2 virus particles and lead to the neutralization of the viral spike. Neutralizing antibodies are critical for the efficacy of any SARS-CoV-2 vaccine. Over the past two years, SARS-CoV-2-neutralizing antibodies have been developed for preventive or therapeutic uses [106,107,108]
. Most of the neutralizing antibodies target the S protein; their neutralization potency and breadth vary according to recognition epitopes. These findings have prompted an intense effort to identify potential immunodominant epitopes that are recognized by broadly neutralizing the antibodies that could be used as templates for SARS-CoV-2 vaccine design.
5.3. Prevaccination Antibody Screening
Notably, limited-resource countries (LRCs) remain at the back of the line when it comes to new technologies, infrastructure, and public health control measures such as vaccines. Evidently, COVID-19 mass vaccination is currently not applicable in most of the LRCs, where a lack of resources is placing enormous pressure on governments to accelerate the mass vaccination strategies. Accordingly, the establishment of new strategies that maximize the number of individuals who get vaccines without losing the efficacy of immune protection is urgently needed. The limited availability of authorized SARS-CoV-2 vaccines has led to widespread consideration of a single vaccine dose for people with past SARS-CoV-2 infection 
6. Resistance to SARS-CoV-2 Antibodies
Resistance to the SARS-CoV-2’s antibody is now a living fact due to the introduction of new virus variants, and this places an enormous load on the vaccination process. As such, recent vaccines have been developed to contradict the virus that was first discovered in late 2019 in Wuhan, China 
. However, emerged variants such as the South African-Beta (B.1.351) and the UK-Alpha (B.1.1.7) have demonstrated extensive mutations in their S proteins, and these variants have been demonstrated to be highly contiguous 
. Indeed, almost all monoclonal antibodies directed against the S protein’s N-terminal domain failed to recognize Alpha, although antibodies directed against the receptor-binding region were more effective 
. Nonetheless, the variation exhibited reduced affinity for plasma from patients who have recovered from SARS-CoV-2 or sera from those who have been immunized against SARS-CoV-2. Both monoclonal antibodies directed against the N-terminal domain and several separate monoclonal antibodies directed against the receptor-binding motif are very ineffective against the mutated Beta, and this resistance might have been acquired from the mutated E484K substitution. Furthermore, Beta is much more resistant to neutralization by convalescent plasma (9.4-fold) and serum (10.3–12.4-fold) from BNT162b2-immunized individuals 
. Furthermore, the Beta SARS-CoV-2 variants, which had various changes in their S proteins, were resistant to 17 neutralizing monoclonal antibodies as well as sera from convalescent patients and vaccinated mice, which were unable to neutralize the variants 
Subsequent studies indicated that certain Omicron variants are now resistant to antibodies elicited by vaccine doses. Remarkably, Omicron S proteins evaded blockage by antibodies obtained from persons inoculated with the BioNTech-Pfizer vaccine (BNT162b2) or convalescent patients with 12- to 44-fold more efficiency than the Delta (B.1.617.2) variant S protein 
Furthermore, the antibodies ReGN10933, REGN10987, and JS016 and serum from vaccinated participants neutralized the S protein of numerous wild-type variations, including Alpha, Beta, Gamma, and Delta. Regn10987 antibodies, for example, were the most effective in neutralizing the aforementioned variations. In addition, the Regn10933 and JS016 antibodies were both effective, although their reactions to the S protein were quite different 
. These two antibodies, however, were unable to neutralize the Beta version. Unexpectedly, none of these antibodies neutralized the Omicron variant 
. This result implies that these antibodies cannot be used to combat the ongoing Omicron variant pandemic or any developing variation.
Despite immunocompromised individuals potentially being more susceptible to SARS-CoV-2 viruses with unusual manifestations, prolonged immunosuppression treatment may provide some defense from severe COVID-19 disease consequences. However, the possibility of immunocompromised people acquiring abnormally severe COVID-19 is currently elusive 
. Recent evidence shows that immunocompromised people may benefit greatly from convalescent plasma therapy [127,128,129,130]
, and that mutations and newly emerged virus strains will lead to more severe complications in the group. New variants, however, can appear in this patient population as a result of the selection pressure brought on by a severe viral infection 
. The majority of people who have severe SARS-CoV-2 have immune system issues, thus the virus may live on for a very long period. Patients with impaired immune systems have been demonstrated to have varying SARS-CoV-2 evolution patterns 
. How selection forces and evolutionary processes interact during chronic infection is an issue that has yet to be resolved. With that in mind, due to mutations (Q493KRBD
) in the S protein in immunocompromised individuals, antibodies that were recovered from a healthy COVID-19 convalescent donor were shown to be ineffective in providing protection against SARS-CoV-2. In particular, the Q493KRBD
mutation 15-fold reduced the effectiveness of REGN10933 pseudotype neutralization, but the Q493K/RRBD
mutations almost completely imparted resistance to healthy COVID-19 convalescent donor IgG 
7. Limitations of Antibody Tests
Although antibody tests are useful in COVID-19 case management and in vaccinations effectiveness, a number of drawbacks occur. The significant limitation is that antibodies may be present at undetectable levels in early days, thereby influencing the potency of any serodiagnostic tests and their effectiveness for diagnosing SARS-CoV-2 infection [135,136]
. In this situation, a false-negative serological result from individuals with replicating and shedding viruses can have serious public health consequences 
. Another limitation is the unknown duration at which IgM or IgG antibodies remain detectable after the virus has been cleared from the body. Furthermore, variations in antigens and methodologies used in IgM and IgG detection kits are essential, and they affect the sensitivity and specificity of the tests 
. Unfortunately, the inaccuracy of antibody tests is unavoidable and will inevitably lead to false-positive or -negative results and disease misclassifications, particularly if these tests are not properly conducted and interpreted 
. Moreover, the proven cross-reactivity of SARS-CoV-2 antibody tests with other coronaviruses is difficult to avoid 
. Indeed, increasing the levels and duration of antibodies after SARS-CoV-2 vaccination to provide full protection and diagnostic opportunity against the current variants and those that can emerge in the future is still the primary objective in the upcoming studies.