Infectious diseases impose a major health threat globally, leading to 15 million deaths annually [1] . Although the percentage of mortality due to infectious diseases has declined, numerous new infectious diseases have been identified and reported recently. The novel coronavirus disease , caused by the SARS-CoV-2 virus, was firstly identified in Wuhan, China, in late December 2019 as an outbreak of unusual viral pneumonia [2] . Later, the World Health Organization (WHO) declared a public health emergency worldwide, and the total number of infected cases reached 4.4 million by May 2020 [3] . Consequently, educational institutions, business centers, public transport, and other social interaction were locked down points to prevent the spread of COVID-19 and ease the burden on health facilities. SARS-CoV-2 is an enveloped positive-sense single-stranded RNA virus with six open reading frames (ORFs) that codes for structural proteins, including surface (S), envelope (E), membrane (M), and nucleocapsid N proteins [4] . Based on the genomic structures and phylogenetic analysis of SARS-CoV-2, the virus belongs to genera Betacoronavirus, which includes SARS-CoV and MERS-CoV.
Yet, SARS-CoV-2 has differences in its genomic that can influence its pathogenesis.
The most effective approach to prevent and mitigate the adverse consequences of this viral pandemic requires the development of effective surveillance programs, incorporated with laboratory preparedness. Diagnostic laboratory tests play a significant role in the rapid and accurate detection of new viruses [5, 6] . Currently, real-time reverse-transcription polymerase chain reaction (RT-PCR) testing is the main technique used for the diagnosis of COVID-19.
However, false-negative RT-PCR results occur in up to 30% of COVID-19 patients [7] [8] [9] . This could be due to the collection of inappropriate or insufficient sample, inaccurate conditions of sample transportation and storage, as well as collecting the sample too late in the disease process.
On the other hand, serology testing could cover this gap since detecting SARS-CoV-2 IgG antibodies could indicate recovery or immunity from COVID-19 infection. Besides, IgM could be detected in the acute phase of infections. Although, manual enzyme-linked immunoassay (ELISA) kits could be subjected to non-specific binding and cross-reactivity with other coronaviruses such as MERS-CoV and SARS-CoV-1, most commercially available antibodies utilize lateral flow assays (LFA) [10] . However, recently ELISA and automated-based assays were also introduced.
The diagnostic performance, including sensitivity and specificity, of these assays, were better than the LFA [11] . It worth mentioning that there is a high percent of COVID-19 asymptomatic patients who could transmit the infection to all communities. For instance, the asymptomatic ratio of COVID-19 was estimated to be 41.6% of Japanese individuals who were evacuated from China [12] . Similarly, 72% of people infected with COVID-19 on board the Diamond Princess cruise ship were asymptomatic [13] . However, the extent of viral transmission from the asymptomatic cases is not clear yet. The positive RT-PCR results only imply the potential infectivity. A prospective study was published on March 28 in which the viral load and clinical manifestations of 2,147 close contacts of symptomatic and asymptomatic COVID-19 cases were followed up [15] .
The study concluded that the virus infection rate of close contacts with asymptomatic patients was 4.11%.
Since the transmission ability of asymptomatic individuals should not be ignored, it was of interest to conduct a systemic review to paint a picture of the current status and incidence of SARS-CoV-2 in asymptomatic patients. Therefore, this study would give significant insights into COVID-19 infection and help health authorities to determine the need for social distancing close contact restrictions in specific areas or populations.
J o u r n a l P r e -p r o o f
We conducted a systematic review of all literature published on COVID-19 in the asymptomatic patients using four databases: PubMed, Web of Science, Scopus, and ScienceDirect.
The search covered all literature within the databases up to April 2020. The databases were queried with the keywords: "COVID-19", "SARS-CoV-2", "seroprevalence", and "asymptomatic" to ensure complete coverage of all literature. The four databases were searched without filters.
Therefore, results that were letters and commentaries were also included. All retrieved citations were imported into EndNote X8, and duplicates were removed using the EndNote X8 built-in "Find Duplicates" feature. Finally, the titles and abstracts of the remaining citations were screened to remove any irrelevant articles.
The following inclusion criteria were used in study selection: (i) published in a peerreviewed journal, letters, case reports, and commentaries (ii) articles studying the COVID-19 infection in asymptomatic patients, and (iii) articles published in English or at least with an abstract in English. A schematic of the search strategy and study selection process is shown in Figure 1 .
Besides, studies that reported the coinfection of COVID-19 with other viruses as well as comorbidities, such as cancer and cystic fibrosis, were also included in this study. No exclusion criteria were followed unless the studies did not report the incidence of SARS-CoV-2 in asymptomatic patients, published in a non-English language, or do not have full-text access.
The studies included in this systematic review were analyzed two times by the same individual to ensure accurate capture of the information. The analyzed data included the incidence 
The search yielded 505 studies, of which 370 citations remained after removing duplicates ( Figure 1 ). After screening the titles, abstracts, and keywords, 312 citations were excluded. The removed citations included irrelevant studies. The remaining 67 citations were screened against the eligibility criteria. Of these, one study was removed due to the unavailability of full-text access.
Furthermore, three studies were removed two for being published in languages other than English with no English abstract. The remaining 63 studies were included in this study for further analysis, and they consisted of letters to the editor, commentaries, case reports as well as research studies.
The reviewed studies covered SARS-CoV-2 incidence worldwide. Country-wise, the majority of the studies were from China (n = 44) and included different provinces such as Wuhan, Shenzhen, Guangzhou, Beijing, Shanghai, Hunan, Nanjing, Guangdong, Anhui, Hubei, Zhejiang, Jinan, and Hefei ( Table 1 ). The remaining studies were published in Japan (n = 2), Italy (n = 3), Germany (n = 1), Iran (n = 2), and USA (n = 6), which included studies from Texas, Washington, and New York. However, there was a lack of SARS-CoV-2 epidemiological studies from several countries worldwide, tracing the actual incidence of COVID-19, especially in asymptomatic patients.
Looking at all the included studies with a large sample size (n>1000 cases), these studies (Table  1 , highlighted with bold text) estimated that percentage of people contracting SARS-CoV-2 and are likely to be asymptomatic range from (1.2-12.9%). However, the other studies with a smaller sample size (n<1000) reported a much higher incidence and indicated that up to 87.9% of COVID-19 infected individuals could be asymptomatic ( Table 1) . Most of these estimates were based on RT-PCR results. On the other hand, the estimated seroprevalence of antibodies to SARS-CoV-2 was reported to be higher. For instance, a study that was performed on 2,857 blood donors from Rio de Janeiro showed 23.7% of IgM positive cases, 11.4% of IgG positive cases, while both IgM and IgG was detected in 64.9% [16] . This is could due to the limitation of the nasal swab since the PCR diagnostic could be negative though antibody detection is positive. In fact, this finding was reported in a study where four subjects out of 317 asymptomatic participants had negative PCR diagnostic, while antibody testing was positive [17] . Therefore, relying only on molecular testing could significantly underestimate the seroprevalence SARS-CoV-2, especially in asymptomatic individuals.
J o u r n a l P r e -p r o o f
The spread of COVID-19 is an emerging condition with pandemic potential that threatens all countries. Over the last four months, more than three million cases of COVID-19 have been confirmed worldwide. Numerous epidemiologic investigations identified an association with respiratory droplet transmission. Yet, understanding of the transmission risk is incomplete. It worth mention that COVID-19 asymptomatic individuals may pose a significant public health threat. The majority of these patients might be unaware of their disease and, therefore, not isolate themselves or seek treatment. Consequently, unknowingly transmit the virus to others. To the best of our knowledge, this is the first systematic review study that investigated the incidence of SARS-CoV-2 in asymptomatic patients.
A total of 63 out of 505 screened studies reporting COVID-19 asymptomatic patients were included in this review. Epidemiological data, clinical laboratory results, CT image findings, as well as the medical and contact history of the patient are critical knowledge that should be carefully studied when a new infectious disease emerged [59] . Although asymptomatic patients with SARS-CoV-2 were uncommon, studies showed that the prevalence of SARS-CoV-2 in asymptomatic patients is underestimated and might increase. For instance, a review paper showed the rate of asymptomatic individuals with the Middle East Respiratory Syndrome coronavirus (MERS-CoV) ranged from 0% to 28.6% [60] . Besides, it was reported that 75% of COVID-19 infected individuals could be asymptomatic [55] . [19] . Similarly, a study reported in China (Wuhan) showed a 3-years old male asymptomatic patient with positive RT-PCR for SARS-CoV-2 and normal lymphocyte counts and chest CT images [20] . It is unknown yet the reason of having a benign clinical course and low incidence of COVID-19 in children compared to adults. A proposed hypothesis suggested that it might be due to the low expression of ACE2 receptors, high plasticity of their immune system, or to the exposure of other coronaviruses which are generally common in kids [61, 62] .
Besides, children may play a major role in community-based viral transmission. For instance, it was reported that viral shedding in the stool sample could persist for several weeks after diagnosis [63, 64] . Consequently, it poses a threat of viral transmission through the fecal-oral route, particularly for infants and children who are not toilet trained. Most of the reported COVID-19
cases in children were due to close contact with family members with SARS-CoV-2 infection (Table 1 ). Many experts believe that undetermined asymptomatic cases of COVID-19 infection could be an important source of contagion [41] . Therefore, the early identification of the asymptomatic cases, as well as monitoring and tracing close contact, could help in mitigating the spread of COVID-19 infection.
Another factor that increases the asymptomatic rate of COVID-19 is the inaccuracy of diagnostic testing. For instance, a recent article highlighted key important steps to be considered when designing seroprevalence studies, as well as experts' opinion on the recent studies. A major concern raised about the recently published results was the type of antibody test used since most of them inaccurate to support the conclusions [65] . It was reported that the manual ELISA kits are subject to cross-reaction with other coronaviruses such as SARS-CoV-1 and MERS-CoV [10] . Besides, some of the included studies reported that the patients were positive for SARS-CoV-2 IgG, which suggested that the patient was an asymptomatic SARS-CoV-2 carrier. The differential use of serology for confirming acute infection is not appropriate without the additional collaboration of results. Therefore, combining both molecular and serological testing would be the best approach to accurately estimate the prevalence of COVID-19 infection, especially if the patient is at later stages of the infection and does not show symptoms [68] .
Although governments in many countries are planning to conduct largescale seroprevalence surveys, many laboratories try to rely on well-established and validated lab tests, rather than rapid tests. The latter is based on blood collected from finger pricks to detect SARS- Funding: This work is supported by QNRF grant no. RRC-032 grant was given to G.K.N. We would like also to thank Qatar National Library (a member of Qatar Foundation) for sponsoring the publication fees of this article.
The work presented in this manuscript does not involve work with animals or with human subjects, and therefore does not require ethics clearance.
The authors declare no conflict of interest.
J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f [58] 