Coronaviruses: The Common Cold, SARS, and MERS

Coronaviruses of humans were first identified more than 60 years ago from individuals with respiratory infections, mainly mild. Two different viruses, 229E and OC43 were initially recognized. Because of difficulty in isolating them using standard techniques, many of the early studies of their occurrence were based on serology. They were confirmed to be worldwide in distribution, and, in the North Temperate Zone, mainly occurring in the winter season. With the development of the reverse transcriptase polymerase chain reaction (PCR) and related techniques, two additional viruses have been identified, HKU1 and NL63. The four viruses have now been recognized as important in the etiology of common respiratory infections, second only to the rhinoviruses. In 2002, a previously unrecognized betacoronavirus emerged from a zoonotic reservoir in Southern China and spread during the following year to several major cities of the world. The resulting illness was termed Severe Acute Respiratory Syndrome (SARS) because of its potential lethality. More than 8000 probable cases were reported during 2003, mainly from Hong Kong and mainland China, producing social and economic disruption in those areas affected. A constant feature of the outbreak was the importance of nosocomial spread. In spite of an estimated basic reproductive number higher than influenza, the outbreak was contained, in large part because infection was largely clinically overt and transmission occurred 3–4 days after symptom onset. In 2012, another betacoronavirus has emerged in the Arabian Peninsula which produces a somewhat similar illness, termed Middle East Respiratory Syndrome (MERS), marked by extensive nosocomial transmission. Sporadic outbreaks continue to occur in the region. The risk posed by zoonotic coronaviruses was starkly revealed by the emergence and global spread of another novel betacoronavirus SARS-CoV-2, the causative pathogen of Coronavirus Disease 2019 (COVID-19), discussed in a separate chapter.