Coronaviridae

Definition

The viral family Coronaviridae comprises RNA (ribonucleic acid) viruses that infect mammals, birds, and some amphibians worldwide. The viruses pose potential pandemic challenges as several new strains emerge. Infections can range from mild to severe in humans, manifesting as respiratory tract illnesses, enteric infections in infants, and, in rare cases, neurological syndromes.

Overview

Coronaviridae is a family of enveloped, single-stranded, positive-sense RNA viruses. These coronaviruses are named for their “corona” or crown-shaped surface. They are among the largest nonsegmented RNA virus genome known, with a surface of 120 to 160 nanometers (nm) in diameter. Around the crown, viral spikes in the envelope glycoprotein bind to host surface glycoproteins to activate the virus.

Most known coronaviruses belong to the subfamily Orthocoronavirinae and affect mammals or birds. Genera within the subfamily include Alphacoronavirus, Betacoronavirus, Deltacoronavirus, and Gammacoronavirus. Each genus has a natural reservoir in certain species; for example, Gammacoronavirus is found mostly in birds (and is sometimes known as avian coronavirus), while Betacoronavirus originates in bats and rodents. However, what makes coronaviruses particularly notable in epidemiology is their potential to jump from one species to another. At least seven such viruses have been found to infect humans.

A few novel coronaviruses have had serious infectious disease consequences in the human population in the twenty-first century. The SARS-CoV virus (which causes severe acute respiratory syndrome, or SARS) was discovered in February 2003 by World Health Organization (WHO) physician Carlo Urbani. It spread from the Guangdong Province of China, eventually infecting people in over twenty countries in Asia, Europe, South America, and North America. Urbani himself died from SARS in March 2003 after monitoring the early progression of the disease. Within six weeks of the discovery of SARS, thousands of people had become infected throughout the world, causing panic and disruption of national economies. However, the WHO staged an unprecedented rapid global response by issuing a worldwide global threat alert, travel advisories, and daily updates that tracked the spread of SARS. Because of this aggressive public health approach, the SARS epidemic subsided within a few months.

Other human coronaviruses were discovered over the following decade but did not become as widespread a threat. For example, HCoV-NL63 was found to infect mostly children and immunocompromised persons with mild upper respiratory symptoms and serious lower respiratory symptoms, including bronchiolitis and croup, but was determined to have likely circulated in the population for many decades. Other strains were identified as causes of the common cold. More serious was Middle East respiratory syndrome-related coronavirus (MERS-CoV), which was discovered in 2012 as the cause of a serious new respiratory flu-like disease known as Middle East respiratory syndrome (MERS). The highly deadly disease, which originated in camels, spread to over twenty countries but by 2015 was largely contained.

In December 2019, the beginning of an outbreak of a novel coronavirus was reported in Wuhan, China. The virus was eventually named SARS-CoV-2, and the associated disease was labeled COVID-19. By the end of February 2020, the virus had spread to over 80,000 people in over thirty-five countries, including the United States. The disease was soon determined to be highly transmittable via inhalation of respiratory droplets, and as it continued to spread worldwide at an alarming rate, the WHO characterized the outbreak as a pandemic in early March. Many governments began implementing stay-at-home policies and encouraging the practice of "social distancing" to limit close contact to slow the spread of the virus. Nevertheless, the pandemic continued to grow, including with the emergence of new variants of the virus, causing massive global disruptions over the next few years. By February 2023, the number of confirmed cases was 758.39 million, and the death total was 6.86 million globally. Ongoing public health measures, including the development of vaccines, did eventually contribute to declining infection rates, leading the WHO to announce the end of the pandemic declaration in May 2023. However, COVID-19 continued to circulate widely, with occasional significant spikes in infection rates. Some scientists suggested that SARS-CoV-2 might develop into an ongoing or seasonal virus like the common cold or influenza.

Pathogenicity and Clinical Significance

In humans, coronavirus infections often present as a respiratory tract infections that damage the pneumocyte cells, causing alveolar damage and ultimately leading to adult respiratory distress syndrome. Diarrhea can be present; however, the intestinal tissue is not damaged. Other symptoms may include a fever greater than 100.4° Fahrenheit, chills, malaise, dry cough, and chest infiltrates in the lower lobes. However, as in the case of COVID-19, asymptomatic infections are also possible, raising the risk that infected but apparently healthy people could spread the virus without knowing. Tests used to diagnose SARS include a chest X-ray or chest computed tomography scan and a complete blood count to measure white blood count, lymphocytes, and platelets. Nasal and oral swabs were also used to diagnose COVID-19 in a lab and using at-home tests.

Due to the nature of coronavirus transmission, healthcare workers are at high risk for diseases such as SARS and COVID-19 during an outbreak. Healthcare workers should wear protective masks and gowns and keep infected persons in isolation from healthy persons. Most infected persons, however, are not highly infectious outside conditions in which there is a high likelihood of other people handling their body fluids. For example, during the 2003 SARS outbreak, infected persons had flown on several airliners without infecting other passengers. In contrast, certain other people appeared to be “super spreaders,” a pattern often seen in other viral infections.

Drug Susceptibility

The high mutability of coronaviruses makes the development of specific vaccines particularly challenging. Despite years of research, no vaccine has been approved for SARS or strains of the common cold caused by coronaviruses.

The outbreak of COVID-19 brought a fresh wave of research into coronavirus treatment and prevention. In particular, concerted efforts began to develop a potential vaccine, and international research and testing progressed at an unprecedented pace. Early versions of a COVID-19 vaccine were approved in China and Russia within just a few months, initially on an emergency basis only. Other vaccines began to be granted emergency authorization in other countries in late 2020, and wider vaccination campaigns followed in 2021. In the United States, the most commonly administered vaccines were those developed by the companies Moderna and Pfizer–BioNTech, both of which used cutting-edge mRNA vaccine technology. Other effective vaccine types included protein subunit vaccines and vector vaccines.

As with vaccines, direct drug treatment of coronavirus infections is also difficult. No antiviral drugs have conclusively been demonstrated to combat strains of the common cold caused by coronaviruses, for example. Most treatments instead focus on alleviating symptoms. SARS and COVID-19 have shown more susceptibility to certain antiviral medications, including nirmatrelvir with ritonavir (Paxlovid), remdesivir (Veklury), and molnupiravir (Lagevrio). Persons with severe lung inflammation from a coronavirus infection may need high doses of steroids, and those persons whose lungs have sustained damage may need mechanical ventilation for breathing support. Corticosteroids were shown to reduce the death rate in patients critically ill with COVID-19. Persons with SARS can be given antibiotics to treat associated bacterial infections.

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