Asymptomatic Transmission Research Articles

Autoantibodies Directed Against Interferon Alpha, Nuclear Antigens, Cardiolipin, and Beta 2 Glycoprotein 1 Are Not Induced By SARS-CoV-2 or Associated with Long COVID

Autoantibodies (AAbs) directed against interferon alpha (aIFNα), nuclear antigens (ANAs), cardiolipin (aCL), and beta 2 glycoprotein 1 (aβ2GP1), have been demonstrated to significantly correlate with the severity of acute COVID-19. However, whether SARS-CoV-2 infection induces these AAbs and whether they are associated with long COVID remains unclear. The potential induction of aIFNα, ANAs, aCL, and aβ2GP1 by SARS-CoV-2 was assessed by measuring these AAbs in 224 pre- and post-infection paired serum samples from the Johns Hopkins Hospital Emergency Department (JHHED). The relationship between these AAbs and long COVID was assessed using 60 serum samples from participants in the Outpatient SARS-CoV-2 Mild and Asymptomatic Infection Response and Transmission (OutSMART) study. We found no evidence that these AAbs were induced in the JHHED cohort and no significant difference in their prevalence between patients with (n=30) and without (n=30) long COVID in the OutSMART cohort. These findings do not support the hypotheses that SARS-CoV-2 induces these AAbs or that they are related to long COVID.

Update on Vaccine-Derived Poliovirus Outbreaks - Worldwide, January 2023-June 2024.

Circulating vaccine-derived polioviruses (cVDPVs) can emerge and lead to outbreaks of paralytic polio as well as asymptomatic transmission in communities with a high percentage of undervaccinated children. Using data from the World Health Organization Polio Information System and Global Polio Laboratory Network, this report describes global polio outbreaks due to cVDPVs during January 2023-June 2024 and updates previous reports. During the reporting period, 74 cVDPV outbreaks were detected in 39 countries or areas (countries), predominantly in Africa. Among these 74 cVDPV outbreaks, 47 (64%) were new outbreaks, detected in 30 (77%) of the 39 countries. Three countries reported cVDPV type 1 (cVDPV1) outbreaks and 38 countries reported cVDPV type 2 (cVDPV2) outbreaks; two of these countries reported cocirculating cVDPV1 and cVDPV2. In the 38 countries with cVDPV2 transmission, 70 distinct outbreaks were reported. In 15 countries, cVDPV transmission has lasted >1 year into 2024. In Nigeria and Somalia, both countries with security-compromised areas, persistent cVDPV2 transmission has spread to neighboring countries. Delayed implementation of outbreak response campaigns and low-quality campaigns have resulted in further international spread. Countries can control cVDPV outbreaks with timely allocation of resources to implement prompt, high-quality responses after outbreak confirmation. Stopping all cVDPV transmission requires effectively increasing population immunity by overcoming barriers to reaching children.

Mapping the Mpox discourse: A network and sentiment analysis

Mpox, a zoonotic disease re-emerging from animals to humans, poses a risk of evolving into a global pandemic due to its high infectivity and potential asymptomatic transmission. This study maps the structure and configuration of mpox-related discussions on Twitter/X, identifies key influencers and top hashtags, and analyzes public sentiment. Data were collected using NodeXL Pro from May 7, 2022, to January 15, 2023, with the keyword “Monkeypox” and visualized using Gephi. Social network analysis ranked nodes by betweenness centrality scores to highlight key communicators, and the YifanHu layout algorithm visualized the network. Influential users, source topics, and hashtags were identified, and sentiment analysis was conducted using Azure Machine Learning tools. The analysis identified 11,397 mpox-related tweets. The network structure resembled a community with diverse participants. Influential users included health and science journalists, writers, academics, medical doctors, and public figures. News media and organizational websites were the top information sources, emphasizing the need for accessible scientific information. “Monkeypox” and “Mpox” were the most prevalent hashtags. Negative sentiments dominated the discussion. This analysis provides insights into network structure, key influencers, information sources, and public sentiment, aiding tailored health initiatives to address misinformation and advocate valid health information and emergency responses.

Modelling COVID-19 mutant dynamics: understanding the interplay between viral evolution and disease transmission dynamics

Understanding virus mutations is critical for shaping public health interventions. These mutations lead to complex multi-strain dynamics often under-represented in models. Aiming to understand the factors influencing variants’ fitness and evolution, we explore several scenarios of virus spreading to gain qualitative insight into the factors dictating which variants ultimately predominate at the population level. To this end, we propose a two-strain stochastic model that accounts for asymptomatic transmission, mutations and the possibility of disease import. We find that variants with milder symptoms are likely to spread faster than those with severe symptoms. This is because severe variants can prompt affected individuals to seek medical help earlier, potentially leading to quicker identification and isolation of cases. However, milder or asymptomatic cases may spread more widely, making it harder to control the spread. Therefore, increased transmissibility of milder variants can still result in higher hospitalizations and fatalities due to widespread infection. The proposed model highlights the interplay between viral evolution and transmission dynamics. Offering a nuanced view of factors influencing variant spread, the model provides a foundation for further investigation into mitigating strategies and public health interventions.

Otitis media with effusion in patients with COVID-19: A single-center study in China

BackgroundThe swift global spread of coronavirus disease 2019 (COVID-19), a respiratory ailment primarily marked by pulmonary symptoms, has been linked to the involvement of various organs, including the intestines, kidneys, throat, and ears. Otitis media with effusion (OME), often succeeding an upper respiratory tract infection, mirrors its incidence. As a respiratory infectious disease, it prompts the query of whether the COVID-19 pandemic has spurred an uptick in OME and whether the COVID-19 virus persists in middle ear effusion (MEE) for an extended period. MethodsTo gauge the incidence of OME in the population during the COVID-19 pandemic, a tailored questionnaire was disseminated and subsequently analyzed. Assessing the rise in OME incidence during the pandemic, we compared the proportion of OME cases in the otology outpatient department between pandemic and non-pandemic periods. Statistical analysis involved a t-test. Simultaneously, MEE was collected from patients with COVID-19-associated OME during the pandemic to ascertain the presence of SARS-CoV-2 in MEE via polymerase chain reaction. ResultsBased on the questionnaire data, the estimated OME incidence in the population is approximately 31.4 %. In contrast to the non-pandemic period, the percentage variation in the OME outpatient proportion was 71.4 % (P < 0.05). Among the 61 MEE samples, 13 polymerase chain reaction results were positive, constituting 21.31 %. Nasopharyngeal swabs yielded negative results. Notably, only one patient experienced OME recurrence after 1 month of auripuncture. ConclusionsCOVID-19 can trigger an escalation in OME cases. Even when nasopharyngeal swabs show negative results, SARS-CoV-2 can endure in MEE for an extended duration, suggesting the potential for asymptomatic COVID-19 transmission and recurrence within the population.

Evaluating the Usefulness of Population-Wide COVID-19 Testing in the Omicron Era: Insights from a German Model.

The Omicron variant of SARS-CoV-2, which has become dominant worldwide since late 2021, presents a unique challenge due to its high rate of asymptomatic transmission. This study evaluates the efficacy and value of population-wide testing, including self-testing, in the context of COVID-19, particularly under the Omicron variant, using data from Germany. A decision-analytical model and secondary data was used for assessing the impact of systematic screening and testing for COVID-19. Various scenarios were taken into consideration including seasonal patterns of COVID-19 transmission and the potential for annual waves. The model assessed the clinical benefits of testing against the backdrop of vaccine effectiveness, transmission rates, and the potential to prevent severe clinical events, including death, ICU admission, and long COVID syndrome. The study found that the value of mass testing and self-testing for private use was highly contingent on the transmission rate and the scenario of COVID-19 waves (seasonal vs. continuous). For winter waves, a very high incidence rate was required to justify testing, while for continuous waves, testing could be valuable for those in contact with individuals in their last decade of life. The analysis highlighted the limitations of mass testing when community transmission rates were low and the potential value of testing in high-risk contacts or amidst new outbreaks. The findings suggest that the resumption of testing during winter waves is unlikely to provide significant clinical benefits given the current understanding of Omicron's transmission and immunity waning. This study underscores the need for a nuanced approach to COVID-19 testing policies, considering both the epidemiological context and the practical implications of testing strategies.

Pertussis paradox: why are cases increasing despite vaccination efforts?

Introduction Pertussis, a highly transmissible respiratory infection mainly caused by Bordetella pertussis, remains one of the most widespread public health concerns despite the establishment of global vaccination initiatives. The disease manifests as paroxysmal coughing followed by a characteristic high-pitched “whooping” sound, and post-tussive vomiting. Severe complications can include pneumonia, encephalopathy, and even mortality, particularly in infants. Aim of the study This study aims to provide a comprehensive analysis of pertussis, encompassing its clinical presentation, complications, diagnostic methods, epidemiological aspects, and vaccination strategies, with a special focus on the development and potential impact of a novel nasal pertussis vaccine. Brief description of the state of knowledge Diagnosis of pertussis often relies on recognizing typical symptoms supported by laboratory confirmation through culture methods, polymerase chain reaction testing, or enzyme–linked immunosorbent assays. This disease leads to considerable morbidity and mortality worldwide, accompanied by profound medical, social and economic challenges. Summary To address the limitations of existing vaccines, a novel nasal pertussis vaccine has been proposed. This vaccine aims to elicit robust mucosal immune responses, thereby providing enhanced protection against infection and preventing the asymptomatic transmission of Bordetella pertussis. The study offers a detailed examination of pertussis and comprehensive statistics to support the analysis, providing valuable insights into the epidemiology, highlighting the need for improved vaccination strategies to mitigate the disease's impact.

Understanding Nosocomial Amplification by Identifying Important Parameters in a Community-Hospital Model

Background: The phenomena of emerging infectious diseases accelerating once they reach healthcare facilities have been well documented. Outbreaks of MERS-CoV, SARS-CoV, and COVID-19 have led to in-hospital transmission where the initial patient infects healthcare workers, patients, visitors, etc., with infection control policies unable to curtail the spread early on. We refer to this phenomenon as nosocomial amplification. Nosocomial amplification causes an undue burden on a hospital that’s already strained from the pandemic. We aimed to understand which hospital-level parameters impact the community most and vice versa. Methods: We adapted an SEIR compartmental model to have two interconnected units, a community unit and a hospital special care unit, to determine the number of COVID-19 acquisitions in each of them over a hypothetical year. The model was stochastically simulated using Gillespie’s Direct Method for 1000 iterations. A parameter sensitivity analysis assessed the effects each parameter had on the model. The original values of all parameters were allowed to vary +/- 50%. The number of simulation acquisitions was normalized as a percent change from the original model’s mean acquisition. Results: Our analysis found that parameters impacting the community had a disproportionate impact on COVID-19 acquisitions in the hospital as compared to the special care unit, as did the parameters governing the level of asymptomatic transmission. Transmission between healthcare workers facilitated within-hospital transmission even when strict patient-based cohorting and testing were in place. Extensive community-level transmission was also found to readily overwhelm hospital-level infection control at realistic levels of effectiveness and compliance. Conclusion: These findings illustrate that hospitals and the community are tightly linked systems. Hospitals may reintroduce infection into the community that might have contained or mitigated ongoing outbreaks or introduce the disease into a disease-free population; community transmission puts tremendous pressure on infection control. In the future, we can model policies to curb an existing COVID-19 outbreak or subsequent outbreaks to avoid or minimize nosocomial amplification, thus improving the disproportionate burdens on the healthcare system.

A new co-infection model for HBV and HIV with vaccination and asymptomatic transmission using actual data from Taiwan

The co-infection of Human Immunodeficiency Virus (HIV) and Hepatitis B virus (HBV) poses a major threat to public health due to their combined negative impacts on health and increased risk of complications. A novel fractional mathematical model of the dynamics of co-infection between HBV and HIV for Taiwan is presented in this paper. Detailed analyses are conducted on the possible impact of HBV vaccination on the dynamics of HBV and HIV co-infection. The next-generation matrix technique is used to calculate the fundamental reproduction number R 0 = max{R 1, R 2}, where R 1 and R 2 are the reproduction numbers for HBV and HIV, respectively. The disease-free and endemic equilibria of the co-infection model are calculated. An extensive investigation is carried out to determine the local and global stability of the disease-free equilibrium point through Rough Hurtwiz criteria and the construction of Lyapunov function, respectively. We demonstrate that when R 1 < 1 < R 2, HBV infection is eradicated, but HIV remains prevalent. If R 2 < 1 < R 1, the opposite outcome occurs. The real data from 2000-2023 for Taiwan is used to fit the model. The fitting results show how effectively our model handles the data. In addition, numerical simulations are run for different scenarios to observe how the vaccine and fractional parameters changed the model state variables, as well as how the solutions behaved and how quickly they reached the model’s equilibrium points. According to the model’s numerical analysis, greater vaccination efforts against HBV have a positive effect on the propagation of co-infection.

Study on symptomatic and asymptomatic transmissions of COVID-19 including flip bifurcation

The aim of this study is to analyze and investigate the COVID-19 transmission with effect of symptomatic and asymptomatic in the community. Mathematical model is converted into fractional order with the help of fractal fractional definition. The proposed fractional order system is investigated qualitatively as well as quantitatively to identify its stable position. Local stability of the COVID-19 system is verified and test the system is tested with flip bifurcation. Also the system is investigated for global stability using Lyapunov first and second derivative functions to see its rate of spread after recovery. The existence, boundedness and positivity of the COVID-19 are checked which are the key properties for such of type of epidemic problem to identify reliable findings. Effect of global derivative is demonstrated to verify its rate of effects according to their sub compartments to identify in which rate the symptomatic and asymptomatic transmission occurs. Solutions for fractional order system are derived with the help of advanced tool fractal fractional operator with generalized mittag-leffler kernel for different fractional values. Simulations are carried out to see symptomatic as well as asymptomatic effects of COVID-19 in the worldwide using MATLAB Coding. They show the actual behavior of COVID-19 especially for asymptomatic measures which will be helpful in early detection, also which will be helpful to understand the outbreak of COVID-19 as well as for future prediction and better control strategies.

Estimating Asymptomatic and Symptomatic Transmission of the COVID-19 First Few Cases in Selenge Province, Mongolia.

Following the first locally transmitted case in Sukhbaatar soum, Selenge Province, we aimed to investigate the ultimate scale of the epidemic in the scenario of uninterrupted transmission. This was a prospective case study following the locally modified WHO FFX cases generic protocol. A rapid response team collected data from November 14 to 29, 2020. We created a stochastic process to draw many transmission chains from this greater distribution to better understand and make inferences regarding the outbreak under investigation. The majority of the cases involved household transmissions (35, 52.2%), work transmissions (20, 29.9%), index (5, 7.5%), same apartment transmissions (2, 3.0%), school transmissions (2, 3.0%), and random contacts between individuals transmissions (1, 1.5%). The posterior means of the basic reproduction number of both the asymptomatic cases and the presymptomatic cases (1.35 [95% CrI 0.88-1.86] and 1.29 [95% CrI 0.67-2.10], respectively) were lower than that of the symptomatic cases (2.00 [95% Crl 1.38-2.76]). Our study highlights the heterogeneity of COVID-19 transmission across different symptom statuses and underscores the importance of early identification and isolation of symptomatic cases in disease control. Our approach, which combines detailed contact tracing data with advanced statistical methods, can be applied to other infectious diseases, facilitating a more nuanced understanding of disease transmission dynamics.

Secondary attack rates and determinants of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) household transmission in Pakistan: A case-ascertained prospective, longitudinal study

BackgroundHouseholds are considered ideal settings for studying the transmission dynamics of an infectious disease. MethodsA prospective study was conducted, based on the World Health Organization FFX protocol from October 2020 to January,2021. Household contacts of laboratory-confirmed index cases were followed up for their symptomatic history, nasal swabs for RT-PCR,and blood samples for anti-SARS CoV-2 antibodies were collected at enrollment and days 7, 14 and 28. We estimated secondary attack rate (SAR), effective household case cluster size and determinants of secondary infection among susceptible household contacts using multivariable logistic regression. ResultsWe enrolled 77 index cases and their 543 contacts. Out of these, 252 contacts were susceptible at the time of enrollment. There were 77 household clusters, out of which, transmission took place in 20 (25.9%) giving rise to 34 cases. The acquired secondary attack rate (SAR) was 14.0% (95% CI 9.0–18.0). The effective household case cluster size was 0.46 (95%CI 0.33,0.56). Reported symptoms of nausea and vomiting (aOR, 7.9; 95% CI, 1.4–45.5) and fatigue (aOR, 9.3; 95% CI, 3.8–22.7) were associated with SARS-CoV-2 transmission. ConclusionsWe observed a low SARS-CoV-2 secondary attack rate in the backdrop of high seroprevalence and asymptomatic transmission among households in Karachi, Pakistan.

GAME-THEORETICAL MODEL OF COVID-19 VACCINATION IN THE ENDEMIC EQUILIBRIUM

An outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), epi-centred in Hubei Province of the People’s Republic of China, quickly spread worldwide and caused COVID-19 pandemic. It infected hundreds of millions of people and caused millions of deaths. In this paper, we develop a compartmental ODE model of COVID-19 transmission. We consider a possibility of breakthrough infections after the vaccination and account for both symptomatic and asymptomatic infections and transmissions. We also incorporate game theory to study the optimal vaccination decisions from the individuals’ perspective. We show that vaccination alone is unlikely to eliminate COVID-19. To achieve herd immunity, the individuals would have to receive a dose of a vaccine more frequently than once every 3 months. It is therefore crucial to adhere to various guidelines, such as quarantine, isolate and wear a mask if tested positive for COVID-19.

Looking under the lamp-post: quantifying the performance of contact tracing in the United States during the SARS-CoV-2 pandemic

Contact tracing forms a crucial part of the public-health toolbox in mitigating and understanding emergent pathogens and nascent disease outbreaks. Contact tracing in the United States was conducted during the pre-Omicron phase of the ongoing COVID-19 pandemic. This tracing relied on voluntary reporting and responses, often using rapid antigen tests due to lack of accessibility to PCR tests. These limitations, combined with SARS-CoV-2’s propensity for asymptomatic transmission, raise the question “how reliable was contact tracing for COVID-19 in the United States”? We answered this question using a Markov model to examine the efficiency with which transmission could be detected based on the design and response rates of contact tracing studies in the United States. Our results suggest that contact tracing protocols in the U.S. are unlikely to have identified more than 1.65% (95% uncertainty interval: 1.62-1.68%) of transmission events with PCR testing and 1.00% (95% uncertainty interval 0.98-1.02%) with rapid antigen testing. When considering a more robust contact tracing scenario, based on compliance rates in East Asia with PCR testing, this increases to 62.7% (95% uncertainty interval: 62.6-62.8%). We did not assume presence of asymptomatic transmission or superspreading, making our estimates upper bounds on the actual percentages traced. These findings highlight the limitations in interpretability for studies of SARS-CoV-2 disease spread based on U.S. contact tracing and underscore the vulnerability of the population to future disease outbreaks, for SARS-CoV-2 and other pathogens.

An Infectious Disease Model with Asymptomatic Transmission and Waning Immunity

An Infectious Disease Model with Asymptomatic Transmission and Waning Immunity

Public Policy and Lessons from Immunity in a COVID World

To counter perceived viral threats, government organizations have the task of steering public opinion towards behavior that shapes the immunity of the population. This could take the form of vaccine campaigns and/or ‘non-pharmaceutical interventions’. Vaccines are accepted as the most effective means in thwarting the severity of an infection both individually and throughout a population. The success or failure of campaigns however hinges on the messaging of the objectives, public trust, and above all the science that substantiates the campaign. Some consider the recent Coronavirus Disease 2019 Pandemic as one insufficient in clear and consistent messages. At the level of communication, confusion stemmed from the complexities of 1) asymptomatic and presymptomatic transmission, 2) the merits of natural and acquired immunity, 3) The relevance of herd or community immunity, as well as 4) correct information regarding the benefits and risks of necessary vaccination. Here, we review the basis and inconsistencies in the therapeutic and infection messages relevant to immunity concepts that lent to public confusion associated with this and perhaps future pandemics.

Comparative analysis of asymptomatic infection prevalence in Beta, Delta, and Omicron surges of COVID-19

BackgroundThe COVID-19 pandemic, caused by the SARS-CoV-2 virus, has had a devastating impact on the global population, with an estimated 650 million people infected and more than 6.6 million lives lost. Asymptomatic individuals have been shown to play a significant role in the transmission of the virus. Therefore, this study aims to investigate and compare the prevalence of asymptomatic individuals across three waves associated with the Beta, Delta, and Omicron variants of the virus. MethodsThis retrospective study was conducted between December 2020 and March 2022. The study population consisted of passengers on international flights who were referred to the Gerash Clinical and Molecular Diagnosis Laboratory. Real-time PCR was employed for the diagnosis of SARS-CoV-2. ResultsOut of a total of 8592 foreign travelers referred to our laboratory, 139 (1.16 %) tested positive for SARS-CoV-2 infection and were asymptomatic. During the Beta surge, 35 (1.49 %) out of 2335 passengers tested positive for SARS-CoV-2. In the Delta surge, 31 (0.6 %) out of 5127 passengers tested positive. However, during the Omicron surge, a significantly higher number of passengers, specifically 73 (6.46 %) out of 1130, had a positive result for the SARS-CoV-2 test. ConclusionConsidering the significant role of asymptomatic transmission in the spread of COVID-19, it is imperative to reconsider health policies when dealing with future surges of the Omicron subvariants. Additionally, we strongly recommend that the World Health Organization prioritize the development and distribution of second-generation vaccines that target not only disease but also infection prevention.

"Backlash!"? A qualitative exploration of hospice palliative care staff's ongoing experiences of "living with covid".

"Living with covid" has meant that the SARS-CoV-2 virus has become a background concern for many in the United Kingdom. However, people with terminal conditions remain some of those at higher risk of Covid-19 affecting the quality-of-life left, as well as the amount of life. Little is known about how staff manage the ongoing risks and challenges-to themselves and those they seek to support-when providing palliative care in the context of an airborne transmissible virus. To explore the experiences of UK hospice staff of "living with covid" to identify how Covid-19 continues to affect their work and well-being. An interpretivist qualitative interview study. Reflexive thematic analysis of semi-structured online interviews with 12 staff recruited from three hospices in the West Midlands, UK. We explored how participants' accounts of "living with covid" included several ambivalences: Participants not only sought to assert the importance of not forgetting that time but also wished to "move-on." This included moving Covid-19 to the "background" through embedding systemic adaptions and lessons learnt, while also recognizing that they had to address issues relating to Covid-19 "case-by-case." Finally, participants' wish to move-on and a mostly reactive approach to mitigations meant that they were unable to meaningfully reconcile how asymptomatic transmission promotes patients' quality-of-life left. Recollections of the difficulties of the Covid-19 public health emergency were part of a "backlash" to any future consideration of mitigations for airborne transmissible viruses and helped justify a "living with (getting) covid" approach. However, this also created uncertainty of how best to support patients who are vulnerable to having the quality and amount of life left compromised by viral infection. The pandemic has brought renewed impetus to re-examine hospice palliative care's ideals and practices in the context of airborne transmissible viruses.

Severe acute respiratory syndrome coronavirus 2-reactive salivary antibody detection in South Carolina emergency healthcare workers, September 2019-March 2020.

On 19 January 2020, the first case of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection was identified in the United States, with the first cases in South Carolina confirmed on 06 March 2020. Due to initial limited testing capabilities and potential for asymptomatic transmission, it is possible that SARS-CoV-2 may have been present earlier than previously thought, while the immune status of at-risk populations was unknown. Saliva from 55 South Carolina emergency healthcare workers (EHCWs) was collected from September 2019 to March 2020, pre- and post-healthcare shifts, and stored frozen. To determine the presence of SARS-CoV-2-reactive antibodies, saliva-acquired post-shift was analysed by enzyme-linked immunosorbent assay (ELISA) with a repeat of positive or inconclusive results and follow-up testing of pre-shift samples. Two participants were positive for SARS-CoV-2 N/S1-reactive IgG, confirmed by follow-up testing, with S1 receptor binding domain (RBD)-specific IgG present in one individual. Positive samples were collected from medical students working in emergency medical services (EMSs) in October or November 2019. The presence of detectable anti-SARS-CoV-2 antibodies in 2019 suggests that immune responses to the virus existed in South Carolina, and the United States, in a small percentage of EHCWs prior to the earliest documented coronavirus disease 2019 (COVID-19) cases. These findings suggest the feasibility of saliva as a noninvasive tool for surveillance of emerging outbreaks, and EHCWs represent a high-risk population that should be the focus of infectious disease surveillance.

具有接种和无症状传播模型的研究

具有接种和无症状传播模型的研究