The world is presently facing a pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel coronavirus that was first reported from China in December 2019. As on April 26, 2020, the virus has affected 210 countries and territories, with 2,804,796 confirmed cases and 1,93,710 deaths. As per the World Health Organization (WHO) Situation Report – 97, maximum numbers of cases and deaths have been reported from the European region (13,41,851 and 1,22,218, respectively) and region of the Americas (10,94,846 and 56,063, respectively). Moderate numbers of cases have been reported from the Eastern Mediterranean Region (1,60,586; 6887) and Western Pacific Region (1,42,639; 5943), and low number of cases have been reported from Southeast Asia region (43,846; 1747) and African region (20,316; 839).1 India has not been spared, and as on April 27, 2020, 20,835 active cases have been reported; of which, 6184 have been cured or discharged, 872 have died, and 1 has migrated.2
SARS CoV-2 causes coronavirus disease 2019, an illness with a wide spectrum of disease varying from asymptomatic illness, mild uncomplicated illness, mild pneumonia, severe pneumonia, acute respiratory distress syndrome, sepsis, septic shock to death. But this is not first time a virus with predominant respiratory manifestations has caused a pandemic. From the year 2003 onward, to date, four major pandemics have been reported, the comparative features of which are mentioned in Table 1. In the 21st century, the world experienced the first pandemic of severe acute respiratory syndrome (SARS) in 2003, when a previously unrecognized coronavirus (SARS-associated coronavirus [SARS-CoV]) was identified as a result of united global efforts. Mode of transmission was found to be zoonotic transmission from bats to humans, and the pandemic was declared over in 2004. It was predicted at this time that SARS-CoV or similar viruses could reemerge or emerge, based on studies on bats that demonstrated the presence of viruses capable of infecting human cells even in the absence of prior adaptation.3
|Virus||Country of origin||Year of origin||Zoonotic disease (possible reservoir)||Intermediate host||Mortality rates||Most common age-group affected|
|SARS-CoV||Guangdong province, China||2003||Yes (bats)||Palm civets||>10%||Adults|
|Influenza virus (H1N1)||Mexico||2009||Yes (pigs)||None||2.1%||Children and working adults <65 years of age|
|MERS-CoV||Arabian Peninsula||2012||Yes (bats)||Dromedary camels||>35%||Adults >40 years of age|
|SARS-CoV-2||Hubei province, China||2019||Yes (bats)||Not known||Approxiamately 3.3%||Adults|
Human cases of H5N1 avian influenza were first reported in Hong Kong in 1997, and in 2003, three more human cases were reported from Hong Kong and China. In 2004, sporadic cases were reported from Vietnam and Thailand. The year 2005 witnessed a rapid surge in the number of cases even from previously unaffected countries such as Cambodia, Indonesia, China, and Turkey. The increase in the number of human cases was accompanied with outbreaks in poultry that spread westward, reaching Central Asia and Romania. But, probably, because the virus was not a competent human pathogen, a pandemic did not occur. Between 2003 and 2020, the WHO reported 861 human cases of H5N1 avian influenza A from 17 countries, with the last case reported in April 2019 from Nepal. It had a very high case fatality rate of 53%.4
The next pandemic occurred in the year 2009 when H1N1, a novel influenza A virus, emerged in Mexico, US, and then spread throughout the world. Because the virus was different from the circulating H1N1 strains, it was given a new name (H1N1)pdm09 virus. A total of 214 countries were affected worldwide. The virus was a genetically reassorted virus that is believed to have evolved from the triple reassortant North American swine influenza virus and Eurasian swine viruses. It was only after this pandemic in 2011 that the WHO developed a global standardized case definition for severe acute respiratory illness (SARI) for wide implementation. For surveillance purposes, cases were divided into (1) influenza-like illness (ILI), i.e., with acute onset of fever ≥ 38° C and cough, and (2) SARI, i.e., ILI cases that require hospitalization.5 The virus is now a well-established human pathogen, and seasonal outbreaks occur globally causing significant illnesses, hospitalizations, and deaths.
Another epidemic of acute pneumonia occurred in 2012 that originated in Saudi Arabia. The virus was identified as a novel coronavirus (first lineage 2C betacoronavirus known to affect humans) and was termed Middle East respiratory syndrome coronavirus owing to its origin in the Middle East. It was highly pathogenic and had a very high case fatality rate of 35%, but the spread of the virus was limited to 27 countries and did not reach pandemic proportions. The virus originated in bats and probably entered human species via dromedary camels. The first human case was reported from Yemen in a pilot who consumed raw camel milk. Consumption of milk was considered to be a mode of infection, but later, respiratory transmission and direct contact transmission were identified.3
With the ongoing pandemic of SARS-CoV-2, nature has again alarmed human beings that such cross-species virus transmissions will continue to occur in future because of genetic variations in the virus (reassortment or other mutations as seen in coronaviruses and influenza viruses), abundant animal hosts, and the ever-increasing human–animal interface, with its spread facilitated by international travel. Although such pandemics cannot be averted, we can better handle such situations by our preparedness in terms of infrastructure, trained manpower, intersectoral coordination, and political will. This is already being implemented in India, but the present pandemic of SARS-CoV-2 will definitely leave several learning points owing to its uniqueness in several ways such as being so widespread, affecting such a large number of people globally, and continuing for such a long period of time (approximately 5 months to date). The measures taken to combat it are also extreme, be it enormous sample testing, lockdown of several countries, or quarantine of laboratory-confirmed cases and their contacts for a period of at least 14 days. It is also unique in the fact that its behavior varies with the country affected, which is exemplified by the highest mortality rates in Europe and the Americas and lowest mortality rates in the African region, which may in part be explained by multiple clades6 (clade V, S, and G) of SARS-CoV-2 circulating simultaneously.