Covid-19 pandemic: History, aetiology, epidemiology, vaccinology and societal impact
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The virus causing the current novel coronavirus disease in humans is named as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by the International Committee on Taxonomy of Viruses (ICTV). The World Health Organization (WHO) named the disease as coronavirus disease 2019 (COVID-19). On 29 December 2019, the pneumonic disease was first noticed in the Wuhan city of Hubei Province in China affecting people of all age groups. The COVID-19 pandemic is currently affecting 213 countries and territories around the world; it also involved two international conveyances. The SARS-CoV-2 is a novel Coronavirus that had not been seen before, first isolated from people with acute respiratory illness. All features of the novel SARS-CoV-2 are similar to the other coronaviruses occurring in nature in bats and other animal species. Outside the human body, the virus is inactivated by household soap solution and lipid solvents that dissolve its protective envelope layer and make it non-infectious. Genetically, SARS-CoV-2 is closely related to SARS-CoV of 2003, but distinct from Middle East respiratory syndrome-CoV (MERS-CoV) of 2012. All these three respiratory human CoVs (HCoVs) are of bat origin and zoonotic; transmission from bat to human is facilitated by intermediate animal hosts, that, is civet cat for SARS and dromedary camel for MERS. It is not clear as to which animal is the intermediate host to facilitate the jumping of SARS-CoV-2 from bats to human. Ant-eating Pangolins are prime suspect to act as intermediate host, but yet inconclusive. Genetic analysis revealed that SARS-CoV-2 is a beta-coronavirus (genus) and genetically clusters within the subgenus Sarbecovirus (lineage B), together with some bat virus strains with >96% genetic identity. Coronaviruses shift host frequently. Scientists found that the receptor binding domain (RBD) of the SARS-CoV-2 spike protein had evolved to effectively bind to a molecule on different types of human cells known as ACE2 (angiotensin-converting enzyme 2), a receptor involved in regulating blood pressure, to initiate host cell infection cycle. Organs having higher number of ACE2 bearing cells are severely damaged by the virus. The SARS-CoV-2 spike protein is longer than the SARS and MERS-CoVs and is very effective at binding to human cells through ACE2 receptor, and the scientists working on the virus concluded that it was the result of natural selection and not the product of genetic engineering. Furthermore, it has been found that the SARS-CoV-2 backbone differs substantially from those of already known HCoVs. It mostly resembles to related CoVs found in bats and pangolins. Autologous (between viruses affecting same host species) and heterologous (between viruses affecting different host species) genetic recombination help in evolution/emergence of new virus variants/pathogenic species. The COVID-19 affects human of all ages with primarily respiratory sickness of different degree; however, it has been observed to be fatal in elderly people due to Acute Respiratory Distress Syndrome (ARDS) complicated by cytokine storm, as observed earlier in feline infectious peritonitis (FIP), a fatal disease of cats caused by feline CoV (FIPV). Similarly, antibodydependant enhancement (ADE), as described earlier during evaluation of a FIPV vaccine, has been suspected to occur in COVID-19. The present compilation, prepared with the help of data and information available on the Internet and NCBI-PubMed, is an effort for the students in virology, immunology, genetics, pharmacology and epidemiology so that they can be motivated to undertake research programmes to fill the gaps in the available knowledge; particularly in the areas of immunology and vaccinology, epidemiology and therapeutics, etc. Further, real-time surveillance of coronavirus strains circulating in domestic pet, wild and captive animals/mammals, and poultry birds will help in monitoring possible inter species jump of the virus, as it will be of immense help once the current Global COVID-19 pandemic subsides. It is also important to identify intermediate host(s) that might cause possible future zoonoses involving CoVs, and to study virus persistence including carrier status in convalescent individuals that may pose threat in post-COVID-19 pandemic period/scenario in times to come. The pandemic has resulted in travel restrictions, nationwide lockdowns, and home quarantine in several countries in the World to implement social distancing in order to slow the spread/transmission of the virus in human populations. The economic costs and social burdens on individual countries and also global basis, till the pandemic is controlled and slows down, will be enormous and yet to be comprehensively estimated. A number of laboratories around the world are working on the development of dependable diagnostic tests for the detection of virus genome and antibodies in clinical specimens to precisely identify infected individuals and hot spots of the infection, vaccines and antiviral therapeutics, and their success will determine the success of the COVID-19 control programme in hand. Economic and societal impact of the pandemic has also been discussed. It is emphasised to implement ‘One Health’ approach involving human, animal and the environment in controlling the present pandemic and preventing possible future ones for a healthier World. It has been observed by scientists that longitudinal serological studies are required to determine the extent and duration of immunity to SARS-CoV-2.
COVID-19, Coronavirus, Epidemiology, Pandemic, SARS, Vaccine.