1. Introduction
The COVID-19 pandemic has been a colossal test of the strength of the healthcare system around the world, including in Kazakhstan. Unprecedented measures were taken in the Republic to support the industry, which significantly reduced the spread of infection, reduced morbidity, and mortality. The experience of the past months shows that understanding the dynamics by which the disease spreads and making the right decisions is vital in combating and containment of this disease. For more than a hundred years, mathematicians and later epidemiologists have put effort into making models that can predict statistical properties of epidemics. Besides predicting the future state of a pandemic and the number of infections involved, a model to policy makers to take the necessary and optimal decisions is required.
Classical models describe the dynamics of infection spread using systems of differential equations. A significant advantage of models based on the apparatus of differential equations is the possibility of their analytical study. However, SI, SIR, SEIR, SIS and SIRS models fail to effectively model spatial aspects of the spread of an epidemic, the individual contact process, and the effects of individual behaviors, among others [
1]. These shortcomings are partially devoid of modern computer simulation models of the spread of the disease. Cellular automata based models are an alternative to using deterministic differential equations, which use a two-dimensional cellular automaton to model location specific characteristics of the susceptible population together with stochastic parameters which captures the probabilistic nature of disease transmission [
2,
3]. However, the representation of individuals' movement and interactions over the space is no presented. This is an important factor to consider in highly contagious diseases and therefore this methodology gave way to a new approach.
Agent-based models (ABM) are similar to cellular automata-based models but leverage extra tracking of the effect of the social interactions of individual entities. The agent-based model describes a complex system as a collection of many autonomous agents that have their own goals, behavior, and interactions with other agents and the environment. As in the real world, global behavior and trends emerge from the behavior and interactions of individual agents [
4,
5].
Models of interaction between agents within a system play a vital role in dynamic models of infection transmission [
6]. In the past, ABM have been used for tasks such as assessing the spread of smallpox [
7], developing strategies for influenza vaccination [
8], limiting measles transmission through contact tracing and quarantine [
9], assessing the effectiveness of social distancing measures and antiviral prophylaxis against the H5N1 virus influenza A (avian influenza) [
10] and the development of evacuation strategies in case of infection by airborne droplets [
11].
One of the advantages of agent-based modeling is the ease of modeling various scenarios. For example, the scenario of the universal use of masks in combination with social distancing is simulated in the models of B. Braun et al. [
12] and Kai et al. [
13]. Based on the experience of applying this approach, the testing policy [
14], the strategy for reopening public places [
15], the effectiveness of treatment methods [
16], and the development of a vaccination strategy were evaluated [
12].
Some articles in the literature have used agent-based models to simulate the economic and epidemiologic impacts of COVID-19. For example, a study was conducted on the epidemiological and economic impact of the spread of COVID-19 in the United States. The results show that the trade-offs between economic loss, lives saved, and infections averted are non-linear with social distancing adherence and lockdown duration. The industries that have been hit the hardest are not labour-intensive industries such as the agricultural sector and the construction sector, but those in which jobs are highly valued such as professional services [
17].
Inoue and Todo [
18] calculated that a possible one-month lockdown in Tokyo would result in a total production loss of 5.3% of Japan's annual gross domestic product (GDP). Dignum et al. [
19] proposed a tool to analyze the health, social and economic impacts of a pandemic when the government implements a range of interventions such as closing schools, requiring employees to work from home, and providing subsidies to the public. Silva PCL et al. [
20] showed that if it is impossible to introduce a full lockdown, the best solution is the “use of face masks and 50% of social isolation” scenario. This scenario is optimal from the point of view of minimizing the number of deaths, saving business, government, and people revenues.
Zhang T et al. [
21] assessed the need for medical resources based on different scenarios involving COVID-19 spreads and interventions. The authors made recommendations on the main investments in health care and the allocation of resources. The extent and nature of changes in health care utilization during the COVID-19 pandemic were highlighted in a systematic review by Moynihan R et al. [
22].
Other authors have developed COVID-19 strategies that may be most beneficial in resource-limited settings [
23].
One of the goals of modeling is to predict the spread of COVID-19 over time. However, this task faced several problems that did not allow predicting morbidity with a high degree of reliability. Thus, in one of the reviews devoted to the analysis of various models, it is shown that out of 59 predictions in 38% of cases the predicted values were higher than the real ones and in 62% of cases they were lower than the observed values. No differences in accuracy were found between different categories of models, nor within each category [
24].
Among the possible reasons is the problem of the validity of the proposed models. In a literature study by Heath et al. [
25] it was shown that 65% of the agent-based models presented in the articles did not pass rigorous testing. In another review [
26], the author, based on the analysis of 126 agent-based models, showed that most articles (about 87%) do not specify how the presented model or the results were tested. Models are often difficult to test due to unavailability or incompleteness of real data.
Another reason is the high variability in the number of daily cases of COVID-19. Transmission of an infectious disease in a population is a complex phenomenon. His behavior is mainly influenced by interactions between people, and not just by the characteristics of everyone [
27]. Interactions can be viewed as social processes [
28], such as contacts between people, and as place effects [
29]. The complexity of interactions also includes effects over time [
30], where outcomes in the past, present, and future affect the decision context; for example, persons already found to be infected are isolated and no longer represent a source of infection. In addition, other factors influence the rate, such as the seasonality of pneumonia, mobility, transmission frequency, use of masks depending on the weather, social behavior, stress, public health measures, etc. During the pandemic, different preventive measures were taken in different countries and the level of compliance with these measures was different, so the dynamics of transmission of COVID-19 had its own local characteristics.
Therefore, determining the transmission rate is an important step in creating a model for the spread of the disease. Some authors rely on the contact probability matrix [
31], while others determine the physical proximity of agents using geolocation[
13]. Many COVID-19 models have estimated time-varying rates of transmission based on case counts or hospitalization data, or models have inferred changes in transmission rates using estimates of the instantaneous reproduction number of the pathogen [
32,
33,
34].
In this regard, a number of authors consider the stochastic models of the spread of COVID-19. Among them models, which describes a stochastic Markov process [
35], and stochastic calculus [
36,
37], which includes differential equations with stochastic component, such as the Brownian motion (white noise) process [
38,
39,
40]. Scientists developed a stochastic epidemiological model with random fluctuations of parameters [
41], with binomial distributions [
42]. White noise and jumps were introduced, which corresponded to the different disturbances that can occur [
43]. Various scenarios of preventive measures were considered in a stochastic ABM and the influence of the environment on the spread of COVID-19 was analyzed [
16,
44]. One of the advantages of stochastic model is that it takes into consideration various properties of COVID-19. Among them are the unpredictability of sudden outbreaks of flare disease, long periods of asymptomatic course, sudden declines followed by bursts, as well as unimodal and bimodal progression of daily cases of the disease [
45]1.a.i.1.45.
We have created a stochastic agent-based model of the spread of infection in a large city in Kazakhstan. Earlier, the works of Kazakh scientists on modeling COVID-19 were already published in the literature, however, as time has shown, the forecast data had a significant discrepancy with reality.
The main contributions and findings are listed below:
Using the OptQuest optimizer, included in AnyLogic software, to validate a stochastic ABM model for the spread of COVID-19 and use it for short- and long-term forecasting
Determination of age groups and public places of locations most susceptible to the spread of infection in a large settlement of Kazakhstan
Assess how the preventive measures taken by the regulator affect material and human resources compared to six hypothetical scenarios: no intervention, school clause, mask veering, vaccination, combined measures.
4. Discussion
4.1. The Findings and Their Implications
On May 28, 2023, all coronavirus restrictive measures were lifted in Kazakhstan. It is difficult today to assess the economic and social damage caused by the pandemic. Kazakhstan, like many other states, did not yet have experience in dealing with such rapidly spreading infections, with severe complications and high mortality. The pandemic proceeded differently depending on the geographic, economic, and demographic situation of countries. Many other factors, about which there are already enough publications in the scientific literature, also had an influence. In Kazakhstan, from the first days of the appearance of patients, strict restrictive measures were taken, which made it possible to pass this period with relatively small losses. Today it is time to reflect on the experience of the previous three years of fighting the pandemic, evaluate the effectiveness of the strategy used, and prepare a scientific basis for predicting the epidemiological situation in the event of similar situations in the future. Modeling is the best tool for this. We have created a stochastic agent model of the spread of infection, as the most realistic reflection of the daily activity of various segments of the population in the face of many uncertainties. Such uncertainties include adherence to restrictive measures, susceptibility to infection, susceptibility to a vaccine, the duration of the disease, the influence of environmental factors, etc.
One of the tasks that we set ourselves is the task of forecasting based on the created model. Total cases were used as a predictor to predict the spread of COVID-19. Satisfactory results were obtained with long-term (up to 50 days) forecasting in the case of a monotonous change in this indicator. However, the development of events has shown that the dynamics of daily incidence is oscillatory, and periods of relative stability are accompanied by sudden outbreaks, the nature of which has not yet been explained. As a result, inflection points appear on the Total cases curve. On
Figure 4 at least two such points can be identified. In this case, relatively satisfactory results were obtained with short-term forecasting, up to 10 days.
The spread of the infection largely depends on the behavior of various social groups, on their adherence to precautionary measures, on age, health, mentality, marital status, and other factors. Thus, it is known that the most socially active are young people, among them there are more contacts. In some countries, there are many lonely old people, they are more susceptible to severe forms of the disease and their treatment takes longer. Knowledge of such features allows you to prepare in advance for various scenarios. The city of Karaganda has more than 173 thousand households. 35% of families consist of 4 or more people. We noticed that there is a very high risk of domestic infection - up to 50% of infections occur at home. The use of various preventive measures in such cases did not give any improvement. Although wearing a mask is effective in blocking close-contact transmission, people did not always wear a mask in places they frequent daily (e.g., at home, at the office), even when they showed symptoms of illness. There does not appear to be any real means of preventing the spread of infection in the household other than herd immunity.
Another most important place for the spread of infection is public transport (there is only one type in the city - buses), it is used by most of the population, because. The availability of personal transport in the city is only 20%. Reducing the number of bus routes can also be one of the effective measures.
We investigated age dependence in clinical cases also. There were the higher number of patients projected among 18-59 aged population. Further - among children and youth, then the elderly makes up about 12% of total cases. These results diverge from real data - in fact, the proportion of people 60+ was 47%. This discrepancy can be explained by the fact that our simplified model does not take into account that children are less susceptible than adults to infection through contact with an infectious person, which leads to a decrease in cases of the disease among children [
68].
We analyzed the efficiency of several main interventions, such as school closure, mask wearing, vaccination and their combination. According to our data, all these methods have different efficiency. Thus, it is predicted that the most effective is the mask wearing, which can reduce the incidence by 2.2 times. Another measure is the closure of educational institutions. In the literature there is evidence that the school closure is an ineffective measure to protect against the spread of infection. Some studies reported that school closures were associated with no change in transmission. As we noted above it has been found that children may be less likely to transmit COVID-19 than adults, resulting in limited transmission in schools as well as from schoolchildren to adults. Our results are also consistent with these findings - a simulation of this scenario showed a 7% reduction in incidence.
Vaccination is known to help reduce the risk of infection, although the effect may not be very high [
69]. In our model, the use of vaccines reduced the incidence by 35%. Clearly, vaccination should prioritize high-risk workers in health care, basic services, food processing and transport. In addition, people over the age of 60. Ideally, a combination of these measures and strict adherence to all regulations could minimize the damage from the pandemic. Of course, reality is much richer than any models, no matter how complex they would be. The state bodies of Kazakhstan used all available tools to put a barrier to the infection. As a result, the number of cases in the city of Karaganda was 3.2 times less than in the baseline scenario. The mobilization of human resources to fight the pandemic made it possible to avoid a shortage of doctors and technical staff, and significantly reduce their need compared to the baseline scenario.
The bed fund, in general, corresponded to the needs of the situation. The shortage was felt in beds for severe and critical patients. This is understandable, because hospitals were not ready for the influx of such a large number of patients. This problem was solved by reorienting some non-infectious departments to combat COVID-19, opening provisional centers.
The economic effect was also obtained due to lower cash costs for medicines, reagents, medical devices and equipment compared to the hypothetical “do nothing” scenario. Even a simple mask mode can reduce material costs by 17%. In reality, according to our forecast, the savings amounted to 40%.
Clearly, the economic cost of not containing the virus would be unimaginably high. Although the cost of mitigation measures is still a major challenge for the government and other stakeholders. It seems that a complete lockdown is an unnecessary measure. Experience shows that it is fraught with severe post-pandemic social and economic consequences. Therefore, governments must find the optimal balance of freedoms and restrictions and be prepared for various scenarios.
4.2. Limitation
There are some limitations in this study. We could not reflect in our model the diversity of human activity inherent in real life. For simplicity, in this model, we assume that people can travel to no more than one place each day (for students and workers, public transport is the intermediate instance) and spend a certain amount of time there, which is different for different places and age groups. Considered scenarios have been idealized: for instance, special classes for graduates were opened in the schools, the rate of vaccination has been uneven, and adherence to the mask regime is difficult to assess. Some of the model parameters were obtained from previously published studies and, since they differ in different studies, we used average data, which could lead to inaccuracies of the model. Next, our model assumes that after the onset of disease symptoms, the individuals is immediately isolated and who have recovered from COVID-19 cannot be re-infected at least for the duration of the epidemic, and evidence to support this assumption is limited. Also, in our model, we did not consider that diverse segments of the population have different susceptibility to infection. However, these shortcomings do not belittle the significance of the results obtained. In the future, our model may be improved to provide more accurate predictions.