Previous epidemics caused by coronaviruses
Most coronaviruses are adapted to their hosts, whether animal or human, although cases of possible animal-to-human transmission and adaptation have been described in the past two decades, causing two epidemics.
The first such outbreak originated in Guangdong, a southern province of the People’s Republic of China, in mid-November of 2002. The disease was named severe acute respiratory syndrome (SARS). The cause was shown to be a novel coronavirus (SARS-CoV), an animal virus that had crossed the species barrier and infected humans. The most likely reservoir were bats, with evidence that the virus was transmitted to a human through an intermediate host, probably a palm civet or raccoon dog (8,9).
In less than a year, SARS-CoV infected 8098 people in 26 countries, of whom 774 died (10,11). Approximately 25% of the patients developed organ failure, most often acute respiratory distress syndrome (ARDS), requiring admission to an intensive care unit (ICU), while the case fatality rate (CFR) was 9.6%. However, in elderly patients (>60 years), the CFR was over 40%. Poor outcomes were seen in patients with certain comorbidities (diabetes mellitus and hepatitis B virus infection), patients with atypical symptoms, and those with elevated lactic acid dehydrogenase (LDH) values on admission. Interestingly, the course of the disease was biphasic in 80% of the cases, especially those with severe clinical profiles, suggesting that immunological mechanisms, rather than only the direct action of SARS-CoV, are responsible for some of the complications and fatal outcomes (8,9).
Approximately 20% of the reported cases during this epidemic were health care workers. Therefore, in addition to persons exposed to animal sources and infected family members, health care workers were among the most heavily exposed and vulnerable individuals (9,10). During 2004, three minor outbreaks were described among laboratory personnel engaged in coronavirus research. Although several secondary cases, owing to close personal contact with infected patients, were described, there was no further spread of the epidemic. It is not clear how the SARS-CoV eventually disappeared and if it still circulates in nature among animal reservoirs. Despite ongoing surveillance, there have been no reports of SARS in humans worldwide since mid-2004 (11).
In the summer of 2012, another epidemic caused by a novel coronavirus broke out in the Middle East. The disease, often complicated with respiratory and renal failure, was called Middle East respiratory syndrome (MERS), while the novel coronavirus causing it was called Middle East respiratory syndrome coronavirus (MERS-CoV). Although a coronavirus, it is not related to the coronaviruses previously described as human pathogens. However, it is closely related to a coronavirus isolated from dromedary camels and bats, which are considered the primary reservoirs, albeit not the only ones (8,12).
From 2012 to the end of January 2020, over 2500 laboratory-confirmed MERS cases, including 866 associated deaths, were reported worldwide in 27 countries (13). The largest number of such cases has been reported among the elderly, diabetics, and patients with chronic diseases of the heart, lungs, and kidneys. Over 80% of the patients required admission to the ICU, most often due to the development of ARDS, respiratory insufficiency requiring mechanical ventilation, acute kidney injury, or shock. The CFR is around 35%, and even 75% in patients >60 years of age. However, MERS-CoV, unlike its predecessor SARS-CoV, did not disappear, but still circulates among animal and human populations, occasionally causing outbreaks, either in connection with exposure to camels or infected persons (12).
Overall, 19.1% of all MERS cases have been among health care workers, and more than half of all laboratory-confirmed secondary cases were transmitted from human to human in health care settings, at least in part due to shortcomings in infection prevention and control (12,13). Post-exposure prophylaxis with ribavirin and lopinavir/ritonavir decreased the MERS-CoV risk in health care workers by 40% (14).