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COVID-19 pandemic has been raging all around the world for almost a year now, as of November 1, 2020. In this paper, we try to analyze the variation of the COVID-19 pandemic in different countries in the light of some modifications to the susceptible-infected-recovered (SIR) model. The SIR model was modified by taking time-dependent rate parameters. From this modified SIR model, the basic reproduction number, effective reproduction number, herd immunity, and herd immunity threshold are redefined. The re-outbreak of the COVID-19 is a real threat to various countries. We have used the above-mentioned quantities to find the reasons behind the re-outbreak of this disease. Also, the effectiveness of herd immunity to prevent an epidemic has been analyzed with respect to this model. We have also tried to show that there are certain universal aspects in the spread and containment of the disease in various countries for a short period of time. Finally, we have also analyzed the current pandemic situation in India and have attempted to discuss the possibilities in order to predict its future behavior using our model.

Amid growing debate between scientists and policymakers on the trade-off between public safety and reviving economy during the COVID-19 pandemic, the government of Bangladesh decided to relax the countrywide lockdown restrictions from the beginning of June 2020. Instead, the Ministry of Public Affairs officials have declared some parts of the capital city and a few other districts as red zones or high-risk areas based on the number of people infected in the late June 2020. Nonetheless, the COVID-19 infection rate had been increasing in almost every other part of the country. Ironically, rather than ensuring rapid tests and isolation of COVID-19 patients, from the beginning of July 2020, the Directorate General of Health Services restrained the maximum number of tests per laboratory. Thus, the health experts have raised the question of whether the government is heading toward achieving herd immunity instead of containing the COVID-19 pandemic. In this paper, the dynamics of the pandemic due to SARS-CoV-2 in Bangladesh is analyzed with integrated the Unscented Kalman Filter (UKF) in the SIRD model. We demonstrate that the herd immunity threshold can be reduced to 31% than that of 60% by considering age group cluster analysis resulting in a total of 53.0 million susceptible populations. With the data of COVID-19 cases till January, 2021, the time-varying reproduction numbers are used to explain the nature of the pandemic. Based on the estimations of active, severe and critical cases, we discuss a set of policy recommendations to improve the current pandemic control methods in Bangladesh.

We present a comprehensive SEIRDI${}_{\mathrm{\text{CU}}}$ model incorporating six compartments to examine the progression of COVID-19 and evaluate control strategies. Our findings reveal that implementing partial restrictions with varying levels of stringency delays the infection peak by approximately 160 days, alleviating pressure on healthcare systems. Under a herd immunity approach with an 85% immunity threshold, at least 425 days (14 months) are required for a gradual return to normalcy. Accelerated vaccination significantly shortens this timeline, emphasizing the critical role of immunization in societal reopening. Additionally, we highlight the importance of rigorously assessing post-vaccine side effects to uphold ethical and scientific standards. These insights provide actionable guidance for policymakers to balance public health interventions with economic recovery.

The rapid and extraordinary development and deployment of Coronavirus disease 2019 (COVID-19) vaccines worldwide represent an unprecedented achievement in the history of vaccine development. This chapter provides an overview of COVID-19 vaccine strategies, platforms, clinical trials, and regulatory frameworks. Vaccine safety, efficacy, herd immunity, and severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2) variants are also discussed. Real world challenges confronted include the ethics of vaccine allocation, vaccine nationalism, vaccine hesitancy, and vaccine passports.

The aim of this chapter is to discuss the importance and impact of scientific advice on the government’s response to the COVID-19 pandemic. This is a subject worthy of more than one book, so the topic is confined to the question of “herd immunity” or “mitigation”: a strategy considered in the United Kingdom (UK) during the early appearance of the virus in the first months of 2020. This chapter also considers definitions of science and problems within science itself and how these have impacted considerations of herd immunity during the first months of the pandemic. The relationships between the scientists who have advised the government, the politicians involved, and the impact this has had on the scientific advice given and implemented are discussed, as are the problems regarding the concept of “following the science”. The practise of herd immunity is outlined, as are attitudes to these in the UK in the first quarter of 2020. Finally, the position of science and scientists in relation to the responses to COVID-19 is considered in the broader political and ideological context of 21st century Britain.