## Multiscale Computational Models for Respiratory Aerosol Dynamics with Medical Applications

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# Analysis and Numerical Simulations of a Stochastic SEIQR Epidemic System with Quarantine-Adjusted Incidence and Imperfect Vaccination

**Academic Editor:**Xiaole Chen

#### Abstract

This paper considers a high-dimensional stochastic SEIQR (susceptible-exposed-infected-quarantined-recovered) epidemic model with quarantine-adjusted incidence and the imperfect vaccination. The main aim of this study is to investigate stochastic effects on the SEIQR epidemic model and obtain its thresholds. We first obtain the sufficient condition for extinction of the disease of the stochastic system. Then, by using the theory of Hasminskii and the Lyapunov analysis methods, we show there is a unique stationary distribution of the stochastic system and it has an ergodic property, which means the infectious disease is prevalent. This implies that the stochastic disturbance is conducive to epidemic diseases control. At last, computer numerical simulations are carried out to illustrate our theoretical results.

#### 1. Introduction

Mathematical models for differential equations have been widely applied in various fields [1â€“7]. Specifically, they have had a realistic significance to analyze the dynamical behaviors in the field of mathematical biology [8â€“17], which obtained some novel results.

In fact, the main meaning of the research of infectious disease dynamics is to make people more comprehensively and deeply understand the epidemic regularity of infectious disease; then more effective control strategies are adopted to provide better theoretical support for the prevention and control of epidemics. To this end, many mathematical biology workers considered more realistic factors in the course of the study, such as population size change, migration, cross infection, and other practical factors. In the course of epidemics and outbreaks of infectious diseases, people always take various measures to control the epidemic in order to minimize the harm of epidemic diseases. Quarantine is one of the important means to prevent and control epidemic diseases; it has been used to control contagious diseases with some success. Specifically, during the severe acute respiratory syndrome (SARS) outbreak in 2002, remarkable results were also achieved. Among them, mathematical models have been used to investigate their impact on the dynamics of infectious diseases under quarantine [18â€“22], which attracts deep research interest of many mathematicians and biologists. Recently, Hethcote et al. [21] considered an SIQR (susceptible-infected-quarantined-recovered) model with quarantine-adjusted incidence. The system can be expressed as follows:where the total population size is given by , is the inflow rate corresponding to birth and immigration, and is the outflow rate corresponding to natural death and emigration. Since the quarantine process, using the standard incidence , the contact rate with the quarantined fraction does not occur. Hence the standard incidence is replaced by (it is called quarantine-adjusted incidence); here is the transmission coefficient between susceptible individuals and infected individuals. is the quarantine rate of infected individuals, is the recovery rate of quarantined individuals, and and stand for the rate of disease-related death of infected and quarantined individuals, respectively. is the recovery rate of infected individuals. Furthermore, all the parameters are positive and the region is a positively invariant set of system (1). In the region , they established the basic reproduction number , which determines disease extinction or permanence, whereMeanwhile, they analyzed the global dynamics of system (1) and derived the equilibria (including the disease-free equilibrium and the endemic equilibrium) and their global stability. In addition, the parameter conditions for the existence of a Hopf bifurcation are obtained.

In the real world, with the development of modern medicine, vaccination has become an important strategy for disease prevention and control in addition to quarantine, and numerous scholars have investigated the effect of vaccination on disease [23â€“30]. For another, many infectious diseases incubate inside the hosts for a period of time before becoming infectious, so it is very meaningful to consider the effect of the incubation period. Motivated by the aforementioned work, this paper considers an SEIQR (susceptible-exposed-infected-quarantined-recovered) epidemic model with imperfect vaccination, which is described by the following system:where the total population size is given by , is the vaccine coverage rate, is the vaccine efficacy, and is the rate at which the exposed individuals become infected individuals. Other parameters are the same as in system (1). Now we assume that all the parameters are positive constants here except that , are nonnegative constants. Clearly, the region is a positively invariant set of system (3). For system (3), the basic reproduction number isand it has the following properties:(1)When holds, system (3) has a unique disease-free equilibrium which is globally asymptotically stable in the region . That means the epidemic diseases will die out and the total individuals will become the susceptible and recovered individuals.(2)When holds, system (3) has a unique globally asymptotically stable positive equilibrium in the region , which means the epidemic diseases will persist.

In the natural world, deterministic model is not enough to describe the species activities. Sometimes, the species activities may be disturbed by uncertain environmental noises. Consequently, some parameters should be stochastic [31â€“40]. There is no denying that this phenomenon is ubiquitous in the ecosystem. Therefore numerous scholars have introduced the effect of stochastic perturbation on diseases [41â€“50]. To the best of our knowledge, the research on global dynamics of the stochastic SEIQR epidemic model with imperfect vaccination is not too much yet. In this paper, to make system (3) more reasonable and realistic, we assume the environmental noise is directly proportional to , , , , and . Then corresponding to system (3), a stochastic version can be reached bywhere is the mutually independent standard Wiener process with a.s. is a continuous and bounded function for any and are the intensities of Wiener processes.

In this paper, we are mainly concerned with two interesting problems as follows:(P1)Under what parameter conditions, will the disease die out?(P2)Under what conditions, will system (5) have a unique ergodic stationary distribution?

Throughout this paper, let be a complete probability space with a filtration satisfying the usual conditions (i.e., it is increasing and right continuous while contains all -null sets). Further is defined on the complete probability space.

For simplicity and convenience, we introduce the following notations:(1), .(2)For an integrable function , .(3), .

#### 2. Global Positive Solution

To investigate the dynamical behaviors of a population system, we first concern the global existence and positivity of the solutions of system (5).

Lemma 1. *For any given initial value , system (5) has a unique positive local solution for , where is the explosion time [51].*

Theorem 2. *For any given initial value , system (5) has a unique positive solution on a.s.*

*Proof. *The following proof is divided into two parts.*Part I*. By Lemma 1, it is easy to see that system (5) has a unique positive local solution for any given initial value .*Part II*. Now we prove that the positive solution is global, that is, a.s. Let be sufficiently large such that , , , , and all lie in . For each integer , let us define the stopping time as follows:where we define ( represents the empty set). Evidently, is strictly increasing when . Let ; thus a.s. So we just need to show that a.s. If is untrue, then there exist two constants and such that . Thus there exists such thatDefine a -function byApplying ItÃ´â€™s formula and system (5), we havewhereand here is a positive constant. HenceIntegrating both sides of (11) from 0 to and then taking the expectation, we haveSet and by (7) we can get that . Notice that, for every , there exists , , , , or which equals either or . ThusBy virtue of (12) and (13), one has and here is the indicator function of . Let , which implies is a contradiction. Obviously, we get that . This completes the proof of Theorem 2.

#### 3. Extinction

In this section, we mainly explore the parameter conditions for extinction of the disease in system (5). Before proving the main results, we first give a useful lemma as follows.

Lemma 3. *For any given initial value , the solution of the system (5) has the following properties:Furthermore, when holds, then*

*Proof. *The proof of Lemma 3 is similar to [25, 41]; thus we omit it here.

Theorem 4. *Let . For any given initial value , ifholds, thenMoreover,*

*Proof. *Define a differentiable function byFrom ItÃ´â€™s formula and system (5), we haveIntegrating from 0 to and dividing by on both sides of (22), we have Making use of Lemma 3, we havewhich shows thatFrom the fourth equation of system (5), it is easy to get thatMoreover, integrating from 0 to and dividing by on both sides of the first equation of system (5) yieldand considering (25), (26), and Lemma 3, it then follows thatSimilarly, we also getThe proof of Theorem 4 is complete.

#### 4. Stationary Distribution and Ergodicity

Ergodicity is a significant property in a population system. Recently, it attracts deep research interest of numerous scholars [52, 53]. In this section, based on the theory of Hasminskii et al. [54] and the Lyapunov analysis methods, we study the conditions of the existence of an ergodic stationary distribution.

Assume as a time-homogeneous Markov process in , which is described by the stochastic differential equationand here stands for a -dimensional Euclidean space. The diffusion matrix takes the following form:

*Assumption 5. *Assume that there is a bounded domain with regular boundary such that , satisfying the following properties:(i)In the domain and some neighborhood thereof, the smallest eigenvalue of the diffusion matrix is bounded away from zero.(ii)If , the mean time at which a path issuing from reaches the set is finite, and for every compact subset .

Lemma 6 (see [54]). *When Assumption 5 holds, then the Markov process has a stationary distribution . Furthermore, when is a function integrable with respect to the measure , then for all .*

*Remark 7. *To demonstrate Assumption 5(i) [55], it suffices to demonstrate that is uniformly elliptical in any bounded domain ; herenamely, there exists a positive number such thatTo verify Assumption 5(ii) [56], it suffices to demonstrate that there exist some neighborhood and a nonnegative -function such that , .

Using Lemma 6, we can get the following main results.

Theorem 8. *For any given initial value . Ifholds, then system (5) has a unique stationary distribution and it has ergodic property.*

*Proof. *Define a -function byand here and are positive constants satisfying the following conditions: and we take large enough such that hereObviously, and here . Since is a continuous function, then there exists a unique point in which is the minimum point of . Therefore let us construct a positive-definite -function : by From ItÃ´â€™s formula, we haveSimilarly,and hereWe also haveTherefore,where

Next let us consider the following compact subset : and here is a sufficiently small constant satisfying the following conditions:Thenwith