Hui-Juan Shao, Xue-Feng Ma, Jun Huang, Xiao-Hui Yu, Jiu-Cong Zhang,3?

1. Department of Gastroenterology,The 940 Hospital of Joint Logistic Support Force of PLA,Lanzhou 730050, China

2. Gansu University of Traditional Chinese Medicine,Lanzhou 730000,China;3. The Sixth Division of Infectious Diseases,Huoshenshan Hospital,Wuhan 430050, China

Keywords:COVID-19 Asymptomatic infection Pre-symptomatic infection Characteristics

ABSTRACT The asymptomatic and pre-symptomatic infections of coronavirus disease 2019 (COVID-19)has no perceptible clinical symptoms and signs, and it is not easy to be detected. The virus’transmission of asymptomatic and pre-symptomatic infections carried is concealed, which brings great challenges to the control of the epidemic. Due to the length of the incubation period, some studies have failed to distinguish between asymptomatic and pre-symptomatic infections, resulting in a higher proportion of asymptomatic infections. This article reviews the latest research progress of characteristics, transmission, detection and control of asymptomatic and pre-symptomatic infection.

Since the outbreak of coronavirus disease 2019 (COVID-19),the epidemic has caused a pandemic around the world. Although multi-layered intervention measures have been adopted worldwide,the number of confirmed and cumulative deaths is still increasing.With the continuous in-depth research on COVID-19, more and more evidence shows that a considerable part of the people infected with the virus are in recessive infection, it is asymptomatic and pre-symptomatic infection. Pre-symptomatic infections are similar to asymptomatic infections when they have no clinical manifestations, and they are not easy to be detected. The infection is more concealed, which brings great challenges to the control of the epidemic. Therefore, this article reviews the latest research progress on the characteristics of asymptomatic and pre-symptomatic infections, transmission characteristics, detection and control.

1. The asymptomatic and pre-symptomatic infections of COVID-19

1.1 Overview and definition

Coronavirus disease 2019 (COVID-19) is a severe acute respiratory

infectious disease caused by SARS-CoV-2. SARS-CoV-2 is a β-coronavirus with enveloped, spherical positive single-stranded RNA , Highly contagious. Symptomatic patients with novel coronavirus infections mostly present with fever, cough, sputum expectoration, dyspnea, chest pain, abdominal pain, and diarrhea.However, most asymptomatic and pre-symptomatic infections reported in the literature lack the above clinical features.

The New Coronavirus Pneumonia Prevention and Control Plan(Eighth Edition) proposes that asymptomatic infections of COVID-19 refer to those who have tested positive for the pathogen of the coronavirus but have no relevant clinical manifestations.Asymptomatic infections of COVID-19 mainly through nucleic acid testing, tracing the source of infection, and epidemiological investigations to find entry Personnel, high-risk occupational groups,close contacts and with their close contacts. Pre-symptomatic infection refers to patients who have been confirmed to be infected with SARS-CoV-2 by the laboratory and have no symptoms at the first clinical evaluation but have symptoms at the end of the followup [1] (End of follow-up refers to: after the last contact with a possible case Follow-up for 14 days or longer).

1.2 Reasons for the inability to distinguish between asymptomatic infection and pre-symptomatic patients

Hidden infections bring great difficulties to the discovery of the source of infection. In cross-sectional studies of asymptomatic infections, asymptomatic infections are often confused with presymptomatic infections, leading to a serious overestimation of the proportion of asymptomatic infections [2]. Firstly, those infected with no symptoms at the time of diagnosis, some patients will have clinical symptoms during the follow-up process. If the observation time is not long enough, it is easy to be mistaken for asymptomatic patients. Moreover, due to the different definitions of infected persons in different countries or regions, there are great differences in the evaluation of symptoms. Some studies may mistakenly categorize pre-symptomatic infections as asymptomatic cases.Secondly, the evaluation of symptoms is incomplete. The study by Cevik et al. [3] found that some infected persons also developed other symptoms throughout the course of the disease, including taste and smell disturbances or muscle pain, but these symptoms may be very mild and insidious and would not affect the patient. It is difficult to detect the daily activities of people, and the immune response of some elderly people is impaired, as well as the existence of cognitive impairment and underlying diseases, it is difficult to identify the early symptoms of respiratory virus infection, which may also classify them as asymptomatic infections. There is also little knowledge about the duration of SARS-CoV-2 antibody response, and there may be great limitations in using serological investigations, especially in combination with clinical history to estimate the proportion of asymptomatic infections [4]. In addition,Kang et al.[5]found that the basic characteristic of persons infected by SARS-CoV-2 is that the body will first experience a prolonged presymptomatic stage when an individual is infected with the virus and then enter a completely asymptomatic or symptomatic stage, which is also causes the inability to distinguish between asymptomatic and pre-symptomatic infection. At present, it is worth noting that the asymptomatic infections found are only the current conditions of the patients and cannot be distinguished from pre-symptomatic infection through screening close contacts, tracing the source of infection and testing the key group of SARS-CoV-2. So, it is necessary to further study the natural history of the disease of asymptomatic infections,learn more about the transmission characteristics of SARS-CoV-2,and implement quick and effective diagnostic methods to distinguish between asymptomatic infections and pre-symptomatic infections,which will help to stratify clinical risk and improve prognosis.

2. Characteristics and transmission of asymptomatic infection with COVID-19

Asymptomatic infections have no perceivable clinical symptoms and signs from the onset to the end of COVID-19. They are latent in healthy people and the transmission of the virus carried is insidious,meanwhile due to the subjective nature of symptoms, asymptomatic infections may think that they are not infected with SARS-CoV-2 and will not take the initiative to go to medical institutions to see doctor, and it is difficult to be found in daily testing. The potential transmission of the virus in asymptomatic and pre-symptomatic infections with novel coronavirus has greatly hindered the control of the epidemic, and they are both important sources of the continuous transmission of the SARS-CoV-2 [6]. Oran et al. [7] showed that at least one-third patients with SARS-CoV-2 are asymptomatic infections. Among those who have received a positive PCR test but have no symptoms at the time of the test, nearly three-quarters will be asymptomatic infections during the course of the disease. However,Qiu et al.[8] researched and concluded that potential transmission is an important cause for SARS-CoV-2 community transmission.However, individuals who remain asymptomatic during the infection process have a very low secondary invasion rate, which indicates that the infectivity of asymptomatic individuals is limited. In addition,the results of Huff et al.[9] showed that at the outbreak of COVID-19,the potential transmission in the asymptomatic stage accounted for more than 50% of the total incidence. Even if all symptomatic cases are immediately isolated, this potential transmission can also cause the epidemic continuous development.

3. Characteristics and transmission of presymptomatic infections with COVID-19

Due to the incubation period after infection, some patients will not develop symptoms until the follow-up time is long enough. The follow-up time required for infection before catching symptoms is the longest incubation period of the virus. Cevik et al.[10]found in the SARS-CoV-2 viral load dynamic study of confirmed cases that the peak viral load was detected from the onset of symptoms to the 5th day of onset, indicating that the highest infectivity was within a few days when the onset of symptoms. Related studies have found that the viral load of throat swabs is highest when symptoms appear,and may reach at peak 1-2 days earlier, and 1251 cases indicate that the pre-symptomatic infection rate may be as high as 68% of the total infection rate[11]. Research by He et al.[12] showed that among 100 secondary patients in 47 clusters of coronavirus pneumonia,at least 15% of them may be pre-symptomatic transmission with SARS-CoV-2. Pre-symptomatic and asymptomatic transmission may be different. Although it is difficult to estimate the proportion of pre-symptomatic infections, the virus carried by these patients still spread invisibly. Sah et al.[2] found that the pre-symptomatic stage of SARS-CoV-2 is very contagious (53 cases), and this stage of transmission may account for more than 50% of the disease(16 cases). In addition, studies have shown that the viral load of asymptomatic carriers is equivalent to that of symptomatic patients,and the highest viral excretion before the onset of symptoms suggests that there is higher infectivity before the onset of symptoms[13].

4. The diagnosis method of asymptomatic and presymptomatic infections with COVID-19

For symptomatic infections with coronavirus pneumonia, isolation and close tracking of their contacts are important control strategies,but the presence of pre-symptomatic or asymptomatic infections makes screening difficult. At present, patients diagnosed with coronavirus pneumonia need to undergo SARS-CoV-2 nucleic acid testing. Nasopharyngeal swab nucleic acid testing is currently the most widely used primary diagnostic method for screening infected persons. A document in the Journal of the American Medical Association shows that the positive rate of throat swab nucleic acid is only 62%. For the detection of viral nucleic acid in respiratory samples, differences in the location of the throat swab and its storage method have a certain degree of influence on the results of nucleic acid detection, but related factors are rarely reported. Studies by Schuit et al. [14,15] believe that in the early stage of viral infection,because the number of viruses on nasopharyngeal swabs is small,false negative results may occur. The epidemic results also show that the RT-PCR method is not completely suitable to diagnosis patients in the incubation period.

4.1 Saliva detection

Research by He et al.[16]showed that a large part of the transmission of infections occurs before symptoms appear, which highlights the importance of early detection of infected persons. In a retrospective study of saliva samples, it was found that the sensitivity of saliva detection (83%) is similar to that of nasopharyngeal swabs (84%)[17]. Zhang et al.[18]found that although nasopharyngeal swab nucleic acid has higher sensitivity than saliva detection, it requires well-trained personnel to collect sample of individuals, and it did not reduce recognition frequency of testing required for the presymptomatic stage of the cases carried the virus by at least 33%,50%, and 67%. And coupled with contact tracing and infection prevention, so the 5-day routine saliva testing time provides another screening method. Studies have found that self-collected saliva testing is effective for the detection of coronavirus pneumonia in the early stages of the disease. Saliva maintains high sensitivity and is easier to collect, which minimizes the exposure of medical staff and the need for personal protective equipment. It has become a viable option for detection in the population[19]. Dos et al. [20] used reverse transcription loop-mediated isothermal amplification (RT-LAMP)to detect SARS-CoV-2 in the ribonucleic acid (RNA) of saliva. The sensitivity and specificity of the virus detected in symptom carriers were 80% and 100%, respectively, and the overall accuracy was 99.59%.

4.2 Application of Artificial Intelligence

In addition, the traditional detection and diagnosis methods of infection with SARS-CoV-2 have certain limitations for the management of healthy people. Studies such as D'Haese[21]used the AI model in combination with non-invasive and ready-made technology. This technology can predict the probability of symptoms consistent with virus infection three days before the onset of symptoms, with an accuracy rate of 82%. The model can detect 79%of individuals who will have symptoms and can correctly predict individuals who will not have symptoms in the next three days (97%,negative predictive value).It uses wearable devices and applications to predict viral diseases. The study of symptom occurrence provides a novel digital decision-making tool for public health safety by potentially discovering infected persons. It is worth noting that Mishra et al.[22] in a retrospective study of smart watch data showed that 63% of coronavirus pneumonia cases can pass through a twolevel early warning system based on the extreme acceleration of the resting heart rate relative to the individual's baseline before the onset of symptoms. The wearable devices of real-time detection,tracking and monitoring health can be used for large-scale, real-time detection of respiratory infections before symptoms appear. This continuous monitoring method is expected to be of great help to the early detection of infectious diseases. Wearable devices are an easyto-use, low-cost method that highlights the advantages of combining artificial intelligence and medical care.

4.3 Clinical features

The study by Yu et al.[23]revealed a special type of immunity in patients with pre-symptoms, which is mainly manifested by excessive activation and differentiation of monocytes, possibly due to lymphocyte depletion and immunosuppression. In addition, Yang et al.[24]found that the level of lymphocytes in pre-symptomatic patients was lower than that of asymptomatic patients, suggesting that lymphocyte levels can help to distinguish. The level of white blood cells and neutrophils in pre-symptomatic patients is similar to those of asymptomatic patients. But lower than the level detected at the beginning of symptoms, which indicates that lymphocytes can predict clinical symptoms after SARS-CoV-2 infection, and the levels of white blood cells and neutrophils are more likely to predict disease progression. Shi et al.[25]found that the initial diagnosis model established with 10 laboratory indicators can accurately classify asymptomatic and pre-symptomatic patients. The first 5 indicators in this model are BNP, IgG, IgM, PCT, A/G, which can accurately predict the severity of the disease (AUC=0.75). A hierarchical calculator is provided on the Internet for clinicians to evaluate the progress of disease for patients before symptoms occur.This model can help improve the clinical management of patients with COVID-19 at different disease stages and help predict the progress of the disease. Secondly, the findings of Wadhwa et al.[26]emphasized the practicality of cohort-based testing methods, which can effectively identify pre-symptomatic and asymptomatic cases compared with individual symptom screening. In addition, for highrisk groups with negative SARS-CoV-2 nucleic acid test, chest CT examination is of great significance, which is helpful for the diagnosis of the disease. A literature on 58 cases of asymptomatic infection showed that 94.8% of the infected patients showed characteristic ground-glass density shadow on chest CT. However,some studies have shown that some asymptomatic infections do not have the above-mentioned imaging changes, and the lungs may not have been damaged by the virus[27]. Relevant literature reports that patients with COVID-19 have "ground glass density shadowpaving stone sign-consolidation shadow" on chest CT as the disease progresses. Chest CT plays an important role in the early diagnosis of SARS-CoV-2 infected patients and helps to identify those infected before symptoms show up [28].

5. Advice for asymptomatic and pre-symptomatic infections with COVID-19

During COVID-19 pandemic, the screening of asymptomatic or pre-symptomatic infections is an important challenge. Firstly, it is the definitions of fever and symptoms are different. The coordinated development and standardization of clinical standards between countries and even different states is necessary to reduce the confusion between asymptomatic and pre-symptomatic patients[13]in order to take more reasonable prevention and treatment measures.With regard to controlling the potential transmission of them, this article makes the following recommendations:

The necessity of non-pharmaceutical interventions, such as patient isolation, contact tracing and quarantine, physical distance,sanitation, and ventilation are the main means to control the transmission of SARS-CoV-2. The study by Bushman et al.[29]found that non-pharmaceutical interventions are being introduced and the transmission of symptomatic patients was reduced by 82%,while the pre-symptomatic transmission was reduced by only 16%.The implementation of non-pharmacological interventions not only decreased the transmission rate of SARS-CoV-2 rapidly, but also the time of transmission also changed significantly. Intervention measures, especially case isolation, are highly effective in limiting transmission in the later stages of infection. A lot of relevant literature evidence shows that social isolation plays a key role in controlling the pre-symptomatic transmission of SARS-CoV-2.Contacts should be traced in detail, extended to the pre-symptomatic stage, and prospective studies should be conducted to compare asymptomatic infections. With the characteristics of the secondary morbidity of the infected person before the symptom, to determine the duration of the infection.Zhang et al.[30]found that most secondary infections occur in family clusters and people over 60 seem to be more susceptible to be infected. Therefore, it is recommended that asymptomatic suspected cases be cautiously implemented at home and meanwhile strengthen protection of the susceptible population. Protection is also very important and appropriate activities can enhance immunity. In addition, the development of vaccines should also take into account the hidden transmission of infected persons during the incubation period, so as to expand the coverage of vaccines. Therefore, curbing the spread of the virus no longer only relies on symptoms to identify all infected persons. The most important thing is to control and identify infected persons from the source. Although rapid testing and the development of effective treatments and vaccines can help identify these potential spreaders, we need to reduce the risk of transmission to obtain more time for these ways .

6. Summary

The asymptomatic and pre-symptomatic infected persons of the COVID-19 have no obvious symptoms and signs, but they are spreading concealedly and are not easy to be detected, so it is brings great difficulties to screen infections during the epidemic. Therefore, a more comprehensive understanding of the natural history of asymptomatic and pre-symptomatic patients is important. Meanwhile screening and prevention strategies, further deepening the understanding of the characteristics of SARS-CoV-2 transmission and identifying asymptomatic patients in the incubation period, which will help relevant departments to formulate relevant policies and produce the vaccines to prevent the transmission of the virus from infected persons during the incubation period. It is also to control the epidemic in a timely manner. In addition, future prevention strategies should pay more attention to the combination of medical treatment and artificial intelligence. Wearable devices and software can be used to detect our own health and help control the development of the epidemic.

Conflict of interest

All authors have no conflict of interest.

Author's contribution

Hui-juan Shao is the main author of the review, finishing the collation and analysis of the literature and the writing of the paper;Xue-feng Ma and Jun Huang participate in the analysis of the literature; Xiao-hui Yu and Jiu-cong Zhang guide and revise the paper. All authors have read and agreed to the final text.