The COVID-19 pandemic was an unprecedented event with many repercussions worldwide. Thus, it should not be surprising that the modeling of the COVID-19 pandemic was imperative, making many scientists examine thoroughly this difficult situation, from many aspects. To do so, researchers have employed many different models and techniques from various fields and disciplines. To begin with, many approaches try to model the COVID-19 virus transmission. To give some examples, Najarzadeh et al. [5] used a fuzzy model to simulate the spread of the virus, while another approach implemented the Kalman filter, based on a stochastic epidemiological model [6] to simulate the spread of the pandemic. Furthermore, Matveeva and Leonenko [13] utilized a Gaussian process in a regression model to simulate the COVID-19 spread, highlighting in this way the usefulness of these models. Similarly, El Koufi and El Koufi [12] provided a stochastic model for the modeling of the COVID-19 spread, for the case study of Pakistan. Kalachev et al. [14] utilized three formulations of the Susceptible-Infected-Removed model, a well-established framework in the investigation of epidemics, with application in the case study of the influenza outbreak and the COVID-19 pandemic, providing insights into the model’s performance.
Moreover, Bilgram et al. [9] used a combination of stochastic and machine-learning models to examine the safe and near-optimal strategies for virus exposure. Similarly, Middya and Roy [11] implemented and tested the performance of various deep learning models, regarding the health outcomes of the COVID-19 spread for the case study of India. According to the authors, no model outperforms all other models, implying that each model has its strengths and weak points.
In the same context, Farman et al. [3] modeled the dynamical transmission of COVID-19 utilizing Caputo-Fabrizio fractional operators, proposing a model capable of performing for integer and non-integer scenarios, while Pandey et al. [4] through a similar approach, utilizing the same technique, examined the mathematical modeling of COVID-19 spread for the case-study of India. Abbes et al. [15] utilized the same technique, the Caputo fractional difference operator, to investigate the COVID-19 spread, incorporating non-linear dynamic behaviors. The authors also examine commensurate and incommensurate fractional orders, through several numerical techniques, including phase attractors, maximum Lyapunov exponents, bifurcation diagrams, and C0 algorithm. While Khairulbahri [16] used the same technique, also incorporating behavioral measures, asymptomatic cases, and lockdown measures, to render the model multi-faceted.
Another strand of literature tries to model the chaotic components of the virus spread. For example, Al-Basyouni and Khan [7] utilized linear stability theory to model the chaotic behavior of the pandemic, while Farman et al. [17] used fractal fractional techniques to examine the spread of the Omicron variant COVID-19 virus. Similarly, Khan et al. [8] utilize fractal techniques to model the COVID-19 pandemic. Moreover, Arshad et al. [18] employ a fractional order epidemic model to mathematically assess the spread and its characteristics. Additionally, Li and Guo [19] used the optimal control and cost-effectiveness analysis to create a sufficient model regarding the Omicron strain.
Furthermore, Chen et al. [20] modeled the effect of surveillance and physical distancing measures on the COVID-19 dynamics at a local aspect. We must point out that van der Vegt et al. [21] introduced a package in the R statistical software to model the spread of the COVID-19 pandemic, unveiling the importance of the spread’s modeling. More importantly, there is also a comparison of techniques regarding the modeling and the forecasting of the spread of the pandemic, by investigating and predicting the new cases, for the case study of Nigeria, employing various models, i.e. regression techniques, autoregressive integrated moving average models and machine learning approaches [10]. The overall results provide evidence of the models’ superiority, some of them being also certain regression models.
Moreover, some studies examine the various features and events related to the pandemic, and not the spread itself. More precisely, Song et al. [22] used a stability and optimal control model, also examining the vaccination effect and the isolation delays, regarding the COVID-19 pandemic. Similarly, Tchoumi et al. [23] focused on vaccination and its effect on the limitation of the pandemic [23]. As regards the quarantine effect investigation, Singh et al. [24] propose a dynamical transmission model of a coupled non-linear fractional differential equation in the Atangana-Baleanu Caputo, to examine the COVID-19 transmission in a dynamic framework. Ojo et al. [25] utilized several nonlinear optimal control strategies to develop a robust mathematical model for the investigation of the co-infection of both COVID-19 and influenza.
Additionally, some other approaches utilize advanced modeling [26], while some others develop reactive–diffusion epidemic models to examine the human mobility during COVID-19 [27]. Finally, some studies are limited to even more specific aspects of the COVID-19 pandemic, for example, Rayegan et al. [28] examined the airborne transmission of the virus, only indoors. On the other hand, some studies use mathematical modeling to predict the COVID-19 mortality rate [29].
In this point of reference, the modeling of the COVID-19 confirmed cases, and thus, the evolution of the pandemic is not adequately examined, with many approaches failing to provide a well-stated mathematical model with an empirical evaluation of its validity and superiority. This is the gap in the literature we aim to fill with our approach.