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Coronaviruses and Acute Respiratory Syndromes (COVID-19, MERS, and SARS)


Brenda L. Tesini

, MD, University of Rochester School of Medicine and Dentistry

Last full review/revision Jul 2020| Content last modified Jul 2020
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Coronaviruses are enveloped RNA viruses that cause respiratory illnesses of varying severity from the common cold to fatal pneumonia.

Numerous coronaviruses, first discovered in domestic poultry in the 1930s, cause respiratory, gastrointestinal, liver, and neurologic diseases in animals. Only 7 coronaviruses are known to cause disease in humans. 

Four of the 7 coronaviruses most frequently cause symptoms of the common cold. Coronaviruses 229E and OC43 cause the common cold; the serotypes NL63 and HUK1 have also been associated with the common cold. Rarely, severe lower respiratory tract infections, including pneumonia, can occur, primarily in infants, older people, and the immunocompromised.

Three of the 7 coronaviruses cause much more severe, and sometimes fatal, respiratory infections in humans than other coronaviruses and have caused major outbreaks of deadly pneumonia in the 21st century:

  • SARS-CoV-2 is a novel coronavirus identified as the cause of coronavirus disease 2019 (COVID-19) that began in Wuhan, China in late 2019 and spread worldwide.

  • MERS-CoV was identified in 2012 as the cause of Middle East respiratory syndrome (MERS).

  • SARS-CoV was identified in 2003 as the cause of an outbreak of severe acute respiratory syndrome (SARS) that began in China near the end of 2002.

These coronaviruses that cause severe respiratory infections are zoonotic pathogens, which begin in infected animals and are transmitted from animals to people. SARS-CoV-2 has significant person-to-person transmission.


COVID-19 is an acute, sometimes severe, respiratory illness caused by a novel coronavirus SARS-CoV-2.

COVID-19 was first reported in late 2019 in Wuhan, China and has since spread extensively worldwide. For current information on the number of cases and fatalities, see the Centers for Disease Control and Prevention: 2019 Novel Coronavirus and the World Health Organization's Novel Coronavirus (COVID-2019) situation reports.

Transmission of COVID-19

Early COVID-19 cases were linked to a live animal market in Wuhan, China, suggesting that the virus was initially transmitted from animals to humans. Person-to-person spread occurs through contact with infected secretions, mainly via contact with large respiratory droplets, but it could also occur via contact with a surface contaminated by respiratory droplets and possibly by aerosol transmission of small respiratory droplets. Researchers are still learning how readily this virus spreads from person to person. It is known that symptomatic, as well as asymptomatic and presymptomatic patients, can transmit the virus. The virus appears more transmissible than SARS.

Super-spreaders played an extraordinary role in driving the 2003 SARS outbreak and may also play a significant role in the current COVID-19 outbreak and estimates of transmissibility. A super-spreader is an individual who transmits an infection to a significantly greater number of other people than the average infected person. Persons with minimal or no symptoms may also be able to transmit disease, making it difficult to control the outbreak.

Situations with high risk of transmission include facilities such as nursing homes, long-term care facilities, prisons, and on board ships. Such situations involve high population density and often difficulty in maintaining avoidance precautions. The residents of nursing homes are also at high risk of severe disease because of age and underlying medical disorders.

Quarantine and isolation measures are being applied in an attempt to limit the local, regional, and global spread of this outbreak. Strict adherence to these measures have been successful at controlling the spread of infection in select areas.

Symptoms and Signs

People with COVID-19 may have few to no symptoms, although some become severely ill and die. Symptoms can include

  • Fever

  • Cough

  • Shortness of breath or difficulty breathing

  • Chills or repeated shaking with chills

  • Fatigue

  • Muscle pain

  • Headache

  • Sore throat

  • New loss of smell or taste

  • Congestion or runny nose

  • Nausea, vomiting, and diarrhea

The incubation time ranges from 2 to 14 days after exposure to the virus. The majority of infected people will have no symptoms or mild disease. The risk of serious disease and death in COVID-19 cases increases with age and in people with other serious medical disorders, such as heart, lung, kidney, or liver disease, diabetes, immunocompromising conditions, or severe obesity (body mass index > 40) (1, 2). Severe disease is characterized by dyspnea, hypoxia, and extensive lung involvement on imaging. This can progress to respiratory failure requiring mechanical ventilation, shock, multiorgan failure, and death.

In addition to respiratory disease that can progress to acute respiratory distress syndrome (ARDS) and death, other serious complications include the following:

A rare postinfectious inflammatory syndrome termed multisystem inflammatory syndrome in children (MIS-C) has been observed as a rare complication of SARS-CoV-2 infection. It has features similar to Kawasaki disease or toxic shock syndrome. Children with MIS-C most commonly present with fever, tachycardia, and gastrointestinal symptoms with signs of systemic inflammation. Cases meeting the following criteria should be reported to the Centers for Disease Control and Prevention (CDC) as suspected MIS-C: hospitalization, fever > 24 hours, laboratory evidence of inflammation, signs of ≥ 2 organs involved, and laboratory or epidemiologic association with SARS-CoV-2 infection (3).

Symptoms and signs references


  • Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) testing of upper and lower respiratory secretions

Diagnostic testing for COVID-19 is becoming increasingly available through commercial and hospital-based laboratories in addition to public health laboratories. Point of care antigen detection and PCR-based assays are also commercially available. Antigen detection assays typically are less sensitive than PCR-based assays.

For initial diagnostic testing for COVID-19, the CDC recommends collecting and testing a single upper respiratory specimen. The following are acceptable specimens:

  • A nasopharyngeal specimen collected by a healthcare professional (preferred specimen if available)

  • An oropharyngeal (throat) specimen collected by a healthcare professional

  • A nasal mid-turbinate swab collect by a healthcare professional or by a supervised onsite self-collection (using a flocked tapered swab)

  • An anterior nares specimen collect by a healthcare professional or by onsite or home self-collection (using a flocked or spun polyester swab)

  • A nasopharyngeal wash/aspirate or nasal wash/aspirate specimen collected by a healthcare professional

Refer to accepting laboratory's collection instructions, because not all testing platforms and laboratories may be able to test all specimen types. For nasopharyngeal and oropharyngeal specimens use only synthetic fiber swabs with plastic or wire shafts. Do not use calcium alginate swabs or swabs with wooden shafts, as they may contain substances that inactivate some viruses and inhibit PCR testing. The swabs should be placed immediately into a sterile transport tube containing 2 to 3 mL of either viral transport medium, Amies transport medium, or sterile saline, unless using a test designed to analyze the specimen directly, such as a point-of-care test. Maintain proper infection control when collecting specimens.

The CDC also recommends testing lower respiratory tract specimens, if available. For patients for whom it is clinically indicated (eg, those receiving invasive mechanical ventilation), a lower respiratory tract aspirate or bronchoalveolar lavage sample should be collected and tested as a lower respiratory tract specimen. Collection of sputum should be done only for those patients with productive coughs. Induction of sputum is not recommended. (See CDC: Interim Guidelines for Collecting, Handling, and Testing Clinical Specimens from Persons for Coronavirus Disease 2019.) For biosafety reasons, the CDC recommends local institutions do not attempt to isolate the virus in cell culture or do initial characterization of viral agents in patients suspected of having COVID-19 infection.

SARS-CoV-2 diagnostic testing is becoming more available in the US, and previous restrictions on patient selection for testing are being relaxed. Clinicians should use their judgment as to whether a patient's symptoms and signs are compatible with COVID-19 and whether testing would impact the care of the patient or public health measures. Decision to test may also take into account the local epidemiology of COVID-19, the course of illness, and the patient's epidemiologic factors such as close contact with a confirmed COVID-19 case within 14 days of symptom onset. Clinicians are also encouraged to test for other causes of similar respiratory illness (eg, influenza) if epidemiologically appropriate. Asymptomatic patients may also be candidates for testing based on local public health guidance. (See CDC: Overview of Testing for SARS-CoV-2.)

The CDC suggests that the following are high priority for COVID-19 testing:

  • Hospitalized patients

  • Workers in healthcare facilities, congregate living settings, and first responders who have symptoms

  • Residents in long-term care facilities, other congregate living settings, prisons, and shelters who have symptoms

  • Persons identified through public health cluster and contact investigations

Areas of sustained transmission will vary as the outbreak proceeds. For areas inside the US, clinicians should consult state or local health departments. Cases have been reported in all states. The CDC recommends avoiding all international and cruise ship travel due to the global pandemic; for current information see CDC: Coronavirus Disease 2019 Information for Travel.

Positive test results need to be reported to local and state health departments, and patients require strict isolation at home or in a healthcare facility.

NOTE: Serologic, or antibody, testing should not be used to diagnose acute COVID-19 illness.

Routine laboratory findings for those with more severe disease include lymphopenia as well as less specific findings of elevated aminotransaminase (ALT, AST) levels, elevated lactate dehydrogenase (LDH) levels, D-dimer, ferritin, and elevated inflammatory markers such as C-reactive protein.

Chest imaging findings can be normal with mild disease and increase with increasing severity of the illness. Typical findings are consistent with viral pneumonia and include ground-glass opacities and consolidation on either chest x-ray or chest CT.

The MuLBSTA score may be useful to predict mortality in patients with viral pneumonia due to COVID-19 (1).

Diagnosis reference

  • Chen N, Zhou M, Dong X, et al: Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study. Lancet 395(10223):507-513, 2020. doi: 10.1016/S0140-6736(20)30211-7


  • Supportive

  • Sometimes, remdesivir for severe disease

  • Sometimes, dexamethasone for severe disease

Treatment of COVID-19 is mainly supportive. Over 175 treatment and vaccine clinical trials are currently registered, but data on effective therapy remain sparse. There are currently no treatments approved by the US Food and Drug Administration (FDA) for COVID-19, but the antiviral agent remdesivir has been made available through an FDA emergency use authorization for patients with severe disease (defined as requiring supplemental oxygen, ventilatory, or extracorporeal membrane oxygenation [ECMO] support). Current national guidelines caution against the use of therapeutic agents outside of clinical trials with the exception of remdesivir and dexamethasone (see National Institutes of Health (NIH) COVID-19 Treatment Guidelines and Infectious Diseases Society of America (IDSA) Guidelines on the Treatment and Management of Patients with COVID-19). For each therapeutic agent, the benefits must be weighed against possible risks for each patient.

The NIH COVID-19 Treatment Guidelines recommend using dexamethasone (at a dose of 6 mg once per day for up to 10 days) in patients with COVID-19 who are mechanically ventilated or require supplemental oxygen. Immunomodulatory therapies including immunoglobulin infusion via convalescent plasma and IL-1 and IL-6 inhibitors have been used, but there are insufficient data to recommend their routine use outside of clinical trials. Other drugs that have been used include chloroquine derivatives, azithromycin, and antiretrovirals. There are also insufficient data to support the use of any of these agents outside of clinical trials, and toxicities associated with chloroquine and hydroxychloroquine led to an FDA warning that they not be used outside of the hospital setting or a clinical trial.

Supportive therapy may include critical care management with mechanical ventilation and vasopressor support. Early goals of care discussions are recommended. For patients with severe respiratory failure, ECMO may be considered. The Respiratory Extracorporeal Membrane Oxygenation Survival Prediction (RESP) score developed based on a study of 2355 adult patients with severe acute respiratory failure treated by ECMO from 2000 to 2012 (1) predicts survival in adults receiving ECMO for respiratory failure and may help in the selection of COVID-19 patients for ECMO treatment but is not a substitute for clinical assessment and judgement.

Complications of COVID-19 illness should also be treated as they arise. Hospitalized patients with COVID-19 may be at increased risk for thromboembolic events. Pharmacologic prophylaxis should be given as per hospital guidelines, and a high clinical suspicion for thromboembolic events should be maintained. Therapeutic anticoagulation should be started if there is a high suspicion of thromboembolism and confirmatory imaging could not be obtained.

Drugs such as angiotensin-converting enzyme (ACE) inhibitor or angiotensin II receptor blocker (ARB) therapy should be continued if needed for concomitant medical conditions but not started as treatment for COVID-19. There is no evidence that use of nonsteroidal anti-inflammatory drugs (NSAIDs) is linked to worse outcomes, and either acetaminophen or NSAIDs can be used during the treatment of COVID-19.

Respiratory management of the nonintubated and intubated COVID-19 patient should take into consideration the tendency toward hypoxia. Nonpharmacologic adjunctive measures such as frequent repositioning and ambulation may be helpful. Therapeutic decisions should be made to best manage the patient, but also consider the risk of exposure to healthcare workers and best use of resources. Intubation is a time of particular risk of healthcare provider exposure to infectious aerosols and should be done with extreme care.

To help prevent spread from suspected cases, health care practitioners should use standard, contact, and airborne or droplet precautions with eye protection. Airborne precautions are particularly relevant for patients undergoing aerosol-generating procedures. Patients with respiratory symptoms should be identified and masked immediately upon entry to any healthcare facility. Strategies to monitor and conserve personal protective equipment (PPE) supplies should be considered; tools are available through the CDC. (See CDC: Infection Control Guidance for Healthcare Professionals about Coronavirus.)

Treatment reference

  • Schmidt M, Bailey M, Sheldrake J, et al: Predicting survival after extracorporeal membrane oxygenation for severe acute respiratory failure. The Respiratory Extracorporeal Membrane Oxygenation Survival Prediction (RESP) score. Am J Respir Crit Care Med 189(11):1374-1382, 2014. doi:10.1164/rccm.201311-2023OC

More Information

Middle East Respiratory Syndrome (MERS)

Middle East respiratory syndrome (MERS) is a severe, acute respiratory illness caused by the MERS coronavirus (MERS-CoV).

MERS-CoV infection was first reported in September 2012 in Saudi Arabia, but an outbreak in April 2012 in Jordan was confirmed retrospectively. Through 2019, worldwide, nearly 2500 cases of MERS-CoV infection (with at least 850 related deaths) have been reported from 27 countries; all cases of MERS have been linked through travel to or residence in countries in and near the Arabian Peninsula, with > 80% involving Saudi Arabia. The largest known outbreak of MERS outside the Arabian Peninsula occurred in the Republic of Korea in 2015. The outbreak was associated with a traveler returning from the Arabian Peninsula. Cases have also been confirmed in countries throughout Europe, Asia, North Africa, the Middle East, and the United States in patients who were either transferred there for care or became ill after returning from the Middle East.

Preliminary seroprevalence studies indicate that the infection is not widespread in Saudi Arabia.

The World Health Organization considers the risk of contracting MERS-CoV infection to be very low for pilgrims traveling to Saudi Arabia for Umrah and Hajj. For additional information about pilgrimages to the Middle East, see World-travel advice on MERS-CoV for pilgrimages .

Median age of patients with MERS-CoV is 56 years, and the male:female ratio is about 1.6:1. Infection tends to be more severe in older patients and in patients with a preexisting disorder such as diabetes, a chronic heart disorder, or a chronic renal disorder.

Transmission of MERS-CoV

MERS-CoV may be transmitted from person to person via direct contact, respiratory droplets (particles > 5 micrometers), or aerosols (particles < 5 micrometers). Person-to-person transmission has been established by the development of infection in people whose only risk was close contact with people who had MERS.

The reservoir of MERS-CoV is thought to be dromedary camels, but the mechanism of transmission from camels to humans is unknown. Most reported cases involved direct human-to-human transmission in health care settings. If MERS is suspected in a patient, infection control measures must be initiated promptly to prevent transmission in health care settings.

Symptoms and Signs

The incubation period for MERS-CoV is about 5 days.

Most reported cases have involved severe respiratory illness requiring hospitalization, with a case fatality rate of about 35%; however, at least 21% of patients had mild or no symptoms. Fever, chills, myalgia, and cough are common. Gastrointestinal symptoms (eg, diarrhea, vomiting, abdominal pain) occur in about one third of patients. Manifestations may be severe enough to require treatment in an intensive care unit, but recently, the proportion of such cases has declined sharply.


  • Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) testing of upper and lower respiratory secretions and serum

MERS should be suspected in patients who have an unexplained acute febrile lower respiratory infection and who have had either of the following within 14 days of symptom onset:

  • Travel to or residence in an area where MERS has recently been reported or where transmission could have occurred

  • Contact with a health care facility where MERS has been transmitted

  • Close contact with a patient who was ill with suspected MERS

MERS should also be suspected in patients who have had close contact with a patient with suspected MERS and who have a fever whether they have respiratory symptoms or not.

The most recent recommendations are available from the Centers for Disease Control and Prevention (MERS: Interim Guidance for Healthcare Professionals).

Testing should include real-time RT-PCR testing of upper and lower respiratory secretions, ideally taken from different sites and at different times. Serum should be obtained from patients and from all, even asymptomatic close contacts, including health care workers (to help identify mild or asymptomatic MERS). Serum is obtained immediately after MERS is suspected or after contacts are exposed (acute serum) and 3 to 4 weeks later (convalescent serum). Testing is done at state health departments or the Centers for Disease Control and Prevention.

In all patients, chest imaging detects abnormalities, which may be subtle or extensive, unilateral or bilateral. In some patients, levels of LDH and AST are elevated and/or levels of platelets and lymphocytes are low. A few patients have acute kidney injury. Disseminated intravascular coagulation and hemolysis may develop.


  • Supportive

Treatment of MERS is supportive. To help prevent spread from suspected cases, health care practitioners should use standard, contact, and airborne precautions.

There is no vaccine.

More Information

Severe Acute Respiratory Syndrome (SARS)

Severe acute respiratory syndrome (SARS) is a severe, acute respiratory illness caused by the SARS coronavirus (SARS-CoV).

SARS is much more severe than other coronavirus infections. SARS is an influenza-like illness that occasionally leads to progressively severe respiratory insufficiency.

SARS-CoV was first detected in the Guangdong province of China in November 2002 and subsequently spread to > 30 countries. In this outbreak, > 8000 cases were reported worldwide, with 774 deaths (about a 10% case fatality rate, which varied significantly by age, ranging from < 1% in people ≤ 24 years to > 50% in those ≥ 65 years). The SARS-CoV outbreak was the first time that the Centers for Disease Control and Prevention advised against travel to a region. This outbreak subsided, and no new cases have been identified since 2004. The immediate source was presumed to be civet cats, that were being sold for food in a live-animal market and had likely been infected through contact with a bat before they were captured for sale. Bats are frequent hosts of coronaviruses.

SARS-CoV is transmitted from person to person by close personal contact. It is thought to be transmitted most readily by respiratory droplets produced when an infected person coughs or sneezes.

Diagnosis of SARS is made clinically, and treatment is supportive. Coordination of prompt and rigid infection control practices helped control the 2002 outbreak rapidly.

Although no new cases have been reported since 2004, SARS should not be considered eliminated because the causative virus has an animal reservoir from which it conceivably could reemerge.

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